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

Markus Irsigler and Guenter W. Hein

Abstract:	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. Since both satellite signal failures and multipath result in a distorted correlation function, it should also be feasible to use multi-correlator observations for the detection of multipath signals. Based on this idea, this paper provides a concept to use correlation peak observations for detecting and monitoring multipath signals in real-time and to provide the user with instant information whether or not a satellite signal is affected by multipath. The general idea for the implementation of this real-time multipath monitor is to constantly monitor the correlation function by means of several correlators, to combine the correlator outputs mathematically and to compare them with a threshold that indicates whether or not the combined correlation peak observations exceed their corresponding noise level. If this threshold is exceeded, it is assumed that the incoming signal is not only affected by noise but also by other interfering signals (mainly by multipath, less frequently by signal failures). The paper starts with a description of the general monitoring concept. The second section deals with the question how the correlator outputs can be combined in order to maximize the sensitivity to all types of multipath signals (short-delay/long delay multipath, weak and strong multipath signals). After that, the computation of monitor thresholds is the next important step. The paper presents a first software implementation of the real-time multipath monitor (RTMM), which bases on multicorrelator observations obtained from a NovAtel OEM3 receiver equipped with a special multi-correlator firmware. In order to demonstrate the capability of detecting multipath signals, a GPS hardware simulator (Spirent STR4760) has been used to carry out performance analyses with the aim to determine the minimum relative amplitude a multipath signal must have to be detected by the monitor.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2626 - 2637
Cite this article:	Irsigler, Markus, Hein, Guenter W., "Development of a Real-Time Multipath Monitor Based on Multi-Correlator Observations," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2626-2637.
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