Precise estimation of clock bias difference between multiple GNSS receivers for cooperative positioning

The use of raw-data from multiple GNSS receivers located in a closer vicinity is an emerging concept to enhance the positioning availability in harsh environments, where the number of received measurements by one receiver are not enough to compute a position estimate. Within this modern navigation paradigm, different receivers or users collaborate by exchanging measurements in order to be able to estimate their locations together. However, the cooperation between different receivers at the measurements level needs synchronisation of measurements produced by these different devices. The main issue is that the clock bias and drift of individual receivers with respect to GPS time are not predictable and will produce large errors in measurements and position if the problem of clock difference is not taken into account in the collaborative processing. The existing methods either estimate these clock bias values as part of Position, Velocity and Time (PVT) estimation algorithm or eliminate them by double difference. These approaches necessitate the availability of more measurements to estimate the additional clock bias of each receiver or need to have the same satellites visible to the cooperating receivers. In this paper, we suggest a new approach of clock timing difference compensation when degraded measurements are available from multiple receivers in dense urban environments. These estimates are used to interpolate the raw measurements of multiple receivers to a common reference time. The proposed method is inherently simple, robust and allows us to develop innovative algorithms for cooperative PVT estimation. To illustrate the effectiveness of this method, we present some results by processing real data obtained using low cost commercial GNSS receivers.