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A GPS Overview
70 MiLLennium GPSCard SW Version 4.503/4.52 Command Descriptions Manual Rev 2
(GDOP < 2; no multipath) when SA is off and 40 meters CEP while AS is on. (As the status of selective availability
is generally unknown by the real-time GPS user, the positioning accuracy should be considered to be that of when
AS is on).
The general level of accuracy available from single point operation may be suitable for many types of positioning
such as ocean going vessels, general aviation, and recreational vessels that do not require position accuracies of
better than 100 meters CEP. However, increasingly more and more applications desire and require a much higher
degree of accuracy and position confidence than is possible with single point pseudorange positioning. This is
where differential GPS (DGPS) plays a dominant role in higher accuracy real-time positioning systems.
SINGLE POINT AVERAGING WITH THE GPSCARD
By averaging many GPS measurement epochs over several hours, it is possible to achieve an absolute position
based on the WGS-84 datum to better than five meters. This section attempts to explain how the position averaging
function operates and to provide an indication of the level of accuracy that can be expected versus total averaging
time.
The POSAVE command implements position averaging for reference stations. Position averaging will continue for
a specified number of hours or until the averaged position is within specified accuracy limits. Averaging will stop
when the time limit or
the horizontal standard deviation limit or the vertical standard deviation limit is achieved.
When averaging is complete, the FIX POSITION command will automatically be invoked.
If the maximum time is set to 1 hour or larger, positions will be averaged every 10 minutes and the standard
deviations reported in the PAVA/B log should be correct. If the maximum time is set to less than 1 hour, positions
will be averaged once per minute and the standard deviations reported in the log will likely not be accurate; also,
the optional horizontal and vertical standard deviation limits cannot be used.
If the maximum time that positions are to be measured is set to 24, for example, you can then log PAVA with the
trigger ‘onchanged’ to see the averaging status. i.e.,
posave 24
log com1 pava onchanged
You could initiate differential logging, then issue the POSAVE command followed by the SAVECONFIG
command. This will cause the GPSCard to average positions after every power-on or reset, then invoke the FIX
POSITION command to enable it to send differential corrections.
The position accuracy that may be achieved by these methods will be dependent on many factors: average satellite
geometry, sky visibility at antenna location, satellite health, time of day, etc. The following graph summarizes the
results of several examples of position averaging over different time periods. The intent is to provide an idea of the
relationship between averaging time and position accuracy. All experiments were performed using a single
frequency receiver with an ideal antenna location, see Figure A-5.