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Study on Vehicle tracking system using GPS

Vehicle Tracking GPS

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Study on Vehicle tracking system using GPS

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Vehicle Tracking System Using GPS

All acclamation and appreciation for almighty Allah the most beneficent and merciful who is the entire source of all

kind of wisdom and knowledge.2

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Vehicle Tracking System Using GPS

My special praise of the Holy Prophet Hazrat Muhammad (peace be upon him) the most perfect and exalted among and of every born on the earth. Who is forever

a torch of guidance and knowledge for humanity as a whole?

I thanks to Miss.Sundas, Preston University Lahore for his keen interest,

useful suggestion, consistent encouragement, incentive teaching, and

dynamic supervision throughout the course of his project.

Last but not least, I feel my proud privilege to mention the feeling of obligations toward my affectionate parents and family members, who

inspired me for higher education and supported financially and morally

throughout my study.

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TOOUR WORTHY FATHER

WHO ALWAYS INSPIRE AND ENCOURAGED

ME FOR WHAT I WANT AND GRAFTED IN THE

UNTIRING TO GET ON TO HIGHER IDEAL LIFE

OUR BELOVED MOTHER&

FAMILY MEMBERSWHICH I STRONGLY BELIEVE THATTHEIR PRAYERS ARE WITH ME AND

WILL ALWAYS.

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Electronics TechnologyGroup Members

MuhammadArslan 16D2-311059Hafiz M.Tahir 16D2-311045

Abd-ul-kareem 16D2-311061

Project Submitted To

Miss Sundas5

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Study on Vehicle tracking system using GPS

THIS PROJECT IS SUBMITTED TO

PIMSAT Institute of Higher Education FOR

THE PARTIAL FULFILMENT OF THE REQUIREMENTS FOR AWARDING THE

DEGREE OF B.Tech (Hons)

Electrical

Assigned by: _____________________________ Faculty Member’s Signature

Internal Examiner Sign: ____________________

Name: ___________________External Examiner Sign: _____________________ Name: ____________________

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Table of ContentsCHAPTER 1 INTRODUCTION

1.1 History of Vehicle tracking system1.2 Introduction to Global Positioning System1.3 Brief Introduction of G.P.S tracking system1.4 Block diagram of VTS1.5 How GPS works1.6 Real Time VTS using GSM and GPS Technology

CHAPTER 2 GPS LOCATION2.1 Trilateration - How to GPS determines a location2.2 How to current locations of GPS satellites are determined2.3 Computing the distance between your position and the GPS satellite.2.4 Four Satellites to give a 3D position

CHAPTER 3 DESIGN AND ANALYSIS3.1 Measuring GPS Accuracy3.2 Using Differential GPS to Increase Accuracy3.3 Levels of GPS Accuracy3.4 GPS and canopy3.5 GPS for GIS

CHAPTER 4 MAJOR COMPONENTS OF PROJECT4.1 The list of components use in vehicle tracking system4.2 The GPS Error Budget

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CHAPTER 5 MICROCONTROLLER AT89C515.1 Microcontroller AT89C51 in vehicle tracking system

CHAPTER 6 REFERENCE6.1 Reference

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INTRODUCTIONIt is developed by the U.S. Military commonly known as GPS (Global Positioning Satellite). This type of surveillance system calls for the following four elements. They are:

A TransmitterA SatelliteSender/ReceiverSoftware and Location Decoder

The reason GPS tracking is becoming so popular these days is that it is possible to conduct surveillance that in unmanned. That is, the equipment tracks the movement of the vehicle and its stop locations. Investigative agencies in the USA are starting to offer such services and it's time all investigative agencies become familiar with what this technology is all about.

1.1 A Little History of Vehicle Tracking Non-GPS Tracking SystemsIn the late 1960's and early 1970's, car trackers became available in the private sector. These early systems consisted of a receiver and a sender with at least two antennas that were placed on the vehicle. The surveillance investigator would keep the receiving end of the equipment with him in his vehicle. Basically all these units would do is tell you which direction the vehicle was located in and maybe the distance from the target vehicle. If you lost a vehicle, this type of system made is easier to locate the vehicle but they had their limits. It's important to point out that these units could not reveal the actual location of the target vehicle. They merely indicated which "direction" the target vehicle was in from the surveillance vehicle's location and the distance. One of the major problems with these units is that they had a limited range. Not only that, in huge metropolitan areas with tall buildings and lots of radio traffic, they rarely performed as needed and the range so sort in most instances, they become rather useless.

These non-GPS car tracking systems have been improved over the years and the range on them somewhat expanded but they still have their limitations. Understanding their limitations is important but they can still be used in real time mode to help locate a lost vehicle.

Let us go over each element of what is involved in AVL or GPS tracking.

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Fig 1.1 (a) A Photo of an Early Car Tracking System

Fig 1.1 (b) Pro Track One Non-GPS Tracking System

1.1.1 The Transmitter

An electronic transmitter that sends out signals from the vehicle. Naturally, this equipment will have to be placed somewhere on the vehicle in order for a signal to be transmitted.

Fig 1.1.1(a) The Transmitter

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Vehicle Tracking System Using GPS The typical equipment used for this employs very strong magnetic mount. You walk up to a vehicle and quickly place the entire box on the underside of the vehicle.

1.1.2 The Satellite

A Global positioning satellite obtains signals from the transmitter device placed on the vehicle. The use of only one satellite will give an accuracy rating of about 100 feet. This can be improved by using triangulation math which uses three satellites and gives more pinpointed accuracy down to about ten meters. Up until a short time ago, this was extremely expensive technology. However, the U.S. Department of Defense has placed a series of 24 Global Positioning System (GPS) satellites in orbit around the earth. These GPS satellites broadcast signals that contain time and identifier codes. GPS receivers use an antenna to acquire these signals from multiple satellites to determine a person's position, altitude, speed and direction of travel. These satellites are now being used by the public and can be used as online map plotting tracking in your vehicle as well are used as mobile vehicle locators in surveillance work.

1.1.3 The Sender and Receiver

Here is where options and different configurations come into play. There are what are called real time satellite tracking systems and none real time systems. Real time tracking systems tend to be more expensive than tracking systems that are not real time. With the lower end systems, you cannot plot the location as it actually happens. With real time tracking equipment, you install software on your computer and must have a separate unit attached to your computer that will receive the satellite signal. With this type of set-up, you are in a position to track a vehicle as its moving. The Protrack GPS system is one of the most popular configurations for real time tracking. The black box within the magnetic mount beans a signal to a GPS satellite and then receives its location. That signal is then forwarded through a cell phone connection to the user's computer. In none-real time tracking systems, the satellite signal is send back to the original sending box and stored for latter pickup. With this simpler type of system, you retrieve the box after the surveillance and then load the data from it into your computer. The Car Tracker is one of the more popular non-real time GPS tracking systems on the market today. Understanding the difference between real time GPS tracking and non-real time tracking is simple. Real time tracking let's you obtain the results right now from your computer desktop. Non-real-time tracking requires you to pick up the magnetic mount after the surveillance and obtain the results then.

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Fig 1.1.3(a) The ProTrack GPS System

At first look, real-time tracking seems to be the way to go. However, there are several draw backs to it. First, real-time tracking equipment is going to cost substantially more. Secondly, there are more signals that can get lost in the process. Remember, the signal is sent back to the magnetic mount then forwarded to the subject computer. An extra transmission step is involved. Any time that happens, more things have the potential of going wrong and there will be a greater potential of loss of signal in such a setup.

Power Supply Issues No matter what type of GPS tracking system one uses, some sort of power supply is needed. In the typical fast-action magnetic mount systems, standard self-contained battery power can be used. However, you have a limit to the amount of power time you have. Products like Car Tracker have an optional fuse box hardwire that can be wired into the vehicle power system but this, of course; requires position of the vehicle for such a technique. However, hardwiring can sometimes be employed in situations in which parents want to track their children's location for an extended period of time.

1.1.4 The Software and Location Decoder

A decoder converts the data received into an address. The location information which is generally in latitude and longitude is plotted with mapping software so the user can obtain actual maps of the location of the target vehicle. Thus computer generated map plotting programs are used to convert the latitude and longitude readings into an address on a computer generated digital map. This requires installing specific software onto your computer that will then covert the latitude and longitude data onto the map. Below is a photo of typical mapping software.

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Fig1.1.4(a) Location Software

Fig1.1.4(b) Location Software

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Fig1.1.4© Location Software

The mapping software we use in conjunction with the equipment lets the user zoom in and out of the mapping plots. Many of the better software programs used in conjunction with the GPS hardware such as the software that comes with Car tracker, creates automatically generated reports with times, dates and locations.

1.1.5 InvestigativeBenefits

The benefits of this new technology in surveillance is profoundBy using this technology, an actual physical surveillance by a human is not needed in many instances. When the goal of surveillance is to determine physical location only, this technology can be employed. The transmitter is simply covertly placed on the target vehicle. A computer program is used to collect location data at predetermined intervals. After a few days, the investigator simply has the computer print out the location data of the target vehicle. Of course, this technology will not make human activity in physical surveillance obsolete for obvious reasons any more than online searching made background investigation obsolete.

1.2 What is the Global Positioning System

The Global Positioning System was conceived in 1960 under the auspices of the U.S. Air Force, but in 1974 the other branches of the U.S. military joined the effort. The first satellites were launched into space in 1978. The System was declared fully operational in April 1995.

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Vehicle Tracking System Using GPS The Global Positioning System consists of 24 satellites,that circle the globe once every 12 hours, to provide worldwide position, time and velocity information. GPS makes it possible to precisely identify locations on the earth by measuring distance from the satellites. GPS allows you to record or create locations from places on the earth and help you navigate to and from those places.

