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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