Mixed GPS–BeiDou RTK with inter-systems bias estimation aided by CSAC

Abstract

It is now common to use multiple GNSS constellations (multi-GNSS) for precise positioning. When GPS and BeiDou satellites are used in real-time kinematic GNSS (RTK-GNSS), the normal approach is to construct separate sets of double differences for each satellite constellation. Since QZSS is designed to be the same as GPS in terms of frequency and timing, a QZSS satellite can be used as another GPS satellite for positioning. The normal approach ("normal RTK") requires one primary satellite for each constellation and thus removes one usable satellite. We developed a mixed GPS + BeiDou model by differencing BeiDou carrier phase and pseudorange observations from those of the primary GPS satellites. This approach introduces inter-system (GPS-to-BeiDou) biases, which must be estimated and removed. In addition, the difference between GPS and BeiDou signal frequencies means that receiver clock error has to be estimated precisely in this approach, and a chip-scale atomic clock (CSAC) was utilized to aid this estimation. The proposed mixed GPS + BeiDou RTK method was evaluated using static data and kinematic data. Kinematic data were collected in both normal and dense urban areas in Tokyo using a small car. The kinematic tests were conducted using an external CSAC as an input to the receiver, which allowed precise estimation of receiver clock error while in motion. The resulting fix rate of mixed GPS + BeiDou RTK-GNSS was approximately 10% better than normal RTK-GNSS.