A Report onGlobal Positioning System (GPS)

Prepared by• K.GAUTHAM REDDY - 2011A8PS364G

A Report prepared in partial fulfilment of the requirements of the course

EEE F472: SATELLITE COMMUNICATIONINSTRUCTOR: M.K.Deshmukh

Birla Institute of Technology and Science – Pilani12/04/2014

IntroductionOur ancestors had to go to pretty extreme measures to keep from getting lost. They erected monumental landmarks, laboriously drafted detailed maps and learned to read the stars in the night sky.

Things are much, much easier today. The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil and commercial users around the world. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver.

When people talk about "a GPS," they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else.

Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.

A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.

GPS is comprised of three main segments: space, control and users. The space segment consists of a constellation of U.S. satellites, placed so that at least three satellites are positioned above the horizon from any point on earth. PNT states that as of October 2009, 35 GPS satellites were in use. The control segment includes monitoring stations located worldwide charged with monitoring the GPS system. The user segment is made up of GPS receivers.

GPS provides the user with a precise location by utilizing radio frequencies. The GPS receiver translates the information from at least three GPS satellites to provide the user with a two-dimensional location of latitudinal and longitudinal position on earth. If a fourth satellite is available, then the receiver can provide the user with three-dimensional location information, which includes altitude in addition to latitude and longitude.

Navigation enables a user to process his current location based on GPS data and travel to his desired location, also based on accurate GPS data. Any user with a working GPS receiver can

navigate to a particular destination, whether traveling on foot, by automobile, by airplane or by ship. GPS navigation is even accurate underground.

The standard mode of high accuracy differential positioning requires one reference GPS receiver to be located at a "base station" whose coordinates are known, while the second user GPS receiver simultaneously tracks the same satellite signals. When the carrier phase data from the two receivers is combined and processed, the user receiver's coordinates are determined relative to the reference receiver. However, the use of carrier phase data comes at a cost in terms of overall system complexity because the measurements are ambiguous, requiring the incorporation of an "ambiguity resolution" (AR) algorithm within the data processing software. Developments in GPS user receiver hardware have gone a significant way towards improving the performance of AR.

The distance from the user receiver to the nearest reference receiver may range from a few kilometres to hundreds of kilometres. As the receiver separation increases, the problems of accounting for distance-dependent biases grows and, as a consequence, reliable ambiguity resolution becomes an even greater challenge. On the other hand, developments in "GPS Geodesy" have been so successful in the last 15 years, that relative accuracies of "a few parts per billion" are now possible even without AR. However, for so-called "high productivity" carrier phase-based GPS techniques, AR is crucial when small amounts of data are used

Objectives of GPS:

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. Military actions was the original intent for GPS, however in the 1980s, the U.S. government decided to allow the GPS program to be used by civilians. Weather conditions do not affect the ability for GPS to work. The systems works 24/7 anywhere in the world. There are no subscription fees or setup charges to use GPS.

Main objectives of GPS devices in military:

1) Military GPS user equipment has been integrated into fighters, bombers, tankers, helicopters, ships, submarines, tanks, jeeps, and soldiers' equipment.

2) In addition to basic navigation activities, military applications of GPS include target designation of cruise missiles and precision-guided weapons and close air support.

3) To prevent GPS interception by the enemy, the government controls GPS receiver exports

4) GPS satellites also can contain nuclear detonation detectors.

Main objectives of GPS devices apart from military are to provide:

Automobiles are often equipped GPS receivers.

1) They show moving maps and information about your position on the map, speed you are traveling, buildings, highways, exits etc.

2) Some of the market leaders in this technology are Garmin and Tom Tom, not to mention the built in GPS navigational systems from automotive manufacturers.

For aircraft, GPS provides

1) Continuous, reliable, and accurate positioning information for all phases of flight on a global basis, freely available to all.

2)Safe, flexible, and fuel-efficient routes for airspace service providers and airspace users.

3)Potential decommissioning and reduction of expensive ground based navigation facilities, systems, and services.

4)Increased safety for surface movement operations made possible by situational awareness.

Agriculture

1) GPS provides precision soil sampling, data collection, and data analysis, enable localized variation of chemical applications and planting density to suit specific areas of the field.

2) Ability to work through low visibility field conditions such as rain, dust, fog and darkness increases productivity

Disaster Relief

1) Deliver disaster relief to impacted areas faster, saving lives.

2) Provide position information for mapping of disaster regions where little or no mapping information is available.

3) Example, using the precise position information provided by GPS, scientists can study how strain builds up slowly over time in an attempt to characterize and possibly anticipate earthquakes in the future.

Sports that entail navigation can opt to leave out their compass and other traditional navigation gadgets and go for the digital and technologically advanced gadgets. Sports enthusiasts who are constantly on the move, like mountaineers, hikers or even runners, can sport the GPS sports watch, which works like a small computer.

