GPS-Base User Manual
Revision: 131025
29
The problem of shifting the antenna typically occurs when:
The tripod is knocked over and picked up again. It is hard to get the antenna back
to the same location accurate to 1cm.
If the GPS-Base is used one day, packed up then returned to the same location the
next day. It is very hard to replace the tripod in the same location. It is better to
have a pole that is fixed to the ground if you intend to use the same surveyed
location on several days.
Figure 10. Averaging to a different position example
The problem of averaging to a different position happens each time that the GPS-Base
goes through its averaging process. There is nothing magical about the GPS-Base that
allows it to get its own position accurate to 2cm or better. It is subject to the same
errors that all GPS receivers have and can only average its position to about 1.8m CEP.
If the user is prepared to wait a long time (typically more than 24 hours) then GPS is
able to improve the accuracy of the base-station antenna so it is accurate to 2cm or
better. However, since the timescale for this is long it is not usually practical, except for
permanent installations. (Even when you have a permanent installation it is not required
since all it does is allows you to relate your measurements to a surveyors measurements
and this is rarely required).
To overcome the problem of averaging the save/restore feature of the GPS-Base should
be used. When using the Save/Restore feature the GPS-Base will save the position
where it last averaged and then use this next time (instead of averaging again). This
way the error is the same each time and the repeatability is perfect. You must
remember to put the antenna in the same location each time, accurate to 1cm or better,
when using the Save/Restore feature.