Originally the System was designed only for military applications and it wasn’t until the 1980’s that it was made available for civilian use also.

1.3 The 3 segments of GPS1.3.1 The Space segment: The space segment consists of 24 satellites circling the earth at 12,000 miles in altitude. This high altitude allows the signals to cover a greater area. The satellites are arranged in their orbits so a GPS receiver on earth can always receive a signal from at least four satellites at any given time. Each satellite transmits low radio signals with a unique code on different frequencies, allowing the GPS receiver to identify the signals. The main purpose of these coded signals is to allow for calculating travel time from the satellite to the GPS receiver. The travel time multiplied by the speed of light equals the distance from the satellite to the GPS receiver. Since these are low power signals and won’t travel through solid objects, it is important to have a clear view of the sky.

1.3.2 The Control segment: The control segment tracks the satellites and then provides them with corrected orbital and time information. The control segment consists of four unmanned control stations and one master control station. The four unmanned stations receive data from the satellites and then send that information to the master control station where it is corrected and sent back to the GPS satellites.

1.3.3 The User segment: The user segment consists of the users and their GPS receivers. The number of simultaneous users is limitless.

1.4 How GPS WorksWhen a GPS receiver is turned on, it first downloads orbit information of all the satellites. This process, the first time, can take as long as 12.5 minutes, but once this information is download; it is stored in the receiver’s memory for future use.

Even though the GPS receiver knows the precise location of the satellites in space, it still needs to know the distance from each satellite it is receiving a signal from.

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Vehicle Tracking System Using GPS That distance is calculated, by the receiver, by multiplying the velocity of the transmitted signal by the time it takes the signal to reach the receiver. The receiver already knows the velocity, which is the speed of a radio wave or 186,000 miles per second (the speed of light).

To determine the time part of the formula, the receiver matches the satellites transmitted code to its own code, and by comparing them determines how much it needs to delay its code to match the satellites code. This delayed time is multiplied by the speed of light to get the distance.

The GPS receiver’s clock is less accurate than the atomic clock in the satellite; therefore, each distance measurement must be corrected to account for the GPS receiver’s internal clock error.

Fig 1.4 GPS working

1.4.1 Triangulation

Distance between satellite and receiver = "3" (times the speed of light)

Once both satellite and position are known for at least 4 satellites, the receiver can determine a position by triangulation.

1.5 Sources of GPS Error

GPS receivers have potential position errors due to some of the following sources:

1.5.1 User mistakes account for most GPS errorsIncorrect datum and typographic errors when inputting coordinates into a GPS receiver can result in errors up to many kilometers. Unknowingly relying on less than four satellites for

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Vehicle Tracking System Using GPS determining position coordinates can also result inunreliable position fixes that can easily be off by a distance in excess of a mile.

Even the human body can cause signal interference. Holding a GPS receiver close to the body can block some satellite signals and hinder accurate positioning.

If a GPS receiver must be hand held without benefit of an external antenna, facing to the south can help to alleviate signal blockage caused by the body because the majority of GPS satellites are oriented more in the earth's southern hemisphere. A GPS receiver has no way to identify and correcting user mistakes.

Satellite clock errors:Caused by slight discrepancies in each satellite's four atomic clocks. Errors are monitored and corrected by the Master Control Station.

Orbit errors:

Satellite orbit (referred to as "satellite ephemeris") pertains to the altitude, position and speed of the satellite. Satellite orbits vary due to gravitational pull and solar pressure fluctuations. Orbit errors are also monitored and corrected by the Master Control Station.

Ionosphere interference:The ionosphere is the layer of the atmosphere from 50 to 500 km altitude that consists primarily of ionized air. Ionospheric interference causes the GPS satellite radio signals to be refracted as they pass through the earth's atmosphere - causing the signals to slow down or speed up. This results in inaccurate position measurements by GPS receivers on the ground. Even though the satellite signals contain correction information for ionospheric interference, it can only remove about half of the possible 70 nanoseconds of delay, leaving potentially up to a ten meter horizontal error on the ground. GPS receivers also attempt to "average" the amount of signal speed reduction caused by the atmosphere when they calculate a position fix. But this works only to a point. Fortunately, error caused by atmospheric conditions is usually less than 10 meters. This source of error has been further reduced with the aid of the Wide Area Augmentation System (WAAS), a space and ground based augmentation to the GPS (to be covered later).

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Tropospheric interference: The troposphere is the lower layer of the earth's atmosphere (below 13 km) that experiences the changes in temperature, pressure, and humidity associated with weather changes. GPS errors are largely due to water vapor in this layer of the atmosphere. Tropospheric interference is fairly insignificant to GPS. Receiver noise is simply the electromagnetic field that the receiver's internal electronics generate when it's turned on. Electromagnetic fields tend to distort radio waves. This affects the travel time of the GPS signals before they can be processed by the receiver. Remote antennas can help to alleviate this noise. This error cannot be corrected by the GPS receiver.

Multipath interference is caused by reflected radio signals from surfaces near the GPS receiver that can either interfere with or be mistaken for the true signal that follows an uninterrupted path from a satellite.An example of multipath is the ghosting image that appears on a TV equipped with rabbit ear antennas. Multipath is difficult to detect and sometimes impossible for the user to avoid, or for the receiver to correct. Common sources of multipath include car bodies, buildings, power lines and water. When using GPS in a vehicle, placing an external antenna on the roof of the vehicle will eliminate most signal interference caused by the vehicle. Using a GPS receiver placed on the dashboard will always have some multipath interference.

Fig 1.5.1 Multipath interference

Selective Availability (S/A) was the intentional degradation of the satellite signals by a time varying bias. Selective Availability is controlled by the DOD to limit accuracy for non - U.S. military and government users and was originally instituted for security reasons. In May, 2000,

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Vehicle Tracking System Using GPS bowing to pressure from business and the White House, the Pentagon set Selective Availability to zero. The Pentagon did not turn S/A off, but rather merely reduced the amount of signal interference to zero meters, effectively eliminating intentional position errors.

The Pentagon retains the ability to reactivate S/A without notice to non government GPS users. So it's important to understand what Selective Availability is, and to be aware that it could be reactivated by the U.S. military at any time without prior notification.

Number of satellites visible:The more satellites the receiver can "see", the betterthe accuracy. Signal reception can be blocked by buildings, terrain, electronic interference and sometimes dense foliage. The clearer the view, to the receiver, the better the reception.

Satellite geometry:

This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry exists when the satellites are located in a line or in a tight grouping.

1.6 GPS Terminology 2D Positioning:

In terms of a GPS receiver, this means that the receiver is only able to lock on to three satellites which only allows for a two dimensional position fix. Without an altitude, there may be a substantial error in the horizontal coordinate.

3D Positioning:

Position calculations in three dimensions. The GPS receiver has locked on to 4 satellites. This provides an altitude in a addition to a horizontal coordinate, which means a much more accurate position fix.

Real Time Differential GPS:

Real-time DGPS employs a second, stationary GPS receiver at a precisely measured spot (usually established through traditional survey methods). This receiver corrects any errors found in the GPS signals, including atmospheric distortion, orbital anomalies, Selective Availability (when it existed), and other errors. A DGPS station is able to do this because its computer already knows its precise location, and can easily determine the amount of error provided by the GPS signals. DGPS corrects or reduces the effects of:

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Orbital errors Atmospheric distortion Selective Availability Satellite clock errors Receiver clock errors

DGPS cannot correct for GPS receiver noise in the user's receiver, multipath interference, and user mistakes. In order for DGPS to work properly, both the user's receiver and the DGPS station receiver must be accessing the same satellite signals at the same time.

Fig 1.6(a) Real Time Differential GPS

Wide Area Augmentation System:

The Wide Area Augmentation System (WAAS) is an experimental system designed to enhance and improve satellite navigation over the continental United States, and portions of Mexico and Canada.

Think of WAAS as a highly advanced real-time differential GPS. But instead of using ground based transmitters to broadcast position correction information, WAAS uses its own geostationary satellites in fixed orbit over North America. There are 25 ground reference

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Vehicle Tracking System Using GPS stations positioned across the United States (including three in southern Alaska, and one each in Puerto Rico and Hawaii) that monitor GPS satellite signals.

These stations continuously receive and correct GPS satellite information against their own known precise positions. Each WAAS ground station (referred to as a Wide Area Reference Station, or WRS) then sends its corrected GPS data to one of two master control stations located on the Pacific and Atlantic coasts of the U.S. These master control stations create a correction message that weeds out atmospheric distortion, GPS satellite orbit and clock errors and time errors. This message is then broadcast to the two WAAS satellites.

These in turn re- broadcast the correction information using the basic GPS signal structure to any WAAS capable GPS receiver.

Unlike differential GPS which requires additional equipment to work, the WAAS is available to anyone equipped with a WAAS capable GPS receiver in much of the United States and portions of Mexico. However, the System has its limitations at this time, including poor coverage over portions of the northern United States, and very slow signal acquisition time. WAAS capable GPS receivers are now widely available.

Fig 1.6(b) Wide Area Augmentation System

Waypoint (Landmark)

A waypoint is based on geographic coordinate values entered into the receiver's

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Vehicle Tracking System Using GPS memory. These coordinates represents either a saved receiver position fix, or user entered coordinates. A Waypoint will have a designated alphanumeric name, or a user supplied name. Once entered and saved it remains static in the GPS receiver's memory until edited or deleted. To turn a position fix into a waypoint is simply a matter of saving the receiver's current position as a waypoint. The receiver must be locked on to enough satellites to provide at least a two-dimensional (2D), or three-dimensional (3D) fix. The receiver will give the position coordinates an alpha-numeric name, or the user can designate a unique name. Once this happens the position fix becomes a waypoint with static coordinates saved in memory.

