Performance Of A Partially Coherent Ultra-Tightly Coupled Gnss/Ins Pedestrian Navigation System Enabling Coherent Integration Times Of Several Seconds To Track Gnss Signals Down To 1.5 Dbhz.

The paper describes a prototype of an ultra-tightly coupled GNSS/INS system. A MEMS IMU is used and it is argued that the canonical way of ultra-tight coupling (i.e. an IMU strapdown navigation solution plus an - IMU bias estimating - error state Kalman filter, which is updated with GNSS correlation values) is difficult to realize due to the MEMS gyro bias instability. Instead the strapdown calculation is replaced by a step detection and dead reckoning algorithm and the error state Kalman filter is updated with (non-coherent) code pseudoranges. To aid signal processing, the strapdown calculation is maintained but the strapdown trajectory is sub-divided into segments of e. g. 100 ms - 2 s duration. Those segments aid signal processing in a coherent way and allow (together with assistance data and a stable oscillator) coherent integration times of the same duration. We will argue that the long coherent integration mitigates three major problems of indoor signal processing: squaring loss, multipath and cross-correlations and allows tracking GNSS signals of around 1.5 dBHz or below. The paper also contains a technical description of the built prototype and presents end-to-end performance results for simulated Galileo E1 signals and real GPS C/A signals.