Differential Correction of GPS Positions

1
Differential Correction of GPS Positions
2
Differential Correction

• The use of a GPS receiver located at a fixed and
  known point to remove or minimize the effect of
  some of the sources of position error.

3
Differential Correction Base Station Parameters
Antenna Height
Height above Ellipsoid
Ellipsoid Surface
Latitude
Longitude
4
Differential Correction

• Differential correction can be done by combining
  transmitted position corrections (real time
  DGPS), or corrections stored for processing at a
  later time (post processing).

5
Differential Correction Signal

• Uses standardized output RTCM-104
• Binary code, not directly readable
• Receivers often provide RTCM104 on a RS-232
  serial output port

6
Real Time DGPS
GPS Signals
Base Station
GPS Signals
RTCM-104
Mobile Receiver
3 m (10)
7
Differential Correction Sources

• Commercial wide area
• Coast Guard beacons
• FAA Wide Area Augmentation System (WAAS)
• NASA Global Differential GPS (GDGPS) delivered by
  Internet
• Cellular telephone
• Local base station
• Commercial FM side band

8
Omnistar Coverage Area
Source www.omnistar.com
9
Nation-wide DGPS

• Coast Guard is working toward a nation-wide
  system of beacons. Completion date is
  undetermined.

10
Coast Guard Beacon Coverage http//www.navcen.uscg
.mil/
11
Latest staus http//WWW.NAVCEN.USCG.MIL/dgps/cov
erage/
12
Real-time DGPS via Internet

• NASA global network of reference stations
• Corrections are gathered and distributed via
  Internet
• http//gipsy.jpl.nasa.gov/igdg/system/index.html

13
Post Processing DGPS
Mobile Receiver
Base Station
Roving Position File (50-100 m)
Base Station File
Post Processing Software
Position File (1-3 m)
14
Sources of Base Station Data

• TX Dept. of Transportation (http//www.dot.state.t
  x.us/ insdtdot/orgchart/isd/gps/ gps.htm)
• National Geodetic Survey (http//www.ngs.noaa.gov/
  cgi-cors/ufcors2.prl)

15
Performance of DGPS

• May degrade with distance from base station,
  depending on software used to generate the
  correction
• View plots on OmniStar web site of various
  receiver configurations
• http//www.omnistar.com/current.html

16
GPS Errors Diff. Corrected

• Atmospheric partial
• Clock and Ephemeris yes
• Multipath no
• Receiver noise no
• Selective Availability yes