Accurate Positioning in a Flight Inspection System Using Differential Global Navigation Satellite Systems

The purpose of a flight inspection system is to calibrate and evaluate the performance of aircraft navigation and landing aids to ensure conformance to specifications. This mission requires that the flight inspection platform have a reference position estimate significantly more accurate than that of the facility under inspection, i.e., tenths of meter accuracy over a region of many kilometers, in a dynamic environment. Differential Global Navigation Satellite Systems (DGNSS) have the accuracy potential to be used in real time for ICAO Category III final approach flight inspection. However, this requires that the residual pseudorange errors be very small, and that the values of HDOP and VDOP be appropriately constrained. This paper presents the results achieved by employing several different position estimation techniques for estimating aircraft position during flight tests of an airborne flight inspection system in the final approach mode. These techniques use DGNSS measurements integrated with an Inertial Navigation System (INS), and aIone. Measurements from GPS receivers which employ narrow correlator spacing and carrier-phase tracking techniques are used as an update source to a Kalman filter and as inputs for a nonlinear least squares estimation of aircraft position. The accuracy results show that DGNSS-based position estimate techniques are capable of meeting ICAO requirements for flight inspection of even the most stringent category of precise landing aids.