There is a more specialized GPS system on the market that caters to users who drive cars. This is called a sat-nav or street navigation GPS system. Not only does this type of GPS system tell you where your destination is in detailed directions, it can also tell you your car's mileage, the estimated time of arrival and the speed at which your car is going. It can also employ a voice system to "speak" to you and tell you the directions.

Technological advancements have given way to the integration of GPS into our mobile phones, whether in the form of Personal Digital Assistant (PDA) phones or the standard mobile phone. Most advanced phones have built-in GPS systems with a pre-loaded map or with an additional card slot to accommodate more memory for downloaded maps.

Design challenges involved in the development of GPS

The reason why the actual locational position is significantly less accurate than the data transmitted by the satellite is due to various influences on the signal. These can be collectively termed local and atmospheric effects. Local effects are detrimental conditions on the ground near the receiver or in the receiver’s software while atmospheric effects are problems with the medium through which the signal passes

Local effects:

1) Receiver Clock Error: This is the error in the offset of the GPS measurement of the pseudo random code and the time recorded by the satellite for the data. The receiver attempts to compensate for this with additional measurements but it remains the single largest error that affects positional accuracy

2) Percentage Sky Visible: This is of concern when getting an initial fix and generally causes the second largest error in calculated positions. It is linked to satellite geometry (below) and is a measure of how obscured the sky is. In areas where large parts of the sky are out of sight to the receiver, such as beneath a cliff or when surrounded by buildings, the error in the calculated position will be very large. This is also an issue in areas where the receiver antenna is beneath a thick forest canopy when the signal can be lost altogether

3) Satellite Geometry: GPS receivers are only accurate when the quality of the data they receive is of a high standard. When the satellites being used for determining position are clustered together or all within one hemisphere the quality of the data will be poor. For accurate positions GPS receivers require satellite coverage from across the sky.

4) Multipath Error: When the receiver calculates the length of time the signal has taken to travel from the satellite to use in determining distance to the satellite it assumes the signal has taken the shortest path i.e., a geometric straight line. In actuality the signal may have bounced off a surface before reaching the receiver and the travel time could be slightly longer because of this. In these occasions the receiver will overestimate the distance to the satellite. Atmospheric effects

1) Ionospheric Effects: All GPS signals travel through the charged plasma of the ionosphere. This can cause the signal to be attenuated (slowed down). Any changes in the signal involve changes in the travel time and thus affect calculated positions similar to multipath errors.

2) Tropospheric Effects: The water particles in the upper atmosphere cause very slight changes to the signal. These are very small but can affect minor changes. Error type and its compensation:

Working principle of GPS:

A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. Trilateration in three-dimensional space can be a little tricky, so we'll start with an explanation of simple two-dimensional trilateration.

2-D Trilateration

Imagine you are somewhere in the United States and you are TOTALLY lost -- for whatever reason, you have absolutely no clue where you are. You find a friendly local and ask, "Where am I?" He says, "You are 625 miles from Boise, Idaho."

This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles, like this:

You ask somebody else where you are, and she says, "You are 690 miles from Minneapolis, Minnesota." Now you're getting somewhere. If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points, if you are 625 miles from Boise and 690 miles from Minneapolis.

If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are -- Denver, Colorado.

1 2 3

This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles. In the next section, we'll look at this type of trilateration.

3-D trilateration

Fundamentally, three-dimensional trilateration isn't much different from two-dimensional trilateration, but it's a little trickier to visualize. Imagine the radii from the previous examples going off in all directions. So instead of a series of circles, you get a series of spheres.

If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.

The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.

In order to make this simple calculation, then, the GPS receiver has to know two things:

1) The location of at least three satellites above you

2) The distance between you and each of those satellites

The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signals from the GPS satellites. Better units have multiple receivers, so they can pick up signals from several satellites simultaneously.

Radio waves are electromagnetic energy, which means they travel at the speed of light (about 186,000 miles per second, 300,000 km per second in a vacuum). The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive.

GPS receiver design

GPS use satellite data to calculate an accurate position on the earth. These calculations can relate the user’s position to almost any map projection within milli-seconds. All GPS work in a similar manner but they often look very different and have different software. The most significant difference between GPS receivers is the number of satellites they can simultaneously communicate with. Most receivers are described as 12 channel meaning they can communicate with 12 satellites. Older models may be 8 or even 5 channel with more modern receivers capable of communicating with 14 – 20. Given the current makeup of the GPS satellite’s constellation 12 channel is more than adequate.

Almost all units have an LCD screen or at least software that links to a PC/PDA with an output screen. The unit might have several different pages that can be displayed on screen but usually the default page is very similar. Commonly on starting a receiver you will be presented with a map of the satellites in view. The GPS receiver shows a view of the sky split into four quadrants. These represent the NE, SE, SW, NW parts of the sky, with the concentric circles representing

the horizon at 90° from the zenith, with the inner circles representing 60° and 30°. The cross at the centre represents the zenith. The dots/circles represent the satellites and the bars at the bottom represent satellite signal strength. The higher the bar the stronger the signal. This display is typical of a 12 channel set. The dots and bars will commonly be labelled with a number to represent the identity of the satellite.The bars are commonly either hollow or solid (usually white or black on a monochrome display). Hollow lines represent a satellite for which the Ephemeris data is not known. It is therefore not being used to calculate a position. Black bars represent “Fixed” satellites whose ephemeris data has been collected successfully. These satellites are thus available for calculating a position. This is not consistent across all models and some may use grey bars as well as hollow bars to represent satellites not yet fixed.

