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Globalpositioning system

ABSTRACT

The Global Positioning System is a navigation system developed

by United states Department of Defense. Earlier time it was developed

for military uses only. Nowadays it is open for common people also.

Navigation in Global Positioning System is done with the help of

satellites. A total of 32 satellites are now on service.

Besides the military uses GPS service can be used for lost and stolen

things recovery, root mapping etc. Other nations are also trying to build

up their on satellite navigation systems IRNSS of India is a proposed

project on satellite navigation system.

Dept. of IS&E, JNNCE 1

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ACKNOWLEDGEMENT

The constant encouragement of all sources has gone a long way in the

accomplishment of this seminar. It is our pleasant duty to thank all those

who have been helpful in various ways towards successful completion of

this seminar. This seminar was completed in due time as we were

constantly supported by our teachers and friends.

I wish to express my deep sense of acknowledgement and

respect to my guide Mr. Syed Aftab Ahmed , lecturer, Department of

IS&E and Mrs. Sunitha G P , Asst. professor, for their encouragement

throughout the completion of our seminar. I am very much grateful toour respected HOD Dr.S N Jagadeesha for his encouragement.

I sincerely thank the whole teaching and non-teaching staff of

Information Science and Engineering department.

MANOJ KUMAR


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CONTENTS

1 Before GPS.1

2 Introduction to GPS3

3 Technical Description5

4 How GPS Works 8

5 Accuracy and Error Sources.13

6 Application15

Conclusion


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Refererence

1. BEFORE GPS:

For thousands of years, speed to a walking pace and

landmark were used to find locations. At sea, early navigators limited

their voyages to coastal routes to avoid becoming lost. New methods for

determining position arose as trade between distant ports increased.

Polaris, the North Star, was used to determine north-south distance

(latitude) in the hemisphere. But mariners also had to find latitude when

sailing in the southern hemisphere, and they lacked a method for

determining east-west position (longitude). The solution, celestial

navigation, required accurate time. In the late 18th century this led to the

development of the marine chronometer, an accurate sea-going

timepiece. Beginning in the 19th century the U.S Naval Observatory, the

nations official timekeeper, provided accurate time for navigators from

an array of chronometers.

1. a. Electronic Innovations:


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Electronic navigation introduced all-weather capability,

ease of use, and eventually, increased accuracy. In the 1930s radio

beacons were used to provide bearings from airfields.

During World War II radio navigation system were developed, the

best-known being LORAN, or LONG RANGE AID TONAVIGATION. Positions were determined by the timing of signals

received from different LORAN transmitter stations. In the 1960s the

OMEGA SYSTEM provided worldwide electronic navigation coverage

for the first time.

In the mid- 1960s the U.S Navys Navigation system (NAVSAT), also

known as TRANSIT, was developed to provide positions for ships and

submarine.

1.b. TRANSITE

Satellite:

Fig 1: The first

Electronic navigation

TRANSIT Satellite


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TRANSIT was the first operational satellite positioning system. Six

satellites gave worldwide coverage every 90 minutes and provided

positions that were accurate to within 200 meters (660 feet). Position

Were obtained by measuring the Doppler shift of the satellite signal.

TRANSIT was effective, but it was limited by low accuracy and lack of

24-hour availability. The TRANSIT system operated until 1996.

2. INTRODUCTION TO GPS:

The Global Positioning System (GPS) is the only fully

functional Global Navigation Satellite System (GNSS). The GPS uses a

constellation of at least 24 (32 by March 2008) Medium Earth Orbit

satellites that transmit precise microwave signals, that enable GPS

receivers to determine their location, speed, direction, and time. GPS

was developed by the United States Department of Defense. Its official

name is NAVSTAR-GPS. Although NAVSTAR-GPS is not an

acronym, a few acronyms have been created for it. The GPS satellite

constellation is managed by the United States Air Force 50th space wing.