A waypoint can also be created from coordinates derived other than by GPS. Coordinates on a map can become a waypoint. Coordinates radioed from person in a remote location to another person can also become a waypoint once they are programmed into a GPS receiver.

Route

Routes are just a sequence of waypoints. Once you pass one waypoint, the next waypoint will be navigated to. When you first activate a route, the GPS receiver will assume that the first leg is A to B. B is the waypoint being navigated to and A is the anchor point that defines the first leg of the route.

Fig 1.6(c) Routes

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Vehicle Tracking System Using GPS Track Log

A track log is the GPS units' record of your travel or where you have been. As you move along your every movement is being stored. Receivers with a Track Back feature will allow you to reverse your route taking you back the same way you originally traveled. Most GPS receivers show you your track as you move along on a map screen.

Go to Function

The Goto function gives GPS receivers the capability of leading you to any place you have specified. You simply enter the coordinate of your desired destination into the GPS receiver as a waypoint and then, by using the Goto function, you tell the receiver to guide you there. The receiver guides you to your destination using a steering screen. There are several different versions of a steering screen, but they do the same thing of pointing you in the direction you need to go to get from your present position to the waypoint you selected.

Note: The following terms are used by Garmin in its GPS manuals. Other GPS receiver manufacturers may use different terminology.

Active from waypoint is your starting waypoint, or the receiver's last waypoint in an active route.

Active GOTO waypoint is your designated destination in the receiver, whether in an active route, or as a single waypoint.

Active leg is always a straight line between the last waypoint and the GOTO waypoint. A GPS receiver always plots the most efficient, straight-line course of travel between two points - the active leg. If the receiver is following a route, the active leg will be the desired track between the last waypoint in the route, and the next waypoint in the route. If the receiver has deviated from the route, the receiver selects the closest leg to its position and makes it the active leg in the route (the next waypoint in the route list becomes the GOTO destination waypoint).

Bearing (BRG)/Desired Track (DTK): In GPS the term bearing is used in place of 28 azimuths. As used in GPS, bearing is the compass direction (expressed in degrees) from your present position to your desired destination waypoint, or the compass direction between any two waypoints.

Tracking (TRK)/ Heading (HDG) is the direction you are moving, expressed in degrees from north. The direction you are actually traveling.

Course Made Good (CMG) or Course Over Ground (COG) is your present direction of travel expressed in degrees from north. It is not necessarily the most direct path to a GOTO waypoint.

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Vehicle Tracking System Using GPS If traveling from one waypoint to another (using GOTO), then XTE (see below) will show the distance of deviation of your actual route from the active leg (a straight line) between those waypoints.

Crosstrack Error (XTE) is the distance off the desired track (active leg) on either side of the active leg. It's the linear difference between the Desired Track (DTK) and your actual Course Made Good (CMG).

Course Deviation Indicator (CDI) graphically shows the amount and direction of XTE.

Desired track (DTK) is shown in degrees from north. It's a function of GOTO. DTK is measured along the active leg (a straight line between two waypoints in a route), or from your current position to a designated GOTO waypoint (when not navigating a route).

Speed Over Ground (SOG) is the velocity you are traveling.

Velocity Made Good (VMG): Velocity made good is the speed at which you approach your destination. If you are directly on course, VMG is the same value as SOG, but if you stray from course, VMG decreases and is less than SOG.

Estimated Position Error (EPE)A measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality.

Estimated Time Enroute (ETE)The time left to your destination based upon your present speed and course.

Estimated Time of Arrival (ETA)The time of day of your arrival at a destination

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Fig 1.6 (d) GPS Navigation

GPS Receiver Inputs

The following are inputs that are needed before you use your GPS receiver.

Position Format: Input what units you want your position. Examples: Latitude and Longitude Degrees - Minutes - Seconds (had mm' ss.s", N 43 - 40'- 55.8" E) UTM (11T 0557442m E 4836621m N)

Map Datum:Make sure the map datum in your GPS receiver matches the map datum of the map you are using when you are going to plot points on that map. Some common map data used are WGS 84, NAD 27 and NAD 83.

Distance:Input distance units such as: Nautical or Statute miles, Metric, Yards Elevation:Input elevation units in feet or meters

North Reference: Input the North reference you want to use. Examples: True, Magnetic or Grid

Time: Input time format for 12 or 24 hour and input the correct time zone.

The NAVSTAR Global Positioning System (GPS) is a satellite-based radio-positioning and time- transfer system designed, financed, deployed, and operated by the U.S. Department of Defense. GPS has also demonstrated a significant benefit to the civilian community who are applying GPS to a rapidly expanding number of applications. What attracts us to GPS is:

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The relatively high positioning accuracies, from tens of meters down to the millimeter. The capability of determining velocity and time, to an accuracy commensurate with

position. The signals are available to users anywhere on the globe: in the air, on the ground, or at sea.

It is a positioning system with no user charges that simply requires the use of relatively lowcost hardware.

It is an all-weather system, available 24 hours a day.

The position information is in three dimensions, that is, vertical as well as horizontal informationis provided.

The number of civilian users is already significantly greater than that of the military users. However, for the time being the U.S. military still operates several "levers" with which they control the performance of GPS (Section 1.2.3). Nevertheless, despite the handicap of GPS being a military system there continues to be tremendous product innovation within the civilian sector, and it is ironic that this innovative drive is partly directed to developing technology and procedures to overcome some of the constraints to GPS performance which have been applied by the system's military operators.

2.1 Introduction to the System Components

2.1.1 System Design Considerations

Development work on GPS commenced within the U.S. Department of Defense in 1973, the motivation being to develop an all-weather, 24-hour, global positioning system to support the positioning requirements for the armed forces of the U.S. and its allies. (For a background to the development of the GPS system the reader is referred to [1].) The system was therefore designed to replace the large variety of navigational systems already in use, and great emphasis was placed on the system's reliability and survivability. In short, a number of stringent conditions had to be met:

Suitable for all classes of platform: aircraft (jet to helicopter), ship, land (vehicle-mounted to handheld) and space (missiles and satellites), able to handle a wide variety of dynamics, real-time positioning, velocity and time determination capability to an appropriate accuracy, the positioning results were to be available on a single global geodetic datum, highest accuracy to be restricted to a certain class of user, resistant to jamming (intentional and unintentional), redundancy provisions to ensure the survivability of the system, passive positioning system that does not require the transmission of signals from the user to the satellite(s), able to provide the service to an unlimited number of users, low cost, low power, therefore as much complexity as

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Vehicle Tracking System Using GPS possible should be built into the satellite segment, and total replacement of the Transit1 satellite and other terrestrial nevoid systems.

This led to a design based on the following essential concepts:

A one-way ranging system, in which the satellites transmit signals, but are unaware of who is using the signal (no receiving function). As a result the user (or listener) cannot easily be: (a) detected by the enemy (military context), or (b) charged for using the system (civilian context). Use of the latest atomic clock and microwave transmission technology, including spread- spectrum techniques. A system that makes range-like measurements with the aid of pseudo-random binary codes modulated on carrier signals. Satellite signals that are unaffected by cloud and rain. A multiple satellite system which ensures there is always a sufficient number of satellites visible simultaneously anywhere on the globe, and at any time. Positioning accuracy degradation that is graceful.

What was perhaps unforeseen by the system designers was the power of product innovation, which has added significantly to the versatility of the GPS as a system for precise positioning and navigation. For example, GPS is able to support a number of positioning and measurement modes in order to satisfy simultaneously a variety of users, from those satisfied with general navigational accuracies (tens of meters) to those demanding very high (sub-centimeter) relative positioning accuracies. It has now so penetrated certain applications areas that it is difficult for us to imagine life without GPS!

Rarely have so many seemingly unrelated technological advances been required to make a complex system such as GPS work. Briefly they are:

Space System Reliability: The U.S. space program had by 1973 demonstrated the reliability of space hardware. In particular, the Transit system had offered important lessons. The Transit satellites were originally designed to last 2-3 years in orbit, yet some of the satellites have operated well beyond their design life. In fact Transit continued to perform reliably for over 25 years.

Atomic Clock Technology: With the development of atomic clocks a new era of precise time- keeping had commenced. However, before the GPS program was launched these precise clocks had never been tested in space. The development of reliable, stable, compact, space-qualified atomic frequency oscillators (rubidium, and then cesium) was therefore a significant technological breakthrough. The advanced clocks now being used on the GPS satellites routinely achieve long-term frequency stability in the range of a few parts in 1014per day (about 1 sec in 3,000,000 years!). This long-term stability is one of the keys to GPS, as it allows

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Vehicle Tracking System Using GPS for the autonomous, synchronized generation and transmission of accurate timing signals by each of the GPS satellites without continuous monitoring from the ground.

Quartz Crystal Oscillator Technology: In order to keep the cost of user equipment down, quartz crystal oscillators were proposed (similar to those used in modern digital watches), rather than using atomic clocks as in the GPS satellites. Besides their low cost, quartz oscillators have excellent short-term stability. However, their long-term drift must be accounted for as part of the user position determination process.

Precise Satellite Tracking and Orbit Determination: Successful operation of GPS, as well as the Transit system, depends on the precise knowledge and prediction of a satellite's position with respect to an earth-fixed reference system. Tracking data collected by ground monitor stations is analyzed to determine the satellite orbit over the period of tracking (typically one week). This reference ephemeris is extrapolated into the future and the data is then up-loaded to the satellites. Prediction accuracies of the satellite coordinates, for one day, at the few meter level have been demonstrated.

Spread-Spectrum Technology: The ability to track and obtain any selected GPS satellite signal (a receiver will be required to track a number of satellites at the same time), in the presence of considerable ambient noise is a critical technology. This is now possible using spread-spectrum and pseudo-random-noise coding techniques.

