GPSHow it WorksFor a full tutorial on GPS and its applications visit the Trimble Website

What is GPS?

• Global Positioning System is a worldwide radio-navigation system formed from a constellation of satellites

• Dependent on line of sight

GPS Systems

• NAVSTAR (Navigation System Timing and Ranging)

– United States

• GLONASS– Russian Global Navigation SS

• GALILEO– European Union

• COMPASS (BeiDou)– Chinese Navigation System

• IRNSS– Indian Regional Navigational SS

Components of GPS

• Space– Satellites

• Control– Ground control and antennas and

monitoring stations

• User– Military, civil, commercial, scientific

Satellites (SV’s)

• 32 satellites• Orbit at 12,600

miles• Six orbital planes• Each SV has a ½

sidereal day orbital period

• ~ Nine satellites are visible at any given time Garmin

Control Segment

Users

• Civilian– Cartography, Cellular networks, fleet

tracking, navigationetc.

• Military– Navigation, target tracking, munitions

guidance, and reconnaissance– Nuclear detonation detectors

How do we determine our position?• We have to determine our precise

distance to at least 4 GPS satellites– We need extremely accurate clocks

• In addition to distance we need to know exactly where the satellites are in space.

• Lastly, we correct for any delays the signal experiences as it travels through the atmosphere.

How GPS works

Information sent in the GPS signal• GPS timing signal (PRC)• Ephemeris

– Satellite orbit information– Updated every 2 hours valid for 4

hours

• Almanac– Orbit and status for all 32 satellites– Error corrections

Sync Satellite and Receiver

• We need to have an accurate measure of the radio signal travel time

• Atomic Clock– Use oscillations of an atom for timing

• A fourth measurement is used to determine the position– The receiver looks for a single correction

factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point

Correcting Errors

• Charge particles in the ionoshpere and water vapor in troposhpere slows the GPS signal

• Multipath error• Tiny errors that occur at the

satellite• Selective Availability

Differential GPS

• Relies on the operation of two receivers– One is stationary and its location is know

precisely– The second is roving and making

measurements (this is YOU!)• The reference station “knows” where

it is located– It uses that information to correct the

timing from the GPS satellite• This error correction is then sent to

the rover to correct the measurements

Wide Area Augmentation System

• WAAS – Uses ground stations to measure the

variation in the GPS system– It corrects for this error and sends the

correction to WAAS satellites

• Must have a specially equipped receiver to obtain the WAAS signal

• Typically accurate to within 1 meter

Real Time Kinematic

• RTK– Based on the use

of carrier phase measurements

– RTK systems use a base station that transmits the measure carrier phase to mobile individuals

– Can result in cm accuracy