1. Differential GPS
An Introduction
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2. How does it work
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3. 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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4. Method of Differential Correction
Local GPS calculates corrected position
Corrected pseudorange = pseudorange measured + pseudorange correction
PR(t) = PRM(t) + PRC(t)
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5. 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
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6. Typical System Diagram
Vehicle Mounted GPS Unit
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7. 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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8. 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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9. 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
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10. How to determine Health of the NAVSTAR and Coast Guard Systems
Check the Coast Guard web site for status
WAAS – Wide Area Augmentation System
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