Large-Scale Integrated Circuit Technology: To realize the desired low cost, low power and small size necessary for much of the user equipment, the GPS program relies heavily on the successful application of VLSI circuits, and powerful computing capabilities built onto them. The GPS system consists of three segments (figure 1.1). (Good general references on the GPS system are [2, 3].):

The Space Segment: comprising the satellites and the transmitted signals.

The Control Segment: the ground facilities carrying out the task of satellite tracking, orbit computations, telemetry and supervision necessary for the daily control of the space segment.

The User Segment: the entire spectrum of applications equipment and computational techniques that is available to the users.

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Figure 2.1: GPS System Elements.

2.1.2 The Space Segment

The Space Segment consists of the constellation of spacecraft and the signals broadcast by them which allow users to determine position, velocity and time. The basic functions of the satellites are to:

Receive and store data transmitted by the Control Segment stations. Maintain accurate time by means of several onboard atomic clocks. Transmit information and signals to users on two L-band frequencies. Provide a stable platform and orbit for the L-band transmitters.

Several constellations of GPS satellites have been deployed, and several more are planned. The experimental satellites, the so-called "Block I" satellites, were built by the Rockwell Corporation. The first was launched in February 1978, and the last of the eleven satellite series (one exploded on the launch pad) was launched in 1985. The operational series of GPS satellites, the "Block II" and "Block IIA" satellites were also built by the Rockwell Corporation. The 20 replacement "Block IIR" series of satellites, first launched in 1997, are built by the General Electric Corporation (now the Lockheed Martin Corporation). The "Block IIF" series are still in the design phase and may, for example, incorporate an additional civilian transmission frequency. They are planned for launch from 2005 onwards. The operational satellite I.D.s are separated into three space vehicle numbering series: SVN 13 through 21 for the Block II, SVN 22 through 40 for Block IIA, and SVN 41 and above for the Block IIR satellites.

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Vehicle Tracking System Using GPS The current status of the GPS constellation and such details as the launch and official commissioning date, the orbital plane and position within the plane, the satellite I.D. number(s), etc., can be obtained from several electronic GPS information sources on the Internet. Section 2.2.1 describes the general satellite orbit characteristics.

Figure 2.2: GPS Satellite Signal Components

Each GPS satellite transmits a unique navigational signal centre on two L-band frequencies of the electromagnetic spectrum, permitting the ionosphere propagation effect on the signals to be eliminated. At these frequencies the signals are highly directional and so are easily reflected or blocked by solid objects. Clouds are easily penetrated, but the signals may be blocked by foliage (the extent of blockage is dependent on the type and density of the leaves and branches, and whether they are wet or dry). The signal is transmitted with enough power to ensure a minimum signal power level of - 160dBw at the earth's surface (the maximum it is likely to reach is about -153dBw, see [3]). The satellite signal consists of the following components (figure 2.2):

The two L-band carrier waves. The ranging codes modulated on the carrier waves. The so-called "navigation message".

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Vehicle Tracking System Using GPS Modulated onto the carrier waves are the PRN ranging codes and navigation message for the user. The primary function of the ranging codes is to permit the signal transit time (from satellite to receiver) to be determined. The transit time when multiplied by the velocity of light then gives a measure of the receiver-satellite "range" (in reality the measurement process is considerably more complex). The navigation message contains the satellite orbit information, satellite clock parameters, and pertinent general system information necessary for real-time navigation to be performed. All signal components are derived from the output of a highly stable atomic clock. Each GPS satellite is equipped with several cesium and rubidium atomic clocks.

2.1.3 The Control Segment

The Control Segment consists of facilities necessary for satellite health monitoring, telemetry, tracking, command and control, satellite orbit and clock data computations, and data uplinking. There are five ground facility stations: Hawaii, Colorado Springs, Ascension Island, Diego Garcia and Kwajalein. All are owned and operated by the U.S. Department of Defense and perform the following functions:

• All five stations are Monitor Stations, equipped with GPS receivers to track the satellites. The resultant tracking data is sent to the Master Control Station.

• Colorado Springs is the Master Control Station (MCS), where the tracking data are processed

in order to compute the satellite ephemerides and satellite clock corrections. It is also the station that initiates all operations of the space segment, such as spacecraft manoeuvring, signal encryption, satellite clock-keeping, etc.

Three of the stations (Ascension Is., Diego Garcia, and Kwajalein) are Upload Stations allowing for the uplink of data to the satellites. The data includes the orbit and clock correction information transmitted within the navigation message, as well as command telemetry from the MCS.

Overall operation of the Control and Space Segments is the responsibility of the U.S. Air Force Space Command, Second Space Wing, Satellite Control Squadron at the Falcon Air Force Base, Colorado.

Each of the upload stations can view all the satellites once a day. All satellites are therefore in contact with an upload station three times a day, and new navigation messages as well as command telemetry can be transmitted to the GPS satellites every eight hours if necessary. The computation of: (a) the satellite orbits or "ephemerides", and (b) the determination of the

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Vehicle Tracking System Using GPS satellite clock errors, are the most important function of the Control Segment. The first is necessary because the GPS satellites function as "orbiting control stations" and their coordinates must be known to a relatively high accuracy, while the latter permit a significant measurement bias to be reduced.

Fig 2.1.3 (a) the Control Segment

The GPS satellites travel at high velocity (of the order of 4 km/sec), but within a more or less regular orbit pattern. After a satellite has separated from its launch rocket and it begins orbiting the earth, it's orbit is defined by its initial position and velocity, and the various force fields acting on the satellite. In the case of the gravitational field for a spherically symmetric body (a reasonable approximation of the earth at the level of about 1 part in 103) this produces an elliptical orbit which is fixed in space the Keplerian ellipse. Due to the effects of the other, non-spherical gravitational components of the earth's gravity field, and non-gravitational forces, which perturb the orbit, the actual trajectory of the satellite departs from the ideal Keplerianellipse.

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Fig 2.1.3 (b) the Control Segment

The most significant forces that influence satellite motion are:

The spherical and non-spherical gravitational attraction of the earth, the gravitational attractions of the sun, moon, and planets (the "third body" effects), atmospheric drag effects, solar radiation pressure (both direct and albedo effects), and the variable part of the earth's gravitational field arising from the solid earth and ocean tides.

To determine the motion of a satellite to a high precision these perturbing forces need to be modeled accurately. If these forces were known perfectly and the initial position and velocity of the satellite were given, then the integration of the Equations of Motion would give the satellite's position and velocity at any time in the future.

However, the perturbing forces are not known to sufficient precision. An "orbit computation" process is therefore performed, in which satellite observations obtained at tracking sites of known position (in the case of GPS, the monitor stations of the Control Segment) are analyzed in order to produce an orbit that is a "best fit" to the available observations. This involves the adjustment of the appropriate parameters of the orbit; possibly together with several

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Vehicle Tracking System Using GPS additional forces model parameters (see [4]). Determining the orbit is a complex procedure, and in the case of GPS satellites this process occurs on a continuous, automatic basis.

The product of the orbit computation process at the MCS is the satellite ephemeris (or trajectory). A satellite ephemeris may be expressed in a number of forms:

A list of 3-D coordinates, and velocities, at regular intervals of time,

TheKeplerian elements at some reference epoch, plus their rate-of-change with time,

A polynomial representation of the trajectory in a suitable reference system, such as along-track, cross-track, or radial components, or satellite position and velocity at some reference time epoch, and requiring these values to be derived for subsequent times by integrating the Equations of Motion.

The GPS broadcast ephemeris, as represented in the navigation message, is actually a combination of all of the above orbit representations ([12]). The orbital ephemerides are expressed in the reference system most appropriate for positioning, which is an earth-fixed reference system such as WGS84. Hence the Control Segment has the function of propagating the satellite datum (Section 1.2.5), which users connect to via the transmitted satellite ephemerides.

The behavior of each GPS satellite clock is monitored against GPS Time, as maintained by an ensemble of atomic clocks at the GPS Master Control Station. The satellite clock bias, drift and drift- rate relative to GPS Time are explicitly determined in the same procedure as the estimation of the satellite ephemeris.

The clock behavior so determined is made available to all GPS users via clock error coefficients (defining the missynchronization with GPS Time) in a polynomial form broadcast in the navigation message. However, what is available to users is really a prediction of the clock behavior for some future time interval.

Due to random deviations -- even cesium and rubidium oscillators are not entirely predictable -- the deterministic models of satellite clock error are only accurate to about 20 nanoseconds. This is not precise enough for accurate range measurement (see Section 1.3.6).

As the GPS system matures we can expect that the satellites will operate with greater independence from the ground-based Control Segment, without significant degradation in performance. For example the "Block IIR" and "Block IIF" satellites will have a crosslink capability enabling between-satellite communication and ranging.

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Vehicle Tracking System Using GPS The satellites will talk to each other! This data will be processed to produce the ephemeris information within the space segment, with relatively little operator control having to be exercised.

1.1.4 The User Segment -- The Applications

This is the part of the GPS system with which we are most concerned -- the space and control segments being largely transparent to the operations of the navigation function. Of interest is the range of GPS:

• Applications,

• Equipment, and

• Positioning strategies.

The "engine" of commercial GPS product development is, without doubt, the user applications. Each day new applications are being identified, each with its unique requirements with regards to: accuracy of the results, reliability, operational constraints, user hardware, data processing algorithms, latency of the GPS results, etc. To make sense of the bewildering range of GPS applications it may be useful to classify them according to the following:

Land, Sea and Air Navigation and Tracking, including enrooted as well as precise navigation, collision avoidance, cargo monitoring, vehicle tracking, search and rescue operations, etc. While the accuracy requirement may be modest and the user hardware is generally comparatively low cost, the reliability, integrity and speed with which the results are needed are generally high.

