Differential GPS An Introduction

How does it work

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

Method of Differential Correction Local GPS calculates corrected position Corrected pseudorange = pseudorange measured + pseudorange correction PR(t) = PRM(t) + PRC(t)

Local GPS Communications NMEA 0183 used to communicate fix data from GPS devices –Serial character data –Baud Rate4800 –Data Bits8(d7=0) –ParityNone –Stop BitsOne(or more) RTC SC 104 used to communicate differential data –Serial character data

Typical System Diagram

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)

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

NMEA 0183 Example Global Positioning Fix Data $GPGGA,120757, ,N, ,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

How to determine Health of the NAVSTAR and Coast Guard Systems Check the Coast Guard web site for status –