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Similar satellite navigation systems include the Russian GLONASS

(incomplete as of 2008), the upcoming European Galileo positioning

system, the proposed COMPASS navigation system of China, and

IRNSS of India. Following the shooting down of Korean Air Lines

Flight 007 in 1983, President Ronald Reagan issued a directive making

the system available free for civilian use as a common good. Since then,

GPS has become a widely used aid to navigation worldwide, and a

useful tool for map-making, land surveying, commerce, scientific uses,

and hobbies such as geocaching. GPS also provides a precise time

reference used in many applications including scientific study of

Earthquakes and synchronization of telecommunications network.

A GPS receiver calculates its position by carefully timing the

signals sent by the constellation of GPS satellites high above the Earth.

Each satellite continually transmits messages containing the time the

message was sent, a precise orbit for the satellite sending the message

(the ephemeris), and the general system health and rough orbits of allGPS satellites (the almanac). These signals travel at the speed of light

through outer space, and slightly slower through the atmosphere. The

receiver uses the arrival time of each message to measure the distance to


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each satellite, from which it determines the position of the receiver using

geometry and trigonometry. The resulting coordinates are converted to

more user-friendly forms such as latitude and longitude, or location on a

map, and then displayed to the user.

It might seem that three satellites would be enough to

solve for a position, since space has three dimensions. However, a three

satellite solution requires the time be known to a nanosecond or so, far

better than any non-laboratory clock can provide. Using four or more

satellites allows the receiver to solve for time as well as geographical

position, eliminating the need for a super accurate clock. In other words,the receiver uses four measurements to solve for four variables: x, y, z,

and t. While many GPS applications have no particular use for the

computed time, it is used in some GPS applications such as time

transfer.

3. TECHNICAL DESCRIPTION

The Global Positioning System consists of 3 segments:


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Space Segment: It is a constellation of 24 satellites, which orbit the

earth every 12 hours.

Control Segment: The Control Segment comprises a series of

monitoring stations at different locations around

the world, with the master control facility located

at Schriever Air Force Base in Colorado USA.

User Segment: The User Segment is made up of all the GPS

receivers in the worlds aircraft, ships, cars, and

handheld units.

3. a. Space Segment:

The space segment comprises the orbiting GPS satellites or

Space Vehicles in GPS parlance. The GPS design originally called for

24 SVs, eight each in three circular orbital planes but this was modified

to six planes with four satellites each. The orbital planes are centered on

the Earth, not rotating with respect to the distant stars. The six planes

have approximately 55 inclination (tilt relative to Earth's equator) and

are separated by 60 right ascension of the ascending node. (angle along


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the equator from a reference point to the orbit's intersection). The orbits

are arranged so that at least six satellites are always within line of sight

from almost everywhere on Earth's surface.

Fig 2: Orbital planes indicating constellations

Orbiting at an altitude of approximately 20,200 kilometers

(12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km


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(16,500 mi or 14,400 NM)), each SV makes two complete orbits each

sidereal day.The ground track of each satellite therefore repeats each

(sidereal) day. This was very helpful during development, since even

with just four satellites, correct alignment means all four are visible from

one spot for a few hours each day. For military operations, the ground

track repeats can be used to ensure good coverage in combat zones.

As of September 2007, there are 31 actively broadcasting satellites

in the GPS constellation. The additional satellites improve the precision

of GPS receiver calculations by providing redundant measurements.

With the increased number of satellites, the constellation was changed to

a no uniform arrangement. Such an arrangement was shown to improve

reliability and availability of the system, relative to a uniform system,

when multiple satellites fail.

3. b. Control segment:

The flight paths of the satellites are tracked by US Air Force

monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego

Garcia, and Colorado Springs, Colorado, along with monitor stations


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operated by the National Geospatial-Intelligence Agency (NGA). The

tracking information is sent to the Air Force Space Commands master

control station at Schriever Air Force Base in Colorado Springs, which

is operated by the 2nd Space Operations Squadron (2 SOPS) of the

United States Air Force (USAF). Then 2 SOPS contacts each GPS

satellite regularly with a navigational update (using the ground antennas

at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs).

These updates synchronize the atomic clocks on board the satellites to

within a few nanoseconds of each other, and adjust the ephemeris of

each satellite's internal orbital model. The updates are created by a

Kalman filter which uses inputs from the ground monitoring stations,

space weather information and various other inputs.