Surveying and Mapping, on land, at sea and from the air. Includes geophysical and resource surveys, GIS data capture surveys, etc. The applications are of relatively high accuracy, for positioning in both the static and moving receiver mode, and generally require specialized hardware and data processing software.

Military Applications. Although these are largely mirrored by civilian applications, the military

GPS systems are generally developed to "military specifications" and a greater emphasis is placed on system reliability.

Recreational Uses, on land, at sea and in the air. The primary requirement is for low cost instruments which are very easy to use.

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Vehicle Tracking System Using GPS Other specialized uses, such as time transfer, attitude determination, spacecraft

operations, atmospheric studies, etc. Obviously such applications require specially developed, high cost systems, often with additional demanding requirements such as real-time operation, etc.

GPS user equipment has undergone an extensive program of development, both in the military and civilian area. In this context, GPS "equipment" refers to the combination of:

hardware, software, and Operational procedures or requirements.

While the military R&D programs have concentrated on achieving a high degree of miniaturization, modularization and reliability, the civilian user equipment manufacturers have, in addition, sought to bring down costs and to develop features that enhance the capabilities of the positioning system. The following general remarks can be made. Civilian users have, from the earliest days of GPS availability, demanded ever increasing levels of performance, in particular higher accuracy and improved reliability. This is particularly true of the survey user seeking levels of accuracy several orders of magnitudehigher than that of the navigation user. In some respects the GPS user technology is being driven by the precise positioning market -- in much the same way that automotive technology often benefits from car racing. Yet another major influence on the development of GPS equipment has been the increasing variety of civilian applications. For although there may exist a similar positioning accuracy requirement across many user applications, to address a particular application in the most satisfactory manner, a specific combination of hardware and software features is often required.

It is expected that the worldwide market for GPS receiver equipment will grow from about US$1 billion at the present time, to over US$8 billion by the year 2000! Market surveys suggest that the greatest growth is expected to be in the commercial and consumer markets such as ITS applications, integration of GPS and cellular phones, and portable GPS for outdoor recreation and similar activities. These could account for more than 60% of the GPS market by the turn of the century. There are at present over 100 manufacturers of GPS instruments of varying kinds -- GPS instrumentation is discussed further in Section 1.4.

1.1.5 The User Segment -- Positioning Principles

The basic concept of GPS positioning is that of positioning-by-ranges. The geometrical principles of positioning can be demonstrated in terms of the intersection of locii. In the two-dimensional case, a measured range to a known point constrains the position to lie on circle with the

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Vehicle Tracking System Using GPS measured range as radius. In three dimensions a measured range to a known point constrains the position in 3-D space toile on the surface of a sphere centered at the known point, with radius being the measured distance.

In the case of GPS, the distance measurement is made to a satellite with known position (coordinates are obtained from the satellite ephemeris data transmitted within the navigation message), however the principle applies to any range measuring positioning system, terrestrial or satellite-based.

In two dimensions, position can be defined as the intersection of two circles, involving distances d1 and d2 to two known points, as shown in figure 1.5. Note that there are two possible solutions, only one of which is correct. In general one solution can be discarded rather easily through apriori knowledge of approximate position and velocity. Another possibility is to measure another range to a third point and if all ranges are measured without error the intersection of three LOPs is a single uniquely defined point.

Fig 2.1.5 positioning-by-ranges

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Figure 2.1.5(b): Surfaces of Position for Range Measurements.

In the three-dimensional case, the intersection of three spheres describes two points in space, only one of which is correct (figure 1.6). Hence, a minimum of three ranges are required, to three separated known points, in order to solve the 3-D position problem. The quality of the positioning solution is dependent, amongst other things, on the accuracy with which the ranges can be measured and the geometry of the intersection.

Fig 2.1.5(c) Intersection of Surfaces of Position Based on Range Measurements

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Vehicle Tracking System Using GPS If the point being positioned is stationary, the two (or three) ranges do not need to be measured simultaneously. If the point is moving however, all ranges must be measured simultaneously (or over an interval of time during which the point has not moved by an amount greater than the uncertainty of the "fix"). Because the GPS constellation was designed to ensure at least four satellites are always visible anywhere on the earth, satellite positioning using simultaneously measured ranges is the basic positioning strategy for most navigation applications.

However, there is still the issue of how to account for measurement biases, as the technology used for making GPS range measurements does not give calibrated distance from the receiver to the satellite. Disturbing influences and errors in fact contaminate the range measurements to an unacceptable degree (in effect the radii of the spheres are incorrect -- Section 1.3.5), and hence the basic positioning principle is modified in several ways to satisfy the varying levels of accuracies required by different applications.

1.2 GPS Satellite Constellation and Signals

1.2.1 GPS Constellation Design

The operational Block II/IIA satellite constellation was to be fully deployed by the late 1980's. However, a number of factors, the main one being the Space Shuttle Challenger disaster (28 January 1986), has meant that the GPS system only became operational in the 1990's as far as most users were concerned. Full Operational Capability was declared on 17 July 1995 -- 24 Block II/IIA satellites operating satisfactorily. At an altitude of approximately 20,200km, a constellation of 24 functioning GPS satellites is sufficient to ensure that there will always be at least four satellites visible, at all unobstructed sites on the globe. Typically there are 6 to 10 satellites visible most of the day. The U.S. Department of Defense has undertaken to guarantee 24 satellite coverage 70% of the time, and 21 satellite coverage 98% of the time.

As the GPS satellites are in nearly circular orbits, at an altitude of approximately 20,200km above the earth (figure 1.7), this has a number of consequences:

Their orbital period is approximately 11hrs 58mins, so that each satellite makes two revolutions in one sidereal day (the period taken for the earth to complete one rotation about its axis with respect to the stars).

At the end of a sidereal day (23hrs 56mins in length) the satellites are again over the same position on earth.

Reckoned in terms of a solar day (24hrs in length), the satellites are in the same position in the sky about four minutes earlier each day.

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Vehicle Tracking System Using GPS The orbit groundtrack approximately repeats each day, except that there is a very small

drift of the orbital plane to the west which is arrested by periodic manoeuvres.

2.1.5(d) The GPS Constellation "Birdcage"

The following general remarks can be made with regard to satellite constellation design for navigation purposes:

• The higher a satellite, the longer it is visible above the horizon (the extreme case is the geostationary satellites).

• The higher a satellite, the better the coverage due to longer fly-over passes and extended visibility of the satellite across large areas of the earth.

• The higher a satellite, the less the rate-of-change of distance, and the lower the Doppler frequency of a transmitted signal.

• The greater the angle of inclination, the more northerly the track of the sub-satellite point across the surface of the earth.

• No satellite can be seen simultaneously from all locations on the earth.

• Depending on the positioning principles being employed, there may be a requirement for observations to be made to more than one satellite simultaneously from more than one ground station.

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Vehicle Tracking System Using GPS The Block II satellites are deployed in six orbital planes at 60o intervals about the equator, with each containing four satellites. The satellites can be moved round their orbits if it becomes necessary to "cover" for a failed satellite. The orbital planes are inclined at an angle of 55o to the equatorial plane.

As the satellites are at an altitude of more than three times the earth's radius, a particular satellite may be above an observer's horizon for many hours, perhaps 6-7 hours or more in the one pass. At various times of the day, and at various locations on the surface of the earth, the number of satellites and the length of time they are above an observer's horizon will vary. Although at certain times of the day there may be up to 12 satellites visible simultaneously, there are nevertheless occasional periods of degraded satellite coverage (though naturally their frequency and duration will increase if satellites fail). "Degraded satellite coverage" is generally defined in terms of the magnitude of the Dilution of Precision (DOP) factor, a measure of the quality of satellite geometry. The highest the DOP value, the poorer the satellite geometry. For example, if all the visible satellites are located in the same part of the sky, the intersections of the SOPs will be very obtuse.

1.2.2 GPS Signal Components

The basis of the GPS signal are the two L-band carrier signals. These are generated by multiplying the fundamental frequency fo (10.23MHz) by 154 and 120, yielding the two microwave L-band carrier waves L1 and L2 respectively (figure 1.2). The frequency of the two waves are: fL1 = fo x 154 = 1575.42 MHz, and fL2 = fo x 120 = 1227.6 MHz. These are radio frequency waves capable of transmission through the atmosphere over great distances, but which cannot penetrate solid objects. Note that all GPS satellites transmit carrier waves at the same two L-band frequencies (unlike the GLONASS system, where a different frequency is assigned to each satellite -- see Section 2.3.2). However, the L-band carrier waves themselves carry no information, and must be modified (or modulated) in some way. In the Global Positioning System the L-band carrier waves are modulated by two ranging codes, and the navigation message. The two distinct GPS ranging codes are:

• The C/A code (sometimes referred to as the "clear/access" or "coarse/acquisition" code), sometimes also referred to as the "S code".

• The P code (the "private" or "precise" code) was designed for use only by the military, and other authorized users.

The L1 carrier was designed to be modulated with both the P and C/A codes, whereas the L2 carrier would be modulated only with the P code. Under the policy of Anti-Spoofing (Section

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Vehicle Tracking System Using GPS 1.2.3) the P code is encrypted through modulation by a further secret code (the "W code") to produce a new "Y code". Both carrier signals contain the navigation message.

The C/A and P (or Y) codes provide the means by which a GPS receiver can measure one-way ranges to the satellites. These codes have the characteristics of random noise, but are in fact binary codes generated by mathematical algorithms and are therefore referred to as "pseudo-random-noise" or PRN codes. Both the C/A and P code generating algorithms are known, and are based on a simple Tapped Feedback Shift Register scheme (see, for example [2,3]). One C/A code is assigned to each GPS satellite (the PRN code number is often used as the satellite I.D.). Each C/A code is a 1023 "chip" long binary sequence, generated at a rate of 1.023 million chips per second (that is, 1.023 MHz), thus the entire C/A code sequence repeats every millisecond.

