High Integrity Carrier Phase Based Relative Positioning for Precision Landing using a Robust Nonlinear Filter

Abstract

In this paper, a cascaded ambiguity resolution scheme is integrated into an extended Kalman filter for reliable carrier phase positioning. The state vector includes the user position and velocity, carrier phase ambiguities and ionospheric and tropospheric delays which are determined from satellitesatellite double differenced dual frequency code and carrier phase measurements. The integrity of the state estimation is continuously validated by Autonomous Filter-based Fault Detection, Identification and Model Adaptation (AFDIA). Cascaded ambiguity resolution is applied to fix the widelane ambiguities and then the measurement ambiguities. In this paper, the integer ambiguity resolution is done sequentially with integer decorrelation. This bootstrapping enables a success rate which can be determined analytically. A partial integer decorrelation is used to achieve an optimum trade-off between variance reduction and bias amplification. The AFDIA checks the consistency of the ambiguity resolution and navigation solution at each time step. The algorithm supplies the user also with an error overbound called the Vertical and Horizontal Protection Levels (VPL and HPL). The fixing of the measurement integer ambiguities results in a substantial reduction of the protection levels. Protection levels of less than 50 cm have been observed for simulated dual frequency E1 and E5a Galileo measurements and realistic flight paths.