Improvements in GPS integrity monitoring for non-precision sole means of navigation using hybrid GPS-INS

Abstract : GPS by itself is unsatisfactory as a sole means of navigation for civil aviation users. "Receiver Autonomous Integrity Monitoring" (RAIM) has been proposed whereby a receiver makes use of redundant satellite information to check the integrity of the navigation solution. Two types of algorithms can provide RAIM function: the currently used snapshot methods only process the current measurements and the sequential ones process all past and current measurements. The principal limitation of snapshot RAIM is its availability. Indeed for a Non-Precision Approach (NPA) phase of flight there are periods when the five satellites (with sufficiently good geometry) required for fault detection are not available; these periods sometimes last more than 10 minutes. As well, the fault detection and exclusion function can be unavailable fm mom than an hour. Use of sequential algorithm will with no doubt improve this situation but will probably not be sufficient to allow GPS to meet Civil Aviation requirements. As a solution an attempt could be made to hybridize GPS with an Inertial Navigation System (INS). Two solutions for hybridization are considered. The simplest solution is to use the INS as a primary system of navigation and to update it periodically with the GPS solution. The GPS position must then be carefully monitored by a sequential algorithm that tests the least squares residuals of the GPS solution. The integrity of the INS must then be monitored by another algorithm. The other solution is to hybridize more finely the two systems by using a bank of Kalman filters that take into account all the measurements from GPS satellites and INS. Then a sequential algorithm will try to detect and isolate any faulty GPS channel or INS sensor.