A Real-Time Signal Quality Monitor For GPS Augmentation Systems
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The Local Area Augmentation System (LAAS) and the Wide Area Augmentation System (WAAS) are groundbased applications of differential GPS to support aircraft terminal navigation and precision approach. Due to the safety-critical nature of LAAS and WAAS operations and the resulting stringent integrity requirements on system performance, LAAS and WAAS ground facilities must be able to detect any hazardous signal anomalies within a prescribed time-to-alarm (3 seconds for LAAS Category I precision approach). One class of signal-in-space failures that is difficult to detect without specialized monitoring is deformations of the C/A code signals broadcast by GPS satellites. Since code correlator spacings and RF front-end bandwidths may vary between ground and airborne GPS receivers, these deformations, otherwise known as “evil waveforms,” may produce hazardous differential ranging errors at the aircraft unless the affected satellites are promptly detected and excluded from the broadcast set of differential corrections. This paper describes the construction and testing of a prototype real-time signal quality monitor (RTSQM) described in [1]. The first section describes the implementation of the prototype on a personal computer with multiple serial inputs reading data from customized GPS receivers. The discussion covers several design issues of particular interest, including system timing, simultaneous processing of multiple data streams, and analysis algorithms (including delta and ratio tests using the in-phase values of multiple correlators assigned to each satellite). This system is designed to be integrated into the Stanford LAAS ground system prototype, known as the Integrity Monitor Testbed (IMT). The second section describes testing of the real-time monitor under both nominal and anomalous signal conditions and present results. The overall test setup uses an arbitrary waveform generator (AWG) and RF circuitry to drive two customized receivers. After collecting nominal data to establish realistic thresholds for SQM test metrics, the threat space of a specific failure mode, the second order step anomaly described in the LAAS Ground Facility Specification [2], is explored in detail.
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