GPS Overview Appendix B
MPC User Manual Rev 0D 89
B.5.1.2 Single-Point Averaging with the GPSCard
By averaging many GPS measurement epochs over several hours, it is p ossible to achieve a more
accurate absolute position. 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 AVEPOS 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 optio nal horizon tal and verti cal standard dev iatio n li mi ts cannot
be used.
If the maximum time that positions are to be measured is set to 24, for example, you can then log
AVEPOS with the trigger ‘ontime’ to see the averaging status. i.e.,
posave 24
log com 1 avepos onchange d
You could initiate di fferential logging, then iss ue 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 com mand to enable it to send differential corrections.
The position accuracy that may be ach ieved by these methods will be dependent on many factor s: SA,
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 83.