Reliable RTK positioning with tight coupling of 6 low-cost sensors

Abstract

A tight coupling of GNSS and inertial measurements is needed for both accurate and reliable positioning. The use of Multi-GNSS is recommended to obtain a sufficient number of visible satellites in any outdoor environment. In this paper, we perform a joint GPS/GLONASS ambiguity fixing and a tight coupling of GNSS, 3D accele-rometer, 3D gyroscope, 3D magnetometer, barometer and thermometer measurements. As GLONASS uses FDMA, double difference ambiguities are no longer integer-valued. We derive a transformation for the GLONASS double difference ambiguity term, that recovers the integer property and maintains a full-rank system. The obtained transformation maps the real-valued double difference ambiguity terms into integer-valued double difference ambiguity terms and a common single difference ambiguity term, that is treated as a real-valued parameter. Low-cost GNSS antennas can not suppress multipath and, therefore, require an estimation of multipath errors. We provide a precise model for multipath, that considers an individual amplitude, code delay, phase shift and Doppler shift for each reflected signal, and include it in our sensor fusion. The magnetometer measurements provide a rough attitude information, which makes them very valuable for robust GNSS attitude ambiguity fixing. We verified the performance of our sensor fusion in a test drive on a parking lot. The fixed phase residuals were in the order of a few centimeters for both GPS and GLONASS, which indicates a very precise position estimation.