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Fig 3: GPS Master Control and Monitor Station Network

3. c. User Segment:

The GPS User Segment consists of the GPS receiver and the

user community.

GPS receiver converts SV (satellite vehicle) signals into position,

velocity, and time estimates. Four satellites are required to compute the

four dimensions of X, Y, Z (position) and time.

GPS receivers are used for navigation, positioning, time dissemination,

and other research. The receiver must perform the following task:


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Selecting one or more satellite in view

Acquiring GPS signals

Recovering navigational data

Measuring and tracking

Fig 4: GPS Receivers (User Segment)

4. HOW GPS WORKS:


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As weve said, the complete GPS space system includes 24

satellites, 11,000 nautical (speed unit) miles above the Earth, which take

12 hours each to go around the earth once (one orbit). They are

positioned so that we can receive signals from six of them nearly 100

percent of the time at any point on earth. You need that many signals to

Get the best position information. Satellites are equipped with very

precise clocks that keep accurate time to within three nanoseconds-thats

0.000000003, or three billionths, of a second. This precision timing is

important because the receiver must determine exactly how long it takes

for signals to travel from each GPS satellite. The receiver uses this

information to calculate its position.

The principle behind GPS is the measurement of distance

(or range) between the receiver and the satellites. The satellites also

tell us exactly where they are in their orbits above the earth. It works

something like this: If we know our exact distance from a satellite in

space, we know we are somewhere on the surface of an imaginary

sphere with radius equal to the distance to the satellite radius. If we

know our exact distance from two satellites, we know that we are


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located somewhere on the line where two spheres intersect.

Fig 5: Two sphere surface intersecting in a circle

And, if we take a third measurement, there are only two possible points

where we could be located. One of these is usually impossible, and the

GPS receivers have mathematical methods of eliminating the impossible

location.


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Fig 6: surface of sphere intersecting a circle at two points

Trilateration is used to determine the two points of intersection of three

sphere surfaces corresponding to three satellites. The surface of thesphere corresponding to the fourth satellite or the surface of the earth is

used to determine which of the two intersections provides a valid

estimate of GPS receiver position.


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4. a. Positioning with GPS:

There are essentially two broad categories of GPS

positioning which can be described as a real-time navigation and high

precision carrier phase positioning. Navigation uses a minimum of

four pesudorange measurements to four satellites, which are used to

solve for the three-dimensional coordinates of the receiver and the clock

offset between the receiver and the clock offset between the receiver

oscillator and GPS system time.

Carrier phase observations are used to compute baselines

between two locations. Since the two carriers have short wavelength (19

and 24 cm for L1 and L2 respectively), they cannot be used in the same

manner as the pseudo range. Positions are determined by intersecting

distances between the GPS satellites and the receiver.

Traditionally, the technique is called trilateration.

Distances between the GPS satellites and the receiver are not measured

directly and therefore must be derived.


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5. Accuracy and error sources:

Sources of User Equivalent Range Errors (UERE)

Source Effect

Ionospheric effect 5 m

Ephemeris errors 2.5 m

Satellite clock errors 2 m

Multipath distortion 1 m

Tropospheric effects 0.5 m

Numerical errors 1 m

Table 1.1: Error sources

The position calculated by a GPS receiver requires the

current time, the position of the satellite and the measured delay of the

received signal. The position accuracy is primarily dependent on the

satellite position and signal delay.


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5.1 Atmospheric effects:

Inconsistencies of atmospheric conditions affect the speed of

the GPS signals as they pass through the Earth's atmosphere, especially

the ionosphere. Correcting these errors is a significant challenge to

improving GPS position accuracy. These effects are smallest when the

satellite is directly overhead and become greater for satellites nearer the

horizon since the path through the atmosphere is longer (see air mass).

Once the receiver's approximate location is known, a mathematical

model can be used to estimate and compensate for these errors.