The P code is a far more complex binary sequence of 0's and 1's, being some 267 days long with a chipping rate at the fundamental frequency fo (10.23 MHz). The resolution of this code (length of the

P code chip) is ten times the resolution of the C/A code. Instead of assigning each satellite a unique code, as is the case with the C/A code, the P code is allocated such that each satellite transmits a one week portion of the 267 day long PRN sequence, restarting the code sequence at the end of each week. Further details on how PRN codes are generated are given in, for example, [5,6].

To measure one-way range (from satellite to receiver), a knowledge of the ranging code(s) is required by the user's receiver. Knowing which PRN code is being transmitted by a satellite means that a receiver can generate a local replica of the same code sequence. These PRN codes possess a very important attribute: a given C/A (or P or Y) code will fully correlate with an exact replica of itself only when the two codes are aligned, and has a low degree of correlation with other alignments.

As the same P (or Y) code is synchronously modulated on both carrier waves, any difference in signal transit time of the same PRN sequence is due to the retardation of the two L-band signals by a different amount as they travel through the ionosphere. The effect of the ionosphere on signal propagation is essentially a function of signal frequency (Section 2.1.1), hence measurement on both frequencies is a very effective way of overcoming the ionospheric signal delay.

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Vehicle Tracking System Using GPS 1.2.3 GPS Satellite Ephemerides

How are the GPS satellite ephemerides computed? As the forces of gravitational and non-gravitational origin perturb the motion of the GPS satellites, the coordinates of the satellites in relation to the WGS84 reference system (Section 3.1) must be continually determined through the analysis of tracking data. In the case of the GPS broadcast ephemeris, this procedure is a three-step process ([6]):

• An off-line orbit determination is performed through the analysis of tracking to generate a "reference orbit". This is an initial estimate of the satellite trajectories computed from about one week's of tracking from the five Control Segment monitor stations.

• An on-line daily updating of the reference orbit using a Kalman filter as new data are added. This provides the current estimates of the satellite orbit which is used to predict the future orbit.

• The ephemeris is estimated for 1 to 14 days into the future. To obtain the necessary broadcast information, curve fits are made to 4 to 6 hour portions of the extrapolated ephemeris, and hourly orbit parameters determined.

Note that these parameters are not true Keplerian elements as they only describe the ephemeris over the interval of applicability and not for the whole orbit. (Although only intended for use during the transmission period, they do, however, adequately describe the orbit over intervals of 1.5 to 5 or more hours, with a graceful degradation in accuracy.). The user can derive the earth-centred, earth-fixed WGS84 Cartesian coordinates of the GPS satellite from the broadcast orbital parameters, using an algorithm described in [11], and implemented in every GPS receiver.

1.3 Ranging Using PRN Codes Consider for a moment a perfect system, where all satellite clocks are synchronized to the same time system: the GPS Time (GPST). Furthermore, the ground receiver's clock also maintains the same synchronization, and none of the clocks drift from this GPST scale. Now suppose the satellite starts transmitting its L1 carrier (modulated with the combined C/A PRN code and navigation data). At the same instant, the receiver begins generating the C/A PRN code corresponding to that particular satellite (see figure 1.8). Under these circumstances the satellite and receiver generated C/A codes would be output in unison. When the satellite signal is received, however, it will be lagging the receiver generated code due to the signal transit

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Vehicle Tracking System Using GPS time. Multiplying the time offset required to align the two codes (in effect determining the signal transit time) by the speed of light yields the satellite to receiver distance.

Measuring ranges simultaneously in this fashion to three satellites would fix one's position at the intersection of three spheres of known radii (the satellite ranges), centred at each satellite whose coordinates can be calculated from the navigation message. In reality the situation is more complex:

• GPS receivers are equipped with crystal clocks that do not keep the same time as the more stable satellite clocks (the satellite clocks can be nearly synchronised to GPST using the clock correction model transmitted in the navigation message). Consequently each range is contaminated by the receiver clock error. This range quantity is therefore referred to as pseudo-range, and in order for the user to derive position from pseudo-range data, the receiver equipment is required to track (a minimum of) four satellites, and solve for four unknown quantities: the three-dimensional position components and the receiver-clock offset (from GPST) -- see Section 1.3.5. This is the basis of GPS real-time navigation, and why GPS could be considered an example of a time-difference-of-arrival system.

• There is in fact a 300 km "ambiguity" in the C/A code pseudo-range measurements (300 km is the approximate length of the C/A code sequence). That is, all measured "distances" appear to have a range of 0 to 300 km. This ambiguity is resolved in a number of ways, but the easiest to assume that if the approximate receiver position is known to within say 100 km, the "missing" component of the distance can be determined, and hence the raw pseudo-range measurement can be corrected for this ambiguity to obtain the full satellite-receiver distance.

• Ranging (and hence receiver position determination) can be carried out using the C/A code or the P code. P code ranging can be done on the combination of the two frequencies, hence eliminating the bias due to ionospheric refraction. Furthermore, the C/A code is "coarser", and hence the C/A derived ranges are subject to greater measurement "noise". The absence of a C/A code on L2 is intentional, as one of the accuracy limitations of the GPS system for the general class of civilian users.

• As previously mentioned, this differentiation between ranging codes, and the formulation ofpolicies for their use (in peacetime and in times of global emergencies), is responsible for the provision of two GPS services: The Precise Positioning Service based on P or Y code (dual- frequency) ranging, and the Standard Positioning Service based on single frequency C/A code ranging (Section 2.2.3).

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Fig 1.3 One-Way Ranging Using PRN Codes

1.4 GPS Instrumentation The following components of a generic GPS receiver can be identified (figure 1.9):

• Antenna and Preamplifier: Antennas used for GPS receivers have broadbeam characteristics, thus they do not have to be pointed to the signal source like satellite TV receiving dishes. The antennas are compact and a variety of designs are possible. There is a trend to integrating the antenna assembly with the receiver electronics.

• Radio Frequency Section and Computer Processor: The RF section contains the signal processing electronics. Different receiver types use somewhat different techniques to process the signal. There is a powerful processor onboard not only to carry out computations such as extracting the ephemerides and determining the elevation/azimuth of the satellites, etc., but also to control the tracking and measurement function within modern digital circuits, and in some cases to carry out digital signal processing.

• Control Unit Interface: The control unit enables the operator to interact with the microprocessor. Its size and type varies greatly for different receivers, ranging from a handheld unit to soft keys surrounding an LCD screen fixed to the receiver "box".

• Recording Device: in the case of GPS receivers intended for specialised uses such as the surveying the measured data must be stored in some way for later data processing. In the case of ITS applications such as the logging of vehicle movement, only the GPS-derived coordinates and velocity may be recorded. A variety of storage devices were utilised in the past, including

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Vehicle Tracking System Using GPS cassette and tape recorders, floppy disks and computer tapes, etc., but these days almost all receivers utilise solid state (RAM) memory or removable memory "cards".

• Power Supply: Transportable GPS receivers these days need low voltage DC power. The trend towards more energy efficient instrumentation is a strong one and most GPS receivers operate from a number of power sources, including internal NiCad or Lithium batteries, external batteries such as wet cell car batteries, or from mains power.

Fig 1.4 The Generic GPS Receiver

The antenna and RF technology components are briefly discussed below. For further details the reader is referred to [6,16].

2.4.1 Antennas

The task of the antenna is to convert the energy of the arriving electromagnetic waves into an electric current that can be processed by the receiver electronics. There are a number of special considerations as far as antenna design is concerned:

the antenna must be able to pick and discriminate very weak signals, the antenna may need to operate at just the L1 frequency, or at both the L1 and L2

frequencies, as the signals are right-hand circularly polarised, the GPS antenna must also be right-

handcircularly polarised, antenna gain pattern that enhances the ability of the RF section to discriminate against

multipathsignals (such as left-hand circularly polarised signals), a stable electrical centre, low cost, and reliable.

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Vehicle Tracking System Using GPS There have been several types of GPS antennas used:

monopole or dipole configurations, quadrifilar helices, spiral helices, microstrip, choke ring, and other multipath resistant designs

In short, the antennas are required to be rugged, simple in construction, have stable phase centres, be resistant to multipath, have good gain and pattern coverage characteristics. The microstrip antenna is almost universally used for navigation applications.

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GPS LOCATION2.1 Trilateration - How to GPS determines a

locationIn a nutshell, GPS is based on satellite ranging - calculating the distances between the receiver and the position of 3 or more satellites (4 or more if elevation is desired) and then applying some good old mathematics. Assuming the positions of the satellites are known, the location of the receiver can be calculated by determining the distance from each of the satellites to the receiver. GPS takes these 3 or more known references and measured distances and "triangulates" an additional position.

As an example, assume that I have asked you to find me at a stationary position based upon a few clues which I am willing to give you. First, I tell you that I am exactly 10 miles away from your house. You would know I am somewhere on the perimeter of a sphere that has an origin as your house and a radius of 10 miles. With this information alone, you would have a difficult time to find me since there are an infinite number of locations on the perimeter of that sphere.

Second, I tell you that I am also exactly 12 miles away from the ABC Grocery Store. Now you can define a second sphere with its origin at the store and a radius of 12 miles. You know that I am located somewhere in the space where the perimeters of these two spheres intersect - but there are still many possibilities to define my location.

Adding additional spheres will further reduce the number of possible locations. In fact, a third origin and distance (I tell you am 8 miles away from the City Clock) narrows my position down to just 2 points. By adding one more sphere, you can pinpoint my exact location. Actually, the 4th sphere may not be necessary. One of the possibilities may not make sense, and therefore can be eliminated.

