User's Manual Part 1

Positioning Modes of Operation Chapter 5
OEMV Family Installation and Operation User Manual Rev 5B 75
By averaging many GPS measurement epochs over several hours, it is possible 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 base stations. Position averaging
continues for a specified number of hours or until the averaged position is within specified accuracy
limits. Averaging stops 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 is
automatically invoked.
If the maximum time is set to 1 hour or larger, positions are 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 are averaged once per minute and the standard deviations reported in the log are
not likely to 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 hours, for example, you can then
log AVEPOS with the trigger ‘onchanged’ to see the averaging status:
posave 24
log com1 avepos onchanged
If desired, you could initiate differential logging, then issue the POSAVE command followed by the
SAVECONFIG command. This would cause the receiver 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 depends on many factors: average
satellite geometry, sky visibility at antenna location, satellite health, time of day, and so on. 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 dual frequency receiver with an ideal antenna
location, see Figure 22, Single-Point Averaging (Typical Results) on Page 76. Figure 23 on Page 76
shows the results from the same dual frequency receiver but with WAAS corrections available.