Absolute PCV Field Calibration
Basics and Procedure
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.
The realization has two features to handle the problems "relative" and "multipath influence". First of all, in order to get the absolute PCV information, the test antennas are rotated AND inclined. Secondly, differences between observations with identical multipath effects are used to eliminate site correlations.
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.
The high resolution of the PCV and the possibility to estimate reliable and repeatable azimuthal PCV is based on the automation of the measurement program. With the help of the robot, there are between 4000 and 7000 different positions (standard procedure) of the antenna. The measurement program changes with the actual satellite constellation. Thus, a total coverage of the whole antenna hemisphere is reached. Furthermore, a dynamical elevation mask is used (only satellites in high elevations) within the observation procedure.
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.
More detailed information can be found in several publications.