Combined multi-system GNSS analysis for time and frequency transfer

The Center for Orbit Determination in Europe (CODE) is one of the Analysis Centers (AC) of the International GNSS Service (IGS). It is located at the Astronomical Institute of the University of Bern (AIUB). Since May 2003, CODE provides consistent GPS and GLONASS satellite orbits from a combined analysis using the Bernese GPS Software package. The data of about 30 stations in the IGS network that are equipped with GNSS receivers tracking GPS as well as GLONASS satellites are analyzed for that purpose together with numerous stations that only track the GPS satellites. With the background of the experience in orbit determination for more than one satellite system, we discuss the use of both GPS and GLONASS observations in a combined analysis for geodetic time and frequency transfer using code and phase measurements. The number of satellites that can be used for time transfer is increased when adding the GLONASS observations to the analysis. This may help to improve the redundancy for the receiver clock parameters that are estimated for each station from all satellites in view. GLONASS satellites emit the signals on individual frequencies. This may lead to frequency-dependent biases in the receivers that have been investigated, e.g., in [1]. Of course, these biases must be considered in a combined analysis of GPS and GLONASS code data. Corresponding parameters can be estimated for all GNSS receivers even if they are not connected to an external reference clock. This offers the possibility to investigate the frequency-dependent biases for all receiver types that are represented in the IGS network. The estimated biases can be interpreted as a relative “calibration” of each individual frequency used by a GLONASS satellite with respect to the GPS frequency.