Global Positioning System ( GPS )

A GADGET WHICH

CHANGED THE WAY THE

WORLD OPERATES

Global Positioning System

Seminar by:B V AparnaECECMR College of Engg. And Tech

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.

Four Basic Functions of GPS

Position and coordinates.

The distance and direction between any two waypoints, or a position and a waypoint.

Travel progress reports.

Accurate time measurement.

How does the GPS work?

RequirementsTriangulation from satelliteDistance measurement through travel time of

radio signalsVery accurate timing requiredTo measure distance the location of the

satellite should also be knownFinally delays have to be corrected

GPS Satellite Signal:

L1 freq. (1575.42 Mhz) carries the SPS code and the navigation message.

L2 freq. (1227.60 Mhz) used to measure ionosphere delays by PPS receivers

3 binary code shift L1 and/or L2 carrier phase The C/A code The P code The Navigation message which is a 50 Hz signal

consisting of GPs satellite orbits . Clock correction and other system parameters

Signal generation in a GPS Satellite

L2 outputL2+P+NAV

C/A code

Navigationmessage

L2 1227 Mhz

P code

L11575Mhz 90

SUM

Q

C/A + NAV

P+NAV10.23Mbps

50 bps

1.023Mbps

L1+P+C/A+NAVL1 output

Multipliers

I

I

Triangulation

Position is calculated from distance measurement

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

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

Getting Perfect timing

If the clocks are perfect sync the satellite range will intersect at a single 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

T + 3

T

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.

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.

This information is sent to the three uplink stations

User Segment:

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

The receiver performs following tasks: Selecting one or more satellites Acquiring GPS signals Measuring and tracking Recovering navigation data

User Segment:

There are two services SPS and PPSThe Standard Positioning Service

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

The Precise Position Service PPS is the highest level of dynamic positioning based

on the dual freq P-code Only authorized users, this consists of SPS signal plus

the P code on L1 and L2 and carrier phase measurement on L2

User Segment

Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation.

Control Segment

Space Segment

User Segment

Three Segments of the GPSThree Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

S p ace S eg m en t C on tro l S eg m en t U ser S eg m en t

G P S

Errors in GPS

The GPS is designed as accurate as possible.However, there are still errors and the most significant of these are discussed below:

Atmospheric conditions Ephemeris errors Clock drift/measurement noise Selective availability multipath

Sources of GPS Error

Standard Positioning Service (SPS ): Civilian UsersSource Amount of Error

Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters User error: Up to a kilometer or more

Errors are cumulative and increased by PDOP.

Errors due to geometry

Poor GDOP When angles from

the receiver to the SVs used are similar

Good GDOP When the angles

are different

Sources of Signal Interference causing errorsSources of Signal Interference causing errors

Earth’s Atmosphere

Solid Structures

Metal Electro-magnetic Fields

DGPS

Errors in one position are similar to a local area

High performance GPS receiver at a known location.

Computes errors in the satellite info

DGPS

Data Links Land Links

MF,LF,UHF/VHF freq used Radiolocations,local FM, cellular telephones and marine

radio beacons Satellite links

DGPS corrections on the L band of geostaionary satellites Corrections are determined from a network of reference

Base stations which are monitored by control centers like OmniSTAR and skyFix

Applications of GPS system

Tracking is useful because it enables a central point to monitor the position of several vehicles or people, in real time, without them needing to relay that information explicitly. This can include children, criminals, police and emergency vehicles or military applications.

o GPS vehicle tracking is also used to locate stolen cars, or stolen mobiles.

Once we know our location, we can, of course, find out where we are on a map, and GPS mapping and navigation is perhaps the most well-known of all the applications of GPS. 

Applications of GPS system

A tracking applications are not that much popular as the navigation applications. But, so many people take uses of them. It enables users to find a location of any object that is tagged with a system.

Navigation applications are the most famous GPS applications. The latest releases of those applications allow users to have much advanced features and facilities.

Other common applications:Car navigation,Hand held ,Tracking,GIS,Survey ,Manufacturing,Military Related 1%

Thank you