Determination of Phase Center Corrections for Galileo Signals

Abstract

GNSS are widely used for positioning, navigation and timing (PVT). The quality of results depends on the antenna in use and the capability to take antenna specific effects into account. The most prominent corrections are the direction dependent phase center corrections (PCC), which include corrections for the phase center offset (PCO) and the phase center variations (PCV). These corrections range between a few up to several millimeters for carrierphase observations and up to some decimeters for code observations. In addition, the magnitude of the error depends on the used antenna type and can differ even for different antennas of the same type and manufacturer.

The frequency-dependent PCC are either determined in an anechoic chamber or in the field using a robot (so-called absolute field calibration). Both methods have their advantages and drawbacks. In the Antenna Exchange Format (ANTEX) from the International GNSS Service (IGS), which is widely used, currently only PCC for L1- and L2 frequencies for GPS and GLONASS are officially published. Absolute field calibrations values for new signals like Galileo or GPS L5 are missing. Only some chamber calibration results are available in the European Permanent Network (EPN).

The Institute für Erdmessung (IfE) is one of the the IGS accepted absolute field calibration institutions and provides PCC using the so-called Hannover-Concept. In this approach a robot is used to precisely rotate and tilt the antenna under test. This concepts has now been extended to an experimental approach. The PCC of new signals are estimated in post-processing as spherical harmonics using time differenced single differences. First results show both – a high repeatability of the estimated pattern and an improvement on the observation domain.

In this contribution the theoretical background as well as the extended concept are described. Moreover, patterns for Galileo signals and GPS L5 will be shown and discussed. After a short introduction into the method and the extended Hannover-Concept the robot model and the adjustment concept will be presented. The contribution will show that the estimation of PCC for Galileo signals is feasible with the developed method. This can be described by the root mean square (RMS) of differential pattern (of different calibrations). This indicator for the repeatability show RMS values for the EL1X signal under 0.6 mm for the NOV703GGG antenna and under 0.4 mm for the LEIAR25.R3. The RMS for the EL5X signal is maximal 0.6 mm for the NOV703GGG or 0.65 mm for the LEIAR25.R3. Furthermore, the obtained patterns will be presented and discussed for several antennas typical to IGS stations. For instance the PCV of the LEIAR25.R3 show values in a range of -4 to 7 mm for the EL1X frequency, whereas the Up-component of the PCO is approximately 60 mm. If these PCC are taken into account, the RMS of the single differences (SD) of a short baseline, common clock experiment at the Physikalisch-Technische Bundesanstalt (PTB) can be improved.