GEO-pivoted carrier ambiguity resolution: a method for instantaneous ambiguity resolution in mid-low-latitude regions

The effect of DD ionospheric delays can be unexpectedly large in the range of the equatorial anomaly, that is, in mid-low-latitude regions near noon and/or afternoon, and the large delays cause instantaneous AR to fail even over short baselines. Ionospheric delays can be represented by a function of vertical total electron content values, which often have significant latitudinal gradients in mid-low-latitude regions near noon and/or afternoon. Therefore, a short separation between the pivot and secondary satellites in the latitudinal direction indicates smaller effects of DD ionosphere. In the BeiDou system (BDS), five geostationary earth orbit (GEO) satellites are nearly motionless over the equator. We can use adjacent GEO satellites to form a DD pair whose pivot and secondary satellites are close in latitude (< 4°). Moreover, when inclined geosynchronous orbit or medium earth orbit (IGSO/MEO) satellites approach the equator, the separations between the IGSO/MEO and GEO satellites in the latitudinal direction will be minimal. Therefore, this study proposes a method called GEO-pivoted carrier AR (GEOCAR) for instantaneous AR. This method mitigates the influence of DD ionospheric delays by pivoting GEO satellites in BDS DD pairs and uses a trade-off design between the ionosphere-fixed and ionosphere-weighting models to resolve integer ambiguities of dual-frequency phases. Experimental short-baseline data (< 10 km) collected in mid-low-latitude regions near noon and afternoon are tested with conventional AR and GEOCAR methods. The results show that the GEOCAR can effectively produce higher success percentages than the conventional AR with improvements reaching 68.62% for BDS and 42.55% for BDS/GPS.