A More Accurate Evaluation of GPS C/A Code Self-Interference Considering Critical Satellites

RTCA Special Committee SC-159 is in the process of updating Minimum Operation Performance Standards (MOPS) for airborne GNSS equipment. Part of that process is to update the assessment of GPS C/A code signal self-interference, which is currently documented in RTCA DO-235B. At the same time, C/A code self-interference has been the subject of discussions in various forums and ION papers in the past few years. Some would like to see various applications, including aviation, switch to the future L1C signal. Because of the delays in satellites that will broadcast that signal, plus the time it takes to field new GNSS receivers in aviation, the aviation community will not make that switch any time soon. Thus, the assessment of C/A code signal self-interference is important, especially since other GNSS satellite signals, like Galileo, Beidou, QZSS and the GPS P(Y) code overlay, or will overlay, the C/A code frequency spectrum and will cause some interference to the GPS C/A code. Thus, the purpose of this new more-accurate assessment is to evaluate compatibility in order to accept interference from these other GPS and non-GPS GNSS signals. There are two very important parts of this reassessment that will aid refining the interference margin characterization – a combination of more accurate spectral separation functions coupled with the evaluation of self-interference to critical satellites alone. In previous evaluations, spectral separation took on only one value (in dB/Hz) for each interfering signal. In the new evaluations discussed in this paper, spectral separation varies with Doppler and code phase differences between critical satellites and the interfering satellites.