Showing papers by "Chengfa Gao published in 2018"

Combined GPS and BDS for single-frequency continuous RTK positioning through real-time estimation of differential inter-system biases

Abstract: Double-differenced (DD) ambiguities between overlapping frequencies from different GNSS constellations can be fixed to integers if the associated differential inter-system biases (DISBs) are well known. In this case, only one common pivot satellite is sufficient for inter-system ambiguity resolution. This will be beneficial to ambiguity resolution (AR) and real-time kinematic (RTK) positioning especially when only a few satellites are observed. However, for GPS and current operational BDS-2, there are no overlapping frequencies. Due to the influence of different frequencies, the inter-system DD ambiguities still cannot be fixed to integers even if the DISBs are precisely known. In this contribution, we present an inter-system differencing model for combined GPS and BDS single-frequency RTK positioning through real-time estimation of DISBs. The stability of GPS L1 and BDS B1 DISBs is analyzed with different receiver types. Along with parameterization and using the short-term stability of DISBs, the DD ambiguities between GPS and BDS pivot satellites and the between-receiver single-difference ambiguity of the GPS pivot satellite can be estimable jointly with the differential phase DISB term from epoch to epoch. Then the inter-system differencing model can benefit from the near time-constant DISB parameters and thus has better multi-epoch positioning performance than the classical intra-system differencing model. The combined GPS and BDS single-frequency RTK positioning performance is evaluated with various simulated satellite visibilities. It will be shown that compared with the classical intra-system differencing model, the proposed model can effectively improve the positioning accuracy and reliability, especially for severely obstructed situations with only a few satellites observed.

Gps/bds tight combination carrier differential positioning method

Abstract: Disclosed is a GPS/BDS tight combination carrier differential positioning method, comprising: first constructing, with a GPS as a reference system, a GPS intra-system double-difference ionosphere free combination model and a GPS/BDS inter-system double-difference ionosphere free combination model; then selecting a BDS reference satellite, re-parameterizing the ambiguity of the GPS/BDS inter-system double-difference ionosphere free combination and performing parameter de-correlation; estimating an ionosphere free combination carrier differential inter-system deviation in real time, and performing datum conversion on the ionosphere free combination carrier differential inter-system deviation at an essential moment to achieve the continuous estimability of the ionosphere free combination carrier differential inter-system deviation; and finally, forming an ionosphere free combination using a base carrier observation value with fixed ambiguity and performing tight combination positioning on the inter-system double-difference ionosphere free combination on the basis of the estimated ionosphere free carrier differential inter-system deviation.

A Tightly Combined GPS/Galileo Model for Long Baseline RTK Positioning with Partial Ambiguity Resolution

Abstract: Knowledge of differential inter-system biases (DISBs) is critical to integrate observations from mixed GNSS. If the corresponding DISB could be calibrated in advance, only one pivot satellite is sufficient for ambiguity resolution on overlapping frequencies, which is the so-called tight combining (TC) strategy. Considering that GPS and Galileo transmit signals on two identical frequencies (e.g. L1/E1 and L5/E5a), a tightly combined GPS/Galileo RTK positioning model is proposed in this paper. Traditional DD model has been slightly adjusted to avoid the hand-over problem of reference satellites. The estimation of code and fractional part of phase DISB is archived through zero and ultra-short baselines. Three long baselines were selected to verify the proposed model with DISB calibrated in advance. Moreover, to get better AR performance, a simple but robust procedure of PAR, where the satellite elevation, number of consecutive tracking, success rate and ratio test are all combined to determine the subset of ambiguity, is adopted in the long baseline experiments. Results shows that the code and fractional part of phase DISB is rather stable. The TC strategy do not significantly improve the value of ratio, but shorten the convergence time to reach the 100% success rate. Compared with results of loose combining (LC) strategy, time to first fix (TTFF) is further reduced by 54.3, 72.9, 69.0% respectively under TC strategy corresponding to different long baselines. Besides, TC strategy could slightly improve the fixing rate of epochs. In terms of accuracy, the precision in up direction is worse than that in north and east direction. Once the ambiguity is fixed correctly, both LC and TC strategy can achieve centimeter-level positioning accuracy.

Rapid Ambiguity Resolution Algorithm for Multi-constellation Between Reference Stations Based on Ambiguity Tight Constraint

Abstract: The development of multi-GNSS remarkably increased the number of available satellites, but how to solve the multi-dimensional ambiguity parameters quickly and accurately in network RTK technology is still an issue full of perplexity and significance. To ensure the virtual station have more available satellites under the occlusion environment, a fast ambiguity resolution method for base station based on ambiguity tight constraint was proposed in this paper. Firstly, the optimal subset of ambiguity is selected by partial ambiguity resolution (PAR) strategy, and then impose strong constraints on the ambiguities of these satellites. Finally, update the filter equation and assist in fixing the ambiguity of other satellites. The real measured baseline data which contain GPS, BDS and GLONASS from Tianjin CORS and Curtin University was used in the experiments, and the results illustrated that this method could significantly shorten the initialization time of ambiguity between base stations, accelerate the convergence speed of newly-arisen satellites, and increase the number of available satellites of RTK virtual observations (especially low-elevation angle satellites), that providing a reliable guarantee for high-precision positioning in the occlusion environment, such as the roads in cities.