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Techniques available to sustain dual-frequency ionosphere performance when a dual-frequency airborne Global Positioning System (GPS)/wide area augmentation system (WAAS) user loses all but one GPS frequency while descending into radio frequency interference (RFI) are investigated. We are particularly interested in the case where the user transitions from L1L5 to having L5-only since the uncertainty of the L5-only ionospheric delay estimation is larger than the case for L1-only. The goal is to provide the techniques necessary for single-frequency users to sustain a performance similar to those of dual-frequency users. The proposed techniques are 1) the code and carrier divergence technique, 2) the WAAS ionosphere threat model technique, and 3) the maximum ionospheric delay gradient model technique. The results show that all three proposed techniques provide good ionospheric delay estimation for the full duration of approach.