Techniques for Graceful Reversion from Dual to Single Frequency WAAS

This paper investigates techniques to sustain dualfrequency ionosphere performance when a dual-frequency airborne user loses all but one GPS frequency while descending into the radio frequency interference (RFI) field. In this paper, we are particularly interested in the case where the user transitions from L1-L5 to having L5only. That is because the uncertainty of the L5-only ionospheric delay estimation is larger than the L1-only ionospheric delay estimation. An L1-L5 dual-frequency user has LPV (HAL = 40m, VAL = 50m) [1] precision approach services available 99.9% of time over 100% CONUS, with a nominal σUIRE of 0.32m [2]. An L5 single-frequency user has LPV precision approach services available 99.9% of time over 49.25% CONUS [3]. In this situation, the nominal σUIRE is 6m at coast lines, and 3.5m at the center. In other words, if an L1-L5 dual-frequency user loses L1 GPS frequency due to RFI and instead uses the WAAS grid for ionospheric delay estimation, the loss of CONUS coverage of LPV services will be about 50%. Therefore, the objective of this paper is to find solutions that will sustain a performance similar to the multi-frequency ionospheric delay estimation. Based on the information available to user, there are three techniques to sustain the dual-frequency ionospheric delay estimation. This paper uses a typical precision approach example based on San Francisco International Airport (SFO) to examine the possible solutions, and then uses the MATLAB Algorithm Availability Simulation Tool (MAAST) [4] to measure all airports over CONUS. First, one can use the code-carrier divergence to continue ionospheric delay estimation; this technique would require a robust cycle slip detector. This technique provides good ionospheric delay estimation (better than using the WAAS [5] grid) for the full duration of approach. Second, one can use the WAAS ionospheric threat model to bound the error. This technique requires an ionosphere storm detector. It provides useful ionospheric delay estimation for at least 10 minutes. Third, one can use the maximum ionospheric delay gradient model to estimate ionospheric delay during the ionosphere storm period. This technique should only be used when there is no available ionosphere storm detector. The maximum ionospheric delay gradient technique also provides useful ionospheric delay estimation for at least 10 minutes.