Antenna Reduction - Thales Z-Max Operation And Application Manual

If you plan to set the base system on an unknown point, there are some
disadvantages to this practice that you should be aware of. For every 15 meters of
error between the estimated base coordinates and the true WGS84 base coordinates,
1 part-per-million (ppm) of relative error will be introduced into the computed vector
between the base and rover system, plus the absolute difference between the
computed base station position and the real base station position. For example,
assume that the coordinates assigned to the base point are 30 meters off the true
base position. This 30-meter offset from truth will produce 2 ppm (0.002 m per 1
kilometer or 0.010 ft per mile) of error in the vector between the base and rover. If the
rover is 5 kilometers (3 miles) from the base, this will produce 0.010m (0.030ft) of
error in the vector. In most cases, the base receiver will estimate its position to better
than 30 meters (probably closer to 10-20 meters), but an error of 50 meters is
possible. If you plan to use an estimated position for the base, keep the vector lengths
between the base and rover short and ensure the added error is not significant for the
survey you are performing.

Antenna Reduction

A concept related to the Base Station position is Antenna Reduction. In RTK, the
rover receiver computes the vector from the phase center of the base station antenna
to the phase center of the rover station antenna. Usually, the real position of interest is
not the phase center of the antenna, but the survey mark (or other landmark) over
which the antenna is set up. To compute the position of the mark instead of the
antenna, it is necessary to instruct the receiver to perform an antenna reduction. In an
antenna reduction, the antenna heights are included when computing and outputting
the rover position. Antenna reduction is set using the handheld computer and the field
application software. When using RTK and performing an antenna reduction, the
antenna height of both the base and the rover must be entered (assuming that they
are both over survey marks).
222 Z-Max Surveying System Operation and Applications Manual