Advances in GPS/GLONASS combined solutions

In combining GPS and GLONASS observations in one single or differential position solution we are facing four problems: (1) differences in the time frame, (2) differences in the coordinate datum, (3) Selective Availability (S/A) in GPS (only of minor nature in differential solutions) and (4) the problem of solving carrier phase ambiguities in combined carrier frequency solutions due to different GLONASS frequencies (only in high-precision carrier phase differential solutions). Although progress has been made to determine the coordinate transformation parameters - in recent campaigns (see e.g. Rossbach et al., 1996) there are still uncertainties which result in errors in combined single positioning mode. In addition, in the transformation and combined GPS/GLONASS solutions the effect of sin adds to the remaining errors. This papers addresses a new approach which tries to cope with these two types of errors: S/A and residual uncertainties in the transformation parameters. In addition the authors present their improved solution for the determination of carrier phase ambiguities in a combined differential GPS/GLONASS approach.