Flight Evaluation of a Basic C/A Code Differential GPS for Category I Precision Approach

Flight tests are being conducted by Wilcox Electric to evaluate the capability of local area C/A Code DGPS to provide accuracy suitable for precision approach operations under Category I conditions 200 ft. decision height (DH), 2600 ft. runway visual range (RVR). The Wilcox King Air 300 aircraft which is instrumented for flight inspection of ILS and MLS Landing Systems is equipped with DGPS receiving equipment and additional computing capability which derives GPS based instrument approach paths. converts DGPS latitude/longitude/altitude position information to lateral/vertical deviations from the approach path and drives the appropriate aircraft flight instruments. A mobile ground station using identical DGPS receiving equipment and a VHF data transmitter is employed to send differential corrections to the aircraft system. Other than the differential corrections no other augmentation of the airborne data is employed. Approximately 50 ILS-like approaches have been flown under various I-IDOP and VDOP conditions using the DGPS derived path deviations as guidance to the aircraft. Data is taken simultaneously on aircraft position relative to the desired approach path as determined by a conventional ground based radio telemetering theodolite (RTT), by the DGPS system and by the ILS system installed at the site. In addition to collecting information on the guidance accuracy of the basic DGPS system during landing approaches as well as comparative performance data with respect to ICAO standard ILS the tests are designed to provide information on a number of other factors which are important to the landing requirement. These include the following: l The data is characterized in terms of path following noise (PFN). path following error (PFE) and control motion noise (CMN). This is required in order to define the ability of any system to provide guidance inputs suitable for precision approach. l Data is collected at position update rates of 4 to 5 Hz which is more suitable for precision approach than the conventional 1 Hz update rate common to most GPS receivers today. l The data represents what can be achieved by a minimally equipped aircraft using DGPS alone. The aircraft is flown manually without benefit of augmentation from radar altimeter. inertial reference system or flight management system. This would be typical of most of the general aviation fleet. air taxi and commuter operations as well as older air transport aircraft and helicopters. l The impact on performance of improved receiver signal processing which reduces receiver noise errors and multipath effects over that achieved by most current GPS receivers is evaluated. These are the principal residual errors after local area differential corrections are applied and any improvement will reflect directly into improved accuracy on approach.