Development of a Real-Time Multipath Monitor Based on Multi-Correlator Observations

摘要

So far, multi-correlator observations have primarily been used for the detection of so-called "Evil Waveforms", anomalous signals transmitted from a satellite. The first observation of such an evil waveform was reported in 1993, after an anomalous behavior of SV19 had been detected. The measurement of the power spectrum of this anomalous signal revealed a large spike at the center frequency instead of showing a normal sinc function shape. In 1994, the reason for this anomalous behavior has been identified as a failure in the satellite's signal generation and transmission hardware. The last years have seen much effort to detect such signal failures with the consequence that current satellite based augmentation systems like LAAS or WAAS include Signal Quality Monitoring (SQM) concepts to protect airborne users against such threats (e.g. [ 1 ] [ 4 ],[ 5 ] [ 6 ],[ 7 ]). The detection of satellite signal failures benefits from the fact that such failures result in distortions of the signal's correlation function. Therefore, one possible approach to detect such failures is to monitor the incoming signal by means of several correlators which are placed at distinct locations along the correlation function. By means of this technique, the actual shape of the correlation function can be compared to its "nominal" shape and deviations from the nominal shape can be assigned to signal failures. In order to be able to detect only the satellite signal failures, other influences that also distort the correlation function (mainly multipath) must be considered as well. This is usually done by means of calibration measurements at a distinct site (the future monitor station), where the multipath influences in this specific environment are "recorded" and the required monitor thresholds are adjusted accordingly. This method ensures that only distortions caused by signal failures are detected and that false alarms caused by multipath signals are minimized.