5.2 Multipath effects:

GPS signals can also be affected by multi path issues,

where the radio signals reflect off surrounding terrain; buildings, canyon

walls, hard ground, etc. These delayed signals can cause inaccuracy. A

variety of techniques, most notably narrow correlate spacing, have been


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developed to mitigate multipath errors. For long delay multipath, the

receiver itself can recognize the wayward signal and discard it.

To address shorter delay multipath from the signal reflecting off the

ground, specialized antennas (e.g. a choke ring antenna) may be used to

reduce the signal power as received by the antenna. Short delayreflections are harder to filter out because they interfere with the true

signal, causing effects almost indistinguishable from routine fluctuations

in atmospheric delay.Multipath effects are much less severe in moving

vehicles. When the GPS antenna is moving, the false solutions using

reflected signals quickly fail to converge and only the direct signals

result in stable solutions.

5.3 Ephemeris and clock errors:

While the ephemeris data is transmitted every 30 seconds,

the information itself may be up to two hours old. Data up to four hours

old is considered valid for calculating positions, but may not indicate the

satellites actual position. If a fast TTFF is needed, it is possible to upload

valid ephemeris to a receiver, and in addition to setting the time, a


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position fix can be obtained in under ten seconds. It is feasible to put

such ephemeris data on the web so it can be loaded into mobile GPS

devices.

The satellite's atomic clocks experience noise and clock drift errors. The

navigation message contains corrections for these errors and estimates of

the accuracy of the atomic clock. However, they are based on

observations and may not indicate the clock's current state. These

problems tend to be very small, but may add up to a few meters (10s of

feet) of inaccuracy.

6. Applications

The Global Positioning System, while originally a military project is

considered a dual-use technology, meaning it has significant

applications for both the military and the civilian industry.

6.1 Military:

The military applications of GPS span many purposes: -


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Navigation: GPS allows soldiers to find objectives in the dark or in

unfamiliar territory, and to coordinate the movement of troops and

supplies. The GPS-receivers commanders and soldiers use are

respectively called the Commanders Digital Assistant and the Soldier

Digital Assistant.

Target tracking: Various military weapons systems use GPS to track

potential ground and air targets before they are flagged as hostile. These

weapon systems pass GPS co-ordinates of targets to precision-guided

munitions to allow them to engage the targets accurately.

Missile and projectile guidance: GPS allows accurate targeting of

various military weapons including cruise missiles and precision-guided

munitions. Artillery projectiles with embedded GPS receivers able to

withstand accelerations of 12,000G have been developed for use in 155

mm howitzers.

Search and Rescue: Downed pilots can be located faster if they

have a GPS receiver.

Reconnaissance and Map Creation:The military use GPS

extensively to aid mapping and reconnaissance.


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6.2 Civilian:

Many civilian applications benefit from GPS signals,

using one or more of three basic components of the GPS: absolute

location, relative movement, and time transfer.

The ability to determine the receiver's absolute location allows GPSreceivers to perform as a surveying tool or as an aid to navigation. The

capacity to determine relative movement enables a receiver to calculate

local velocity and orientation, useful in vessels or observations of the

Earth.

GPS functionality has now started to move into mobile phones en masse.

The first handsets with integrated GPS were launched already in the late

1990s, and were available for broader consumer availability on

networks.


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7. CONCLUSION

GPS continues to perform as the world's premier space-based

positioning, navigation, and timing service. Endeavors such as mapping,

aerial refueling, rendezvous operations, geodetic surveying, and search

and rescue operations have all benefited greatly from GPS's accuracy.

GPS capabilities are integrated into nearly all facets of US military

operations. GPS receivers are incorporated into nearly every type of

system used by aircraft, spacecraft, ground vehicles and ships. In

addition, GPS-guided munitions have showcased their increased

accuracy in recent conflicts with unprecedented precision, thus

improving military capability while decreasing the number of weapons

required achieving military objectives.


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REFERENCES

[1].Steven R. Strom. "Charting a Course Toward Global Navigation".

The Aerospace Corporation.2002.

[2].Noe, P.S.; Myers, K.A. "A Position Fixing Algorithm for the Low-

Cost GPS Receiver".IEEE Transactions on Aerospace and Electronic

Systems, 2006.


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