The number, position and strength of signal from the satellites allows the GPS to calculatea rough estimate of the error in its reported position. This error or dilution of precision is a good guide to how accurate any reading would be.

Future Uses

The future of GPS is looking bright. Programs like Google Earth are just the beginning of what can be done with global positioning technology. Militarily, more accurate and faster GPS systems can give troops and commanders up to date analysis of friendly and enemy troop movement. With GPS becoming smaller and more powerful, it can also be used for individual soldiers in the field to triangulate their position. Commercially, the technology has exploded in the past few years, allowing anyone and everyone to afford powerful GPS units in their automobiles and homes. That technology is now making the jump to mobile smart phones.

The future of precise GPS kinematic positioning is dependent on a number of factors, including developments in receiver hardware, carrier phase data processing algorithms and software, operational procedures, the Internet and mobile communications, as well as the augmentation of GPS with pseudolites and inertial navigation systems/sensors, implementation of the WAAS system, the combination of GPS with GLONASS, the development of the Galileo system, and the modernization of GPS to transmit a second and third civilian frequency. All of these will significantly improve the reliability, integrity, and accuracy of the position results.

The GPS modernization program is an ongoing, multibillion-dollar effort to upgrade the GPS space and control segments with new features to improve GPS performance. These features include new civilian and military signals. The U.S. government is well underway on a $5.5 billion project to roll out GPS III, with the goal of making GPS more powerful and more accurate than ever.

In addition to the specific new features noted above, GPS modernization is introducing modern technologies throughout the space and control segments that will enhance overall performance. For example, legacy computers and communications systems are being replaced with a network-centric architecture, allowing more frequent and precise satellite commands that will improve accuracy for everyone.

The GPS modernization program involves a series of consecutive satellite acquisitions, including GPS IIR(M), GPS IIF, and GPS III. It also involves improvements to the GPS control segment, including the Architecture Evolution Plan (AEP) and the Next Generation Operational Control System (OCX).

Lockheed Martin announced it had delivered its first test satellite for GPS III. This satellite isn’t intended to go into space: Instead, it’s a testbed prototype that will be be run through a broad range of tests, including being subjected to very low-temperature conditions and radiation to mimic the effects of being in orbit, along with interference tests. If all goes well, the first launchable GPS III satellite should go into orbit in May 2014.

The U.S. government started gearing up for GPS III all the way back in 1998, and authorized funding for the effort in 2000 — that means some benefits and improvements have begun rolling out already. As originally deployed for civilian use, the GPS system uses one type of radio signal, called L1 C/A. GPS III will add three new civilian signals to that mix — L2C, L1C, and L5 — while keeping the L1 C/A signal operational for a total of four civilian signals.

With GPS tracking systems popping up in cell phones, watches, and shoes, there's no doubt that GPS tracking devices are making their way into all walks of daily life. Considering the increased popularity of GPS tracking systems, what can we expect from the next generation of these tracking devices in

i) Increased Business Use ii) Business Opportunities

iii) Advancements in Software iv) Personal Safety

CONCLUDING REMARKS

In summary:

• Carrier phase-based GPS positioning has evolved rapidly over the last ten years so that it can now position: (a) kinematically, (b) in real-time, and (c) instantaneously.

• There is therefore a blurring of the distinction between precise GPS navigation and GPS surveying.

• If certain conditions are fulfilled, carrier phase-based positioning is almost indistinguishable from pseudo-range-based DGPS, but at a much higher accuracy.

• However, there are very real constraints to the universal use of GPS carrier phase-based positioning.

• If these constraints are accepted, then the trend to very fast OTF-AR is a welcome one.

• Advances in hardware, software and operational procedures has made possible very fast OTF-AR under restrictive conditions of satellite-receiver geometry and baseline length.

• Network-based techniques hold the promise of relaxing one of the critical constraints to very fast OTF-AR, permitting the maximum baseline length to be increased to many tens of kilometres.

• The establishment of continuously-operating GPS reference receiver networks is an important trend as it will permit the gradual implementation of network-based techniques, in the post-processed or real-time mode.

References:

1) http://en.wikipedia.org/wiki/Global_Positioning_System

2) http://electronics.howstuffworks.com/gadgets/travel/gps.htm

3) http://en.wikipedia.org/wiki/GPS_navigation_device

4) http://www.ehow.com/about_5730112_objectives-global-positioning-system.html

5) http://www.ehow.com/about_6595713_purpose-gps-system-started.html

6) http://www.webmapsolutions.com/future-developments-gps-technology