Starting point for the development of this procedure was, that existing
field calibrations were only relative (refering to a reference antenna)
and correlated with the site (multipath influence and satellite constellation,
i.e. northern hole). The goals were

- separation of multipath and PCV
- absolute results independent of a reference antenna
- high resolution of the PCV
- field procedure.

*Antenna Mount*

a) b) c)

During the first development phase of the project we used the two mounts shown above (a, b). In order to achieve a better stability and precision, the current procedure makes use of a robot (c). Considerable efforts were necessary to develop an error model for the robot. The observations for this modell are derived from a tachymetric measurement system (TMS). Beside an exactly known position due to the precise robot and the high number of possible rotations and tilts, another advantage is the possibility of an automation of the calibration procedure.

*Multipath Elimination and PCV Estimation*

The satellite constellation repeats after a mean sidereal day. In case
of identical conditions (geometry, weather, surfaces ...), also multipath
effects repeat with the same period. Thus, with the pre-conditions fulfilled,
day time differences eliminate the multipath influence. Since the PCV information
is elimated also, the antenna has to be rotated and tilted on one of the
two measurement days. The difference of the PCV between the rotated/tilted
position and the fixed position will be the observable for the PCV determination.
**-
NEW -** The multipath elimination approach has been improved within
the second phase of the project. The current calibrations are a real-time
procedure without a static reference day. The multipath conditions between
close measurement epochs are identical. Therefore, the difference between
epochs can eliminate the effects (the realization within the software uses
the correlation in time of the multipath). Finally, with both possibilities
for the multipath elimination/reduction, spherical harmonic functions serve
for the PCV estimation.

*Advantages of the Absolute PCV Field Calibration*

There are several advantages of the absolute PCV field procedure:

- absolute PCV (L1, L2, GPS, Glonass) in real-time
- independent of reference antenna and reference coordinates
- elimination of multipath
- precise robot, exactly known reference point (pivot)
- very good coverage of the whole antenna hemisphere with observations
- automated measurement program
- PCV down to elevation zero
- significant azimuthal PCV.

With the presented procedure (especially through the use of a highly precise and calibrated robot and the automation of the measurements), very precise and reliable PCV corrections can be derived (e.g. see AOAD/M_T results), even down to elevation zero and also for azimuthal PCV. The results have been independently confirmed through measurements

- with different robots
- at different sites
- on different days.

Last update: 17.05.2001 by Geo++