A Simple Method of Signal Quality Monitoring for WAAS LNAV/VNAV

Abstract

The integrity of the Wide Area Augmentation System (WAAS), being developed for the FAA, is of significant concern because of its intended use in commercial aviation navigation applications. The current WAAS plan calls for several upgrades with each upgrade providing improved service coverage and availability at increasingly lower precision approach decision heights. The first planned deployment, LNAV/VNAV, will provide an approach service with both lateral and vertical guidance. One of the threats to the integrity of the WAAS- corrected user position solution is that of an anomalous GPS broadcast signal, also known as an “evil waveform”. Detection of such distortion is made difficult due to the fact that the ranging error caused by such distortion is dependent on the spread spectrum receiver discriminator type, correlator spacing, and bandwidth. This leads to some potentially complex solutions for detection, involving multiple correlator spacings, etc. This paper discusses the threats, detection requirements, and detector design approach used to mitigate the failures of concern to the WAAS LNAV/VNAV system. The approach used takes advantage of another detector already required for another type of satellite failure (code-carrier coherence failure). The analysis includes application of a model for receiver-specific distortion effects. This is used to translate error limits and thresholds between the “user” domain and the “detector” domain. The paper walks through the analysis to select thresholds that meet the allocated integrity (detection) and continuity (false alarm) requirements.