lntegrated GPS/lnertial Fault Detection Availability

Abstract

GPS systems possess a high availability of accurate horizontal positioning but detection and exclusion techniques based on satellite redundancy do not provide the integrity availability sufficient for primary means navigation in the terminal and non precision phases of flight. To achieve primary means integrity availability different types of augmentations such as WAAS, Loran, baro aiding, atomic clock and Inertial aiding are under consideration. Different techniques have been proposed to combine GPS and inertial sensor information and recent published findings seem to indicate that primary means integrity availability is achievable with standard 2 nmi/ h inertial sensor performance. This paper investigates and quantifies what different inertial effects that contribute to enhanced integrity of the integrated GPS/inertial system such as coasting, Schuler feedback etc. A Kalman filter based integration scheme that preserves the integrity information in an optimal fashion is presented and used to quantify integrity performance. The availability of fault detection and exclusion will depend on the geometry of the satellites used for positioning. The availability of satellites in good geometry is in turn a function of the status of the GPS constellation ( failures, maintenance etc. ). Methods for availability calculations have been developed and adopted by the RTCA SC-159. The availability of fault detection and exclusion over a specified region can be calculated for most augmentations but corresponding estimates (conforming with RTCA SC-l 59 guidelines) of the FDE availability provided by GPS/inertial integration techniques have not yet been published. This paper proposes an approximate model, incorporating the main inertial effects contributing to integrity, that can be used to calculate the achievable integrity (horizontal integrity limit) at any geographical location and time. This model is used to estimate the availability of fault detection (FD) for an integrated GPS/lnertial system. The availability of fault detection for the GPS/inertial system is compared to the availability of FD for other augmentations to provide trade off information.