Differential GPS
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Transcript of Differential GPS
2/4/03 1 of 10
Differential GPS
An Introduction
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How does it work
UserGPS
ReferenceStation
RTCM SC104
DifferentialCorrection Signal
PRN 12PRN 7 PRN 16
PRN 3
KnownPosition
Reference
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Method of Differential Correction
– The reference ground station(s) at known locations receive NAVSTAR signals.
– Knowing position of the station, the pseudo-range to each SV is calculated based on the almanac
– The measured pseudorange, PRM(t), is determined
– The pseudorange correction is calculated as the difference
– The pseudorange correction, PRC(t), and the Range Rate Correction RRC(t) are sent from the reference ground station
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Method of Differential Correction
• Local GPS calculates corrected positionCorrected pseudorange = pseudorange measured +
pseudorange correction
PR(t) = PRM(t) + PRC(t)
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Local GPS Communications
• NMEA 0183 used to communicate fix data from GPS devices– Serial character data
– Baud Rate 4800– Data Bits 8(d7=0)– Parity None– Stop Bits One(or more)
• RTC SC 104 used to communicate differential data– Serial character data
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Typical System Diagram
DifferentialCorrectionReceiver
GPSRTCM SC104Serial Data Stream
RTC
M
SC
104
Signal
DGPS Antenna GPS Antenna
TaskController
NMEA 0183Serial Data Stream
GP
S
Satellite
Sig
nal
Vehicle Mounted GPS Unit
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RTCM SC 104
• Specification for the signal used to transmit differential correction to a GPS ground receiver– Format is referred to as the RTCM-104 format
(Radio Technical Commission for Maritime Services Special Committee No. 104)
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RTCM SC 104 Messages• Type 3 - Station coordinates• Type 9 - (Fast Type 1) Correction data
– Scale Factor (.02 m and .002 m/s or .32 m and .032 m/s)
– User Differential Range Error• 1 sigma error is < 1,2,4,8,greater
– Satellite ID - PRN code– PRC - Pseudorange correction– RRC - Range Rate Correction– Issue of Data - Link to the particular GPS data from
which calculations are derived– …
• Sent in 30 bit words including a 5 bit parity
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NMEA 0183 Example
• Global Positioning Fix Data• $GPGGA,120757,5152.985,N,00205.733,W,1,06,2.5,121.9,M,49.4,M,,*52
• Synopsis:– time of fix (hhmmss)– latitude– N/S– longitude– E/W– Fix quality (0=invalid, 1=GPS fix, 2=DGPS fix)– number of satellites being tracked– horizontal dilution of position– altitude above sea level– M (meters)– height of geoid (mean sea level) above WGS84 ellipsoid– time in seconds since last DGPS update– DGPS station ID number– checksum
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How to determine Health of the NAVSTAR and Coast
Guard Systems• Check the Coast Guard web site for status
– http://www.navcen.uscg.gov/– http://www.nis-mirror.com/default.html
• WAAS – Wide Area Augmentation System– http://gps.faa.gov/Programs/WAAS/waas.htm