BDS ambiguity resolution with the modified TCAR method for medium-long baseline

As the BeiDou Navigation Satellite System (BDS) attained initial regional operational status at the end of October 2012, real BDS data are available to investigate triple-frequency ambiguity resolution (AR) performance. In this contribution, analysis of the classical TCAR models and limiting factors is given. It is concluded that the residual double-differencing (DD) ionospheric delay is the key limitation to the medium and long range narrow-lane (NL) ambiguity resolution in step 3 of the classical TCAR method. To improve this algorithm, the third step of the classical TCAR method is modified accordingly. The modified TCAR comprises three major steps. Step 1 and step 2 are the same as the classical TCAR, which is the geometry-free determination of the extra-wide-lane (EWL) and wide-lane (WL) ambiguities. Then we can derive the DD first-order ionospheric delay estimated from the ambiguity-fixed EWL. As the noise term of the estimated DD ionospheric delay is very large, the smooth method is employed to correct the estimated DD ionospheric delay. In step 3, the smooth DD ionospheric delay is used to completely correct the single-epoch DD float NL ambiguity resolution. It is also noted that there exist cycle-slips which influence the process of the ionosphere-smooth method. This is followed by a procedure to repair the cycle-slip. As a result, the modified-TCAR method shows a much better performance than the classical TCAR method over medium and long baseline.