Description of Antenna Calibration
Geodetic and precise GNSS measurements make the exact knowledge of the reception characteristics of the used antennas and therefore a calibration necessary.
Generally, it is differentiated between the antenna offset and the phase center variations (PCV), while the antenna offset represents a kind of mean influence of the phase center variations.
The applied Geo++® calibration method determines the absolute antenna offset in horizontal and vertical position as well as absolute elevation and azimuth dependent PCV for both frequencies. The resulting PCV are completely independent from the used reference antenna and allow the complete modeling of the receiving characteristic of the antenna. This is required for a combined use of different antenna types or for differently orientated antennas. In addition, an analysis of the phase center variations and judgment of the general quality and receiving characteristics of the antenna are possible (azimuth dependency).
Basic aspects of the applied absolute field calibration in real-time are:
- absolute offsets and absolute PCV through observation configuration
- special approach with inclined and rotated antenna (robot)
- elimination of multipath
- coverage of the complete elevation range from 0° to 90°
- significant determination of PCV using a large number of different antenna orientations
- weather independent observations
- simultaneous estimation of L1 and L2 PCV
- shortage of observation procedure and evaluation efforts
Basic concept of the calibration method is a separation between multipath and phase center variation. A special observation procedure with different antenna orientations is used for the the determination of absolute PCV and for multipath elimination.
The processing is done in real-time. Therefore the complete results are directly available after the calibration. The calibration covers the complete receiving area of the antenna down to elevation angles of 0 degree. Hence, antenna calibrated with this method are suited for All-In-View applications (e.g. use on reference stations).
The result is stored in an absolute antenna calibration file, which contains absolute horizontal and vertical offset as well as absolute elevation and azimuth dependent corrections for the calibrated antenna. It can be arranged, that instead of elevation and azimuth dependent corrections only elevation dependent without azimuth dependency are derived. The format description is attached on a separate information sheet. The sign of the offsets are positive for north, east and up. The PCV corrections must be added to the observations (pseudo range). The antenna height must be measured up to the antenna reference point (ARP) of the calibration.
The procedures for the antenna calibration are under steady development and progress. The presented method represents the state of the technique at writ