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What is GPS…

The Global Positioning System (GPS) is a U.S. space-based global navigation satellite system.

It provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis in all weather, day and night, anywhere on or near the Earth which has an unobstructed view of four or more GPS satellites.

Using the GPS the following two values can be determined any where on Earth1.One’s exact location (longitude, latitude and height co-ordinates) accurate to within a range of 20m to approx.1mm.2.The precise time (Universal Time Co-ordinated, UTC) accurate to with in a range of 60ns to approx.5ns. Speed and direction of travel (course) can be derived from these co-ordinates as well as the time. The coordinates and time values are determined by 28 satellites orbiting the Earth.

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Why do we need GPS?

• Trying to figure out where you are is probable man’s oldest pastime.

• Finally US Dept of Defense decided to form a worldwide positioning system.

• Also known as NAVSTAR ( Navigation Satellite Timing and Ranging Global positioning system) provides instantaneous position, velocity and time information.

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Components of the GPS

It is made up of with following segments.

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Space Segment:

• 24 GPS space vehicles(SVs).

• Satellites orbit the earth in 12 hrs.

• 6 orbital planes inclined at 55 degrees with the equator.

• This constellation provides 5 to 8 SVs from any point on the earth.

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Control Segment:

• The control segment comprises of 5 stations.• They measure the distances of the overhead

satellites every 1.5 seconds and send the corrected data to Master control.

• Here the satellite orbit, clock performance and health of the satellite are determined and determines whether repositioning is required.

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User Segment:

• It consists of receivers that decode the signals from the satellites.

• The receiver performs following tasks:o Selecting one or more satelliteso Acquiring GPS signalso Measuring and trackingo Recovering navigation data

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User Segment:

• There are two services SPS and PPS• The Standard Positioning Service

o SPS- is position accuracy based on GPS measurements on single L1 frequency C/A code

o C/A ( coarse /acquisition or clear/access) GPs code sequence of 1023 pseudo random bi phase modulation on L1 freq

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User Segment:

• The Precise Position Serviceo PPS is the highest level of dynamic positioning

based on the dual freq P-codeo The P-code is a very long pseudo-random bi

phase modulation on the GPS carrier which does not repeat for 267 days

o Only authorized users, this consists of SPS signal plus the P code on L1 and L2 and carrier phase measurement on L2

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How does the GPS work?

• Triangulation from satellite• Distance measurement through travel time

of radio signals• Very accurate timing required• To measure distance the location of the

satellite should also be known• Finally delays have to be corrected

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Triangulation

• Position is calculated from distance measurement

• Mathematically we need four satellites but three are sufficient by rejecting the ridiculous answer

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Measuring Distance

• Distance to a satellite is determined by measuring how long a radio signal takes to reach us from the satellite

• Assuming the satellite and receiver clocks are sync. The delay of the code in the receiver multiplied by the speed of light gives us the distance

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Getting Perfect timing

• If the clocks are perfect sync the satellite range will intersect at a point.

• But if imperfect the four satellite will not intersect at the same point.

• The receiver looks for a common correction that will make all the satellite intersect at the same point

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• Let Satellite1 takes 4s to reach receiver.

• The area in the picture above which shaded grey is the region in which GPS signals are supposed to be “realistic”.

Position tracking

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Navigation Message

• Navigation Message is transmitted with the data rate 50bits/sec.

• The frame size:1500bits• 5 sub frames• 1-clock correction

2&3-ephemeric data4&5-almanac data

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• Ephemeris:Highly detailed and valid for 4hours• Almanac:More general and valid for 180 days.

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Error Sources

• 95% due to hardware ,environment and atmosphere

• Intentional signal degradationo Selective availability Selective availability is an artificial falsification of the time in the

L1 signal transmitted by the satellite

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Errors• Satellite errors

o Errors in modeling clock offseto Latency in tracking

• Atmospheric propagation errorso Through the ionosphere,carrier experiences phase

advance and the code experiences group delay.o Dependent on

Geomagnetic latitude Time of the day

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Errors

• Atmospheric errors can be removed byo Dual freq measurement low freq get refracted more than high freq thus

by comparing delays of L1 and L2 errors can be eliminated

• Single freq users model the effects of the ionosphere

Applications…

• MilitaryNavigationTarget trackingMissile and projectile guidanceSearch and Rescue

• CivilianMany civilian applications benefit from GPS signals, using one or more of three basic components of the GPS: absolute location, relative movement, and time transfer.

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References• http://egsc.usgs.gov/isb/pubs/factsheets/fs06299.pdf• http://en.wikipedia.org/wiki/Gold_code• www.surveyingsupplies.com/surveyingsupplies/pdfs/

gpsbasics.pdf• www.gisdevelopment.net/tutorials/tuman004.htm - 7k• www.colorado.edu/geography/gcraft/notes/gps/gps_f.html• www.gps.moi.gov.tw/satellite/english/Satellite/ESatellite_01.htm• www.gps.moi.gov.tw/satellite/english/Satellite/ESatellite_01.htm• www.gmat.unsw.edu.au/snap/gps/gps_survey/chap1/chap1.htm

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