For example, if you know I am above sea level, you can reject a point that has negative elevation. Mathematics and computers allow us to determine the correct point with only 3 satellites Based on this example; you can see that you need to know the following information in order to compute your position:

2.2 How the Current Locations of GPS Satellites are Determined

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Vehicle Tracking System Using GPS GPS satellites are orbiting the Earth at an altitude of 11,000 miles. The DOD can predict the paths of the satellites vs. time with great accuracy. Furthermore, the satellites can be periodically adjusted by huge land-based radar systems. Therefore, the orbits, and thus the locations of the satellites, are known in advance. Today's GPS receivers store this orbit information for all of the GPS satellites in what is known as an almanac. Think of the almanac as a "bus schedule" advising you of where each satellite will be at a particular time. Each GPS satellite continually broadcasts the almanac. Your GPS receiver will automatically collect this information and store it for future reference.

The Department of Defense constantly monitors the orbit of the satellites looking for deviations from predicted values. Any deviations (caused by natural atmospheric phenomenon such as gravity), are known as ephemeriserrors. When ephemeris errors are determined to exist for a satellite, the errors are sent back up to that satellite, which in turn broadcasts the errors as part of the standard message, supplying this information to the GPS receivers.

By using the information from the almanac in conjunction with the ephemeris error data, the position of a GPS satellite can be very precisely determined for a given time.

2.3 Computing the distance between your position and the GPS satellite

GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it takes a radio signal (the GPS signal) to travel from the satellite to the receiver. Radio waves travel at the speed of light, which is about 186,000 miles per second.

So, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal's travel time can be determined.

In order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. The code received from the satellite can be compared with the code generated by the receiver.

By comparing the codes, the time difference between when the satellite generated the code and when the receiver generated the code can be determined. This interval is the travel time of the code. Multiplying this travel time, in seconds, by 186,000 miles per second gives the distance from the receiver position to the satellite in miles.

2.4 Four (4) Satellites to give a 3D positionIn the previous example, you saw that it took only 3 measurements to "triangulate" a 3D position. However, GPS needs a 4th satellite to provide a 3D position. Why??

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Vehicle Tracking System Using GPS Three measurements can be used to locate a point, assuming the GPS receiver and satellite clocks are precisely and continually synchronized, thereby allowing the distance calculations to be accurately determined. Unfortunately, it is impossible to synchronize these two clocks, since the clocks in GPS receivers are not as accurate as the very precise and expensive atomic clocks in the satellites. The GPS signals travel from the satellite to the receiver very fast, so if the two clocks are off by only a small fraction, the determined position data may be considerably distorted.

The atomic clocks aboard the satellites maintain their time to a very high degree of accuracy. However, there will always be a slight variation in clock rates from satellite to satellite. Close monitoring of the clock of each satellite from the ground permits the control station to insert a message in the signal of each satellite which precisely describes the drift rate of that satellite's clock. The insertion of the drift rate effectively synchronizes all of the GPS satellite clocks.

The same procedure cannot be applied to the clock in a GPS receiver. Therefore, a fourth variable (in addition to x, y and z), time, must be determined in order to calculate a precise location. Mathematically, to solve for four unknowns (x,y,z, and t), there must be four equations. In determining GPS positions, the four equations are represented by signals from four different satellites.

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DESIGN AND ANALYSIS3.1 Measuring GPS Accuracy

As discussed above, there are several external sources which introduce errors into a GPS position. While the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometry, of the group of satellites (constellation) from which signals are being received. The geometry of the constellation is evaluated for several factors, all of which fall into the category of Dilution Of Precision, or DOP.

Fig 4.1(a) Dilution Of Precision

DOP: DOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites you use for the measurement. Different satellite geometries can magnify or lessen the errors in the error budget described above. A greater angle between the satellites lowers the DOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement configuration.

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Fig 4.1 (b) Dilution Of Precision

Some GPS receivers can analyze the positions of the satellites available, based upon the almanac, and choose those satellites with the best geometry in order to make the DOP as low as possible. Another important GPS receiver feature is to be able to ignore or eliminate GPS readings with DOP values that exceed user-defined limits. Other GPS receivers may have the ability to use all of the satellites in view, thus minimizing the DOP as much as possible.

3.2 Using Differential GPS to Increase Accuracy

As powerful as GPS is, +/-50 - 100 meters of uncertainty is not acceptable in many applications. How can we obtain higher accuracies?

A technique called differential correction is necessary to get accuracies within 1 -5 meters, or even better, with advanced equipment. Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point (typically it is a surveyed benchmark). Because the physical location of the base station is known, a correction factor can be computed by comparing the known location with the GPS location determined by using the satellites.

The differential correction process takes this correction factor and applies it to the GPS data collected by a GPS receiver in the field. Differential correction eliminates most of the errors listed in the GPS Error Budget discussed earlier. After differential correction, the GPS Error Budget changes as follows:

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Source Uncorrected With DifferentialIonosphere 0-30 meters mostly removedTroposphere 0-30 meters All RemovedSignal Noise 0-10 meters All RemovedEphemeris Data 1-5 meters All RemovedClock Drift 0-1.5 meters All RemovedMultipath 0-1 meters Not RemovedSA 0-70 meters All Removed

By eliminating many of the above errors, differential correction allows GPS positions to be computed at a much higher level of accuracy.

3.3 Levels of GPS AccuracyThere are three types of GPS receivers which are available in today's marketplace. Each of the three types offers different levels of accuracy, and has different requirements to obtain those accuracies. To this point, the discussion in this book has focused on Coarse Acquisition (C/A code) GPS receivers. The two remaining types of GPS receiver are Carrier Phase receivers and Dual Frequency receivers.

3.3.1 C/A Code receivers

C/A Code receivers typically provide 1-5 meter GPS position accuracy with differential correction. C/A Code GPS receivers provide a sufficient degree of accuracy to make them useful in most GIS applications.

C/A Code receivers can provide 1-5 meter GPS position accuracy with an occupation time of 1 second. Longer occupation times (up to 3 minutes) will provide GPS position accuracies consistently within 1-3 meters. Recent advances in GPS receiver design will now allow a C/A Code receiver to provide sub-meter accuracy, down to 30 cm.

3.3.2 Carrier Phase receivers

Carrier Phase receivers typically provide 10-30 cm GPS position accuracy with differential correction. Carrier Phase receivers provide the higher level of accuracy demanded by certain GIS applications.

Carrier Phase receivers measure the distance from the receiver to the satellites by counting the number of waves that carry the C/A Code signal. This method of determining position is much more accurate; however, it does require a substantially higher occupation time to attain 10-30

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Vehicle Tracking System Using GPS cm accuracy. Initializing a Carrier Phase GPS job on a known point requires an occupation time of about 5 minutes. Initializing a Carrier Phase GPS job on an unknown point requires an occupation time of about 30-40 minutes.

Additional requirements, such as maintaining the same satellite constellation throughout the job, performance under canopy and the need to be very close to a base station, limit the applicability of Carrier Phase GPS receivers to many GIS applications.

3.3.3 Dual-Frequency receivers

Dual-Frequency receivers are capable of providing sub-centimeter GPS position accuracy with differential correction. Dual-Frequency receivers provide "survey grade" accuracies not often required for GIS applications.

Dual-Frequency receivers receive signals from the satellites on two frequencies simultaneously. Receiving GPS signals on two frequencies simultaneously allows the receiver to determine very precise positions.

3.4 GPS and CanopyGPS receivers require a line of sight to the satellites in order to obtain a signal representative of the true distance from the satellite to the receiver. Therefore, any object in the path of the signal has the potential to interfere with the reception of that signal. Objects which can block a GPS signal include tree canopy, buildings and terrain features.

Further, reflective surfaces can cause the GPS signals to bounce before arriving at a receiver, thus causing an error in the distance calculation. This problem, known as multipath, can be caused by a variety of materials including water, glass and metal. The water contained in the leaves of vegetation can produce multipath error. In some instances, operating under heavy, wet forest canopy can degrade the ability of a GPS receiver to track satellites.

There are several data collection techniques which can mitigate the effects of signal blockage by tree canopy or other objects. For example, many GPS receivers can be instructed to track only the highest satellites in the sky, as opposed to those satellites which provide the best DOP. Increasing the elevation of the GPS antenna can also dramatically increase the ability of the receiver to track satellites.

Unfortunately, there will be locations where GPS signals simply are not available due to obstruction. In these cases, there are additional techniques which can help to solve the problem. Some GPS receivers have the ability to collect an offset point, which involves recording a GPS position at a location where GPS signals are available

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Vehicle Tracking System Using GPS while also recording the distance, bearing and slope from the GPS antenna to the position of interest where the GPS signals are not available. This technique is useful for avoiding a dense timber stand or building.

Further, a traditional traverse program can be used to manually enter a series of bearings and ranges to generate positions until satellite signals can again be received. This position data can then be used to augment position data collected with the GPS receiver.

3.5 GPS for GISUp to this point, the discussion has focused on describing how GPS determines a location on the surface of the Earth. Now the discussion can shift to the process of describing what is at the location. The "what" is the object or objects which will be mapped? These objects are referred to as "Features", and are used to build a GIS. It is the power of GPS to precisely locate these Features which adds so much to the utility of the GIS system. On the other hand, without Feature data, a coordinate location is of little value.

3.5.1 Feature Types

There are three types of Feature which can be mapped: Points, Lines and Areas. A Point Feature is a single GPS coordinate position which is identified with a specific Object. A Line Feature is a collection of GPS positions which are identified with the same Object and linked together to form a line. An Area Feature is very similar to a Line Feature, except that the ends of the line are tied to each other to form a closed area.

