Instantaneous Doppler-aided RTK positioning with single frequency receivers

Instantaneous Real-Time Kinematic (RTK) is one of the most precise GNSS positioning and navigation technologies, with which users can obtain centimeter-level relative positioning accuracy in real-time. This technique relies fundamentally upon the inversion of both carrier-phase and code pseudoranges and successful instantaneous ambiguity resolution. However, in this approach the probability of fixing ambiguities to correct integer values is dominated by the relatively imprecise code pseudorange measurements especially in urban areas and difficult environments where the level of noise and multipath on code pseudoranges are high. This problem may be overcome partially by carrier-smoothing of the code pseudoranges using e.g. the Hatch filter. However, in unfavorable environments frequent carrier-phase outages, cycle slips and anomalies occur due to blockages and foliage, etc. and hence the effectiveness of the conventional carrier-smoothing of the code pseudoranges is limited. On the other hand, a receiver can generate continuous Doppler frequency shift measurements with centimeter-level precision even in severe urban canyons as long as signals are present. The objective of this paper is to show the advantage of adding Doppler frequency shift information to instantaneous RTK positioning and navigation in particular for single frequency users. The aim is to enhance the probability of identifying the correct set of integer ambiguities and hence increase the success rate of the integer ambiguity resolution process in instantaneous RTK. In this paper, a simple combination procedure of the noisy code-based pseudorange measurements and the centimeter-level precision receiver-generated Doppler measurements is used to smooth the code pseudoranges. Doppler-smoothed code pseudoranges are then used to help resolve integer ambiguities in the conventional LAMBDA ambiguity resolution method. Single-epoch single-frequency static trials over varying baselines from few meters up to 40 kilometers have shown improvement in fixing the correct set of integer ambiguity vectors when Doppler-smoothed pseudoranges are used compared to the conventional ambiguity resolution without Doppler-aiding. Relative kinematic positioning with single frequency mass market receivers in urban areas using Virtual Reference Stations (VRS) has also shown improvement.