GPS accuracy

Jim Beach beach at HUH.HARVARD.EDU
Thu Oct 28 15:41:28 CDT 1993

This abstract may be of interest to GPS users who are concerned
about accuracy.  This is from the LTER All Scientists meeting abstracts
which can be found on the (U.S) Long Term Ecological Research Gopher
located at

The senior author is Lolita Krievs (krievs at

> Krievs, Lolita, Stuart Gage, Manuel Colunga and G. Philip Robertson
> POST-PROCESSED GPS DATA W. K. Kellogg Biological Station, Michigan State
> University, MI KBS.

>      Global Positioning Systems (GPS) depend for their accuracy on a
> constellation of satellites orbiting at an altitude of approximately
> 10,000 miles.  GPS receivers translate radio signals emitted by these
> satellites into distance measures to determine receiver locations on
> earth.  Electrical interference in the atmosphere and geographic
> variation in landcover and elevation are two of many potential
> influences that can degrade the precision of single receiver  [7mdata [m
> processing.  The degree of error caused by signal degradation can,
> however, be substantially reduced by GPS differential post-processing.
> Differential post-processing is performed by comparing satellite signals
> simultaneously logged by a mobile and a base GPS receiver and then
> correcting the mobile unit's location by triangulation.  To be
> effective, the base unit must log signals from a known coordinate
> location.  Differential GPS (DGPS) assumes that the difference between
> receiver signal errors associated with upper atmospheric conditions is
> negligible in comparison to the difference in signal errors associated
> with the immediate environment.  The effectiveness of DGPS should also
> depend on the distance separating the mobile and base units, but the
> relationship between separation distance and error reduction is not well
> known for most landscapes.  We attempted to define this relationship by
> surveying locations of first order geodetic controls using a Trimble
> Basic GPS Receiver while simultaneously logging satellite signals with a
> Trimble Pathfinder Community Base Station at KBS.  Geodetic markers were
> chosen along a 300 mile gradient in southwest Michigan.   [7mData [m were
> post-processed using Trimble Software.  Preliminary results indicate
> that locational accuracy decreases significantly with distance from the
> base station; the extent to which this error can be predicted and
> minimized is discussed.


James H. Beach                                         beach at
Data Administrator                                     Tel:   (617) 495-1912
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