Effects of Signal Deformations on Modernized GNSS Signals

Abstract

Satellite-based navigation requires precise knowledge of the structure of the transmitted signals. For GPS, accurate knowledge of the shape of the code correlation peaks is required to ensure no biases are introduced into the position solution. It is generally presumed that all GPS-like satellite signals are virtually identical. However, in 1993 a satellite malfunction introduced significant distortion onto one of the satellite C/A codes. That distortion caused range errors to vary with receiver filter characteristics and code tracking loop implementation. As a result, high-integrity systems such as the Wide Area Augmentation System (WAAS) must implement signal deformation monitors to detect and remove signals that become anomalously distorted. In the future, WAAS will rely on modernized signals from both GPS (L5) and Galileo (E1/L1/E2 and E5A/E5B). This should increase performance for users; however they must still protect against potential signal deformations. Although the International Civil Aviation Organization (ICAO) has agreed on a threat scenario for GPS L1 signals, no such agreement exists for modernized signals. In addition, each of these signals will have different chipping rates and their correlation peak structures will be quite different from that of the GPS C/A code. Their code tracking loop implantations are as yet not well- defined, but may differ somewhat from traditional architectures. An additional complication is the unknown receiver filter characteristics that the new receivers will employ. Each of these factors may render a given signal and/or receiver configuration more or less sensitive to signal deformations. This paper analyzes the range error sensitivity of several modernized signals subjected to distortions of the type considered in the ICAO threat model for signal deformations. To isolate the effects of the signal-in-space deformation errors, it assumes an ideal, wideband receiver filter and basic early-minus-late code tracking implementations for the new codes. It also compares the distortion-induced range errors for the new codes to those currently modeled for the C/A code. Finally, these results are used to motivate threat model refinements and receiver tracking loop constraints that minimize the affects of this error source for the modernized GNSS signals.