Radio Frequency Interference Validation Testing for LAAS using the Stanford Integrity Monitor Testbed

Since GPS signals have very low received power levels (−130 dBm, or −160 dBW), their vulnerability to RF Interference (RFI) is a serious concern. This is particularly true for GPS-based safety-critical systems such as the Local Area Argumentation System (LAAS). The Stanford LAAS Integrity Monitor Testbed (IMT) is a prototype of a LAAS Ground Facility (LGF) that is composed of various monitors to detect possible hazardous anomalies for LAAS users. The goals of this study are to validate the IMT detection algorithms under various RFI conditions, to evaluate the RFI mask specified by the LAAS MOPS, to explore new algorithms for improved RFI detection, and to improve the system robustness under RFI. RFI can be categorized into three main types: broadband, continuous wave (CW), and pulsed interference. Multiple GPS performance metrics may be affected by RFI, including pseudorange and carrier-phase measurement accuracy, phase lock status, acquisition time, etc. The current IMT has several integrity monitors that are designed to detect anomalies that may affect pseudorange and/or carrier phase measurements and their error statistics. Any serious degradation caused by RFI should be detectable before a hazardous error occurs. In addition, Automatic Gain Controller (AGC) outputs from the IMT receivers are sensitive to interference but are not sensitive to other types of failures. In addition to being more sensitive to changes in RFI, they provide a useful metric for Executive Monitoring (EXM) to distinguish between RFI, satellite failures, and IMT receiver failures. A single GPS receiver, one of the three used in the IMT, has been tested under the RFI test conditions specified by FAA AOS-240. These tests are used to validate performance against the LGF Specification RFI requirements. Careful receiver calibration was conducted prior to testing, and each test condition (such as GPS power) was adjusted accordingly. Subsequently, the full IMT is tested with selected RFI scenarios to measure the response of the multiple antenna, multiple receiver LGF implementation. This also tests the ability of EXM to exclude the affected measurements. The AGC outputs are also evaluated and compared with the IMT monitors. We have found that the current receivers tested comply with the MOPS mask with respect to in-band and nearband CW interference. However, the tolerance of out-ofband CW is slightly less than the mask level. Also, it appears that the existing IMT monitors are no better RFI detectors than the receiver loss-of-lock indicator. AGC is sensitive to both wide band noise and CW interference. In addition, AGC is also useful separate RFI from other failure modes that can be detected by the IMT.