gps how it works for a full tutorial on gps and its applications visit the trimble websitetrimble...
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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