3.5.2 Describing Features

As stated above, a Feature is the object which will be mapped by the GPS system. The ability to describe a Feature in terms of a multi-layered database is essential for successful integration with any GIS system. For example, it is possible to map the location of each house on a city block and simply label each coordinate position as a house. However, the addition of information such as color, size, cost, occupants, etc. will provide the ability to sort and classify the houses by these categories.

These categories of descriptions for a Feature are known as Attributes. Attributes can be thought of as questions which are asked about the Feature. Using the example above, the Attributes of the Feature "house" would be "color", "size", "cost" and "occupants".

Logically, each question asked by the Attributes must have an answer. The answers to the questions posed by the Attributes are called Values. In the example above, an appropriate Value (answer) for the Attribute (question) "color" may be "blue".

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The following table illustrates the relationship between Features, Attributes and Values: Feature Attribute ValueHouse Color Blue

Size 3 BDRCost $118KOccupants 5

By collecting the same type of data for each house which is mapped, a database is created. Tying this database to position information is the core philosophy underlying any GIS system.

3.5.3 Feature Lists

The field data entry process can be streamlined by the use of a Feature List. The Feature List is a database which contains a listing of the Features which will be mapped, as well as the associated Attributes for each Feature. In addition, the Feature List contains a selection of appropriate Values for each Attribute. The Feature List can be created on the CMT hand-held GPS data collector, or on a PC. Below is an example of a Feature List as it appears in PC-GPS:

When a Feature List is used in the field, the first step is to select the Feature to be mapped. Once a Feature is selected, the Attributes for that Feature are automatically listed. A Value for each Attribute can then be selected from the displayed list of predetermined Values.

The use of a Feature List streamlines the data entry process and also ensures consistent data entry among different users in the same organization.

3.5.3 Exporting to a GIS System

The final step in incorporating GPS data with a GIS system is to export the GPS and Feature data into the GIS system. During this process, a GIS "layer" is created for each Feature in the GPS job. For example, the process of exporting a GPS job which contains data for House, Road and Lot Features would create a House layer, a Road layer and a Lot layer in the GIS system. These layers can then be incorporated with existing GIS data.

Once the GPS job has been exported, the full power of the GIS system can be used to classify and evaluate the data.

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MAJOR COMPONENTS OF PROJECT

4.1 The list of components use in vehicle tracking system

Micro real-time vehicle tracking system provides you details information’s including vehicle or driver name, ignition status, location, directional heading displayed on a fully featured map, vehicle travel time tracking, vehicle idle time tracking, vehicle mileage tracking report, vehicle speed tracking, location and status tracking.

General

Subscription for online software One Time

GPS and GSM Antenna Internal

Historical Data Yes

Digital Input 5

Analog Input 5

SIM Card Internal

Show current location Yes

Remote Shut down Optional

Replay tracking Yes

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Vehicle Tracking System Using GPS User login Yes

Live Tracking Yes

GEO Zone create Optional

Multi vehicle view on Map alert Only for fleet owner

Battery Disconnect On screen

No Data Transmission On screen

Over speed alert On screen Optional

Ignition ON/OFF On screen

GEO fence in SMS/EMAIL

GEO fence out SMS/EMAIL

4.2 The GPS Error BudgetThe GPS system has been designed to be as nearly accurate as possible. However, there are still errors. Added together, these errors can cause a deviation of +/- 50 -100 meters from the actual GPS receiver position. There are several sources for these errors, the most significant of which are discussed below:

Atmospheric Conditions

The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

Ephemeris Errors/Clock Drift/Measurement Noise

As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 1-5 meters of positional error, clock drift disparity can introduce

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Vehicle Tracking System Using GPS 0-1.5 meters of positional error and measurement noise can introduce 0-10 meters of positional error.

Selective Availability

Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and epherimis signal by the Department of Defense. SA can introduce 0-70 meters of positional error. Fortunately, positional errors caused by SA can be removed by differential correction.

Multipath

A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user.

The chart below lists the most common sources of error in GPS positions. This chart is commonly known as the GPS Error Budget:

GPS Error Budget

Source Uncorrected Error LevelIonosphere 0-30 metersTroposphere 0-30 metersMeasurement Noise 0-10 metersEphemeris Data 1-5 metersClock Drift 0-1.5 metersMultipath 0-1 meterSelective Availability 0-70 meters

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MICROCONTROLLER AT89C5

5.1 Microcontroller AT89C51 in vehicle tracking system

The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of Flash programmable and erasable read only memory (PEROM). The device is manufactured using Atmel's high-density nonvolatile memory technology and is compatible with the industry-standard MCS-51 instruction set and pin out.

The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a highly-flexible and cost-effective solution to many embedded control applications.

The AT89C51 provides the following standard features: 4K bytes of Flash, 128 bytes of RAM, 32 I/O lines, two 16-bit timer/counters, five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The Power-down Mode saves the RAM contents but freezes the oscillator disabling all other chip functions until the next hardware reset.

Pin Description

VCC Supply voltageGND Ground

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Port 0

Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as high- impedance inputs.

Port 0 may also be configured to be the multiplexed low- order address/data bus during accesses to external pro- gram and data memory. In this mode P0 has internal pull-ups. Port 0 also receives the code bytes during Flash programming, and outputs the code bytes during program verification. External pull-ups are required during program verification.

Port 1

Port 1 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 1 output buffers can sink/source four TTL inputs. When 1s are written to Port 1 pins they are pulled high by the

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Vehicle Tracking System Using GPS internal pull-ups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups.

Port 1 also receives the low-order address bytes during Flash programming and verification.

Port 2

Port 2 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 2 output buffers can sink/source four TTL inputs. When 1s are written to Port 2 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups.

Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that uses 16-bit addresses (MOVX @ DPTR). In this application, it uses strong internal pull-ups when emitting 1s. During accesses to external data memory that uses 8-bit addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special Function Register. Port 2 also receives the high-order address bits and some control signals during Flash programming and verification.

Port 3

Port 3 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 3 output buffers can sink/source four TTL inputs. When 1s are written to Port 3 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will source current (IIL) because of the pull-ups. Port 3 also serves the functions of various special features of the AT89C51 as listed below:

Port 3 also receives some control signals for Flash pro- griming and verification.

RST

Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device.

ALE/PROG

Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during Flash programming. In

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Vehicle Tracking System Using GPS normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each access to external Data Memory.

If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.

PSEN

Program Store Enable is the read strobe to external pro- gram memory. When the AT89C51 is executing code from external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory.

EA/VPP

External Access Enable EA must be strapped to GND in order to enable the device to fetch code from external pro- gram memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC for internal program executions. This pin also receives the 12-volt programming enable volt- age (VPP) during Flash programming, for parts that require 12-volt VPP.

XTAL1

Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

XTAL2

Output from the inverting oscillator amplifier.

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REFERENCE http://www.nhdfl.org/library/pdf/Forest%20Protection/Introduction%20to%20Global

%20Positioning%20System.pdf http://www.gmat.unsw.edu.au/snap/gps/gps_notes1.pdf

http://www.ijecse.org/wp-content/uploads/2012/08/Volume-1Number-3PP-1103- 1107.pdf

http://www.atmel.com/images/doc0265.pdf

[1] PARKINSON, B.W., 1994. GPS eyewitness: the early years. GPS World, 5(9), 32-45. [2] KAPLAN, E. (ed.), 1996. Understanding GPS: Principles & Applications. Artech House Publishers, Boston London, 554pp. [3] SPILKER Jr., J.J. & PARKINSON, B.W. (eds.), 1995. Global Positioning Systems: Theory & Applications. American Institute of Aeronautics & Astronautics (AIAA), 1995,

Vol.1(694pp), Vol.2(601pp). [4] SEEBER, G., 1993. Satellite Geodesy: Foundations, Methods & Applications. Walter

de Gruyter, Berlin New York, 531pp. [5] SPILKER Jr., J.J., 1980. GPS signal structure and performance characteristics. In:

Global Positioning System, papers published in Navigation, reprinted by the (U.S.) Inst. of

Navigation, Vol.1, 29-54. [6] WELLS, D.E., BECK, N., DELIKARAOGLOU, D., KLEUSBERG, A., KRAKIWSKY, E.J., LACHAPELKLE, G., LANGLEY, R.B., NAKIBOGLU, M., SCHWARZ, K.P., TRANQUILLA, J.M. &

VANICEK, P., 1987. Guide to GPS Positioning. 2nd. ed. Canadian GPS Associates, Fredericton, New Brunswick, Canada, 600pp.

[7] GEORGIADOU, Y. & DOUCET, K.D., 1990. The issue of Selective Availability. GPS World,

1(5), 53-56.

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Vehicle Tracking System Using GPS [8] N.R.C., 1995. The Global Positioning System: a shared national asset. Rept. by

National Research Council, National Academy Press, 264pp. [9] N.A.P.A, 1995. The Global Positioning System: charting the future. Rept. by National

Academy of Public Administration & the National Research Council, National Academy Press,

332pp. [10] GIBBONS, G., 1996. A national GPS policy. GPS World, 7(5), 48-50. [11] VAN DIERENDONCK, A.J., RUSSELL, S.S., KOPTIZKE, E.R. & BIRNBAUM, M., 1980. The GPS navigation message. In: Global Positioning System, papers published in

Navigation, reprinted by the (U.S.) Inst. of Navigation, Vol.1, 55-73. [12] LANGLEY, R.B., 1991b. The orbits of GPS satellites. GPS World, 2(3), 50-53. [13]

LANGLEY, R.B., 1993. The GPS observables. GPS World, 4(4), 52-59. [14] LANGLEY, R.B., 1991d. Time, clocks, and GPS. GPS World, 2(10), 38-42. [15]

LANGLEY, R.B., 1991c. The mathematics of GPS. GPS World, 2(7), 45-50.

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