The Narrow Bandwidth VLBI (NBV) technique is utilized to track spacecrafts in Japanese lunar exploration. However, the problem of 2π ambiguity affects the phase delay of the carrier waves transmitted by satellites. A probabilistic algorithm called coarse search and fine delay search, in combination with a simple ambiguity judgment procedure, is presented in the current paper to resolve the 2π ambiguity. This method is employed to estimate both the delay and the delay rate from the residual phases obtained using NBVνll. Compared with previous analytic methods, it does not require strict constraint conditions. The ambiguity can be resolved even when the phase variations in the residual phase are 0.2 rad (approximately 11.6 degrees) compared with the less than 4.3 degree phase variations in the analytic method. The method also has the advantage of giving short time variations in the orbital motion of a satellite without ambiguity using NBV.
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