Evaluation of LORAN-C for Non-Precision Approach Applications

Abstract

During the summer of 1979 a flight test program was performed in the western U.S. evaluating Loran-C as a nonprecision approach navigation aid. The objectives of the program were to measure the accuracy and operational suitability of Loran-C in mountainous areas at low altitudes. The equipment used in the test was a commercially available TDL-711 Micro-Navigator built by the Teledyne Corporation. The equipment was installed in a twin engine Piper Aztec. The controls and displays were located in the cockpit where they could be operated by the pilot or copilot. The pilot used the guidance provided by the Micro-Navigator as the sole means of navigation during the instrument approach. Tests were performed in visual meteorological conditions with the copilot watching for traffic and obstacles. Independent aircraft position data was recorded using the Remote Area Precision Positioning System (RAPPS). This system uses multiple L-band range measurements to transponders placed at known ground locations. Data from both the Micro-Navigator and RAPPS were recorded on a microcomputer-based data recording system located in the aircraft cabin. The results of the tests showed that the Loran-C signal-in-space was stable and dependable at all altitudes flown in the test. Significant errors were observed in the conversion of the received time difference values into position and guidance signals. The primary cause of these errors was traced to the propagation model used in the navigation processor. These errors were generally constant in each location, but changed significantly depending upon the Loran-C triad. The errors occur primarily in areas where the distance difference between stations is several hundred miles. Selection of the wrong triad or reversion to an alternate triad during station outages could produce guidance which is in error by several thousand feet or more. However, improved propagation modeling or procedural changes show promise of reducing errors to a point where RNAV accuracy standards can be achieved.