With the gradual operation of BDS and the development of GPS, GLONASS Galileo, IRNSS, etc., the satellite navigation and positioning has entered an era of multi-system co-existence. The multi-system and multi-frequency observations bring the new opportunities for the improvement of positioning accuracy, reliability and other performance. At the same time, the integration of satellite navigation, mobile communication and internet technologies has greatly expanded the application of high-precision satellite positioning. The positioning demands in various industries are undergoing profound changes. Therefore, it is necessary to improve the existing precise positioning technology based on the abundant observations, so that the further demands of GNSS precise positioning can be met better.
Based on the above requirements, this thesis is intended to study the key technologies in precise relative positioning with the fusion of multi-system multi-frequency, which mainly involves: (1) the quality analysis of multi-GNSS observations, (2) the fusion model of observations between different systems, (3) the ambiguity resolution (AR) strategy with multi-system and multi-frequency observations, and (4) the multi-scale positioning model using triple-frequency observations. Based on the innovation or improvement of some algorithms and models, the positioning reliability, real-time, continuity and diversity have been improved.
The main work and contributions are listed as follows:
1. The data quality of each GNSS system and some special problems in observation fusion are analyzed systematically.
(1) The principle of calculating the observation residuals based on the between-station single-difference (SD) model is expounded. For GLONASS which adopts frequency division multiple access (FDMA), a single-difference carrier residual estimation model is proposed by choosing the bases twice. Then the SD carrier residuals can be calculated without considering the SD ambiguity caused by the difference of satellite frequencies. Based on the results of SD residuals from each system, the stochastic models with the elevation for each receiver and each system are established.
(2) The satellite pseudorange bias variation of BDS, the pseudorange inter-frequency bias (IFB) of GLONASS and their effect on the relative positioning are analyzed and verified. The results show that within 250km, the correction of BDS pseudorange bias is less than 10cm, and thus can be neglected for medium baselines. For the GLONASS pseudorange IFB, the results show that even for the receivers with the same type, the effect still cannot be eliminated. For receiver combinations with linear relationship between pseudorange IFB and channel number, the effect of the pseudorange IFB can be absorbed effectively by setting the parameter of IFB rate.
(3) The carrier-phase multipath in triple-frequency observation combinations is studied systematically. Several typical triple-frequency combinations including extra-wide-lane (EWL) combination, ionosphere estimation with ambiguity-corrected EWL/wide-lane (WL) combinations and the geometry-free and ionosphere-free (GIF) combination for narrow-lane (NL) AR, are analyzed in detail. The results show that the multipath-caused errors will increase with the absolute values of combination coefficients and cannot be effectively mitigated in short time, especially for BDS GEO satellites, which shows more systematic characteristics.
2. The tightly combined model between the inter-system observations with different frequencies is proposed.
Aiming at the fusion of different-frequency observations between GPS and BDS system, a method of estimating and analyzing the differential inter-system biases (DISB) is proposed. The results show that when the receiver types are different, the carrier DISB presents a certain low-frequency variation. Fortunately, this variation is very smooth and slow, and changes within 0.1 cycles in a few hours to several days. In general, the carrier and pseudorange DISB are relatively stable. Using this feature, the tightly combined positioning models of GPS and BDS with a common reference satellites are proposed for short and medium baselines respectively. Through the real-time estimation of DISB parameters in multi-epoch mode, the strength of positioning model can be enhanced. Experimental results show that compared with the conventional intra-system differential model, the tightly combined model can effectively improve the positioning accuracy and reliability, especially for the obstructed environments with a small number of satellites available.
3. The AR strategy using multi-system and multi-frequency is optimized.
(1) The influences of the observation noise and the ionospheric delay on each triple-frequency geometry-free (GF) model are analyzed in detail. The properties of the ionosphere-unbiased and biased models for wide-lane (WL) and extra-wide-lane (EWL) are analyzed and compared particularly. The results show that although ionospheric delay remains in the ionosphere-biased models, the influence coefficient is small, so that a high accuracy still can be obtained for medium baselines. The control of the fractional interval can further reduce the probability of the wrong ambiguity fixing.
(2) For the narrow-lane (NL) AR, a geometry-based model with the fusion of triple- and dual-frequency observations is established. In order to solve the problem of large dimension of ambiguities and observations, some optimization strategies are proposed. The performance of AR and positioning is tested and evaluated with real observations. Results show that for short baselines, the triple-frequency performs better than dual-frequency and multi-system performs better than single system, especially for the obstructed environments. In the case of medium baselines, the triple-frequency mode only contributes a little to the NL AR compared with the dual-frequency mode. Generally, the NL AR of BDS is slower than that of GPS due to the slow change of satellite geometry of GEO and IGSO satellites. The combination of BDS and GPS can significantly improve the AR and positioning performance compared with the single system.
(3) Aiming at the triple-frequency WL and EWL AR between long-range reference stations of BDS, a new stepwise ionosphere-free (SIF) method is proposed based on the characteristic that the baseline components are precisely known. Firstly, the (0,-1,1) ambiguity can be solved reliably with single epoch. Then the ambiguity-fixed (0,-1,1) observation are used to form a ionosphere-free carrier observation together with the second wide-lane combination. The results indicate that the proposed SIF method can avoid the influence of ionosphere delay in long-range baselines with smaller observation noises.
4. The multi-scale positioning model based on triple-frequency observations is established and the positioning performance is assessed using baselines with different length.
Based on the advantage that the EWL and WL ambiguities can be fixed easily, two types of multi-scale fast positioning models, i.e. Ionosphere-fixed and Ionosphere-float, are established. The positioning performance is evaluated using BDS real observations with different baseline length. The results show that:
(1) In Ionosphere-fixed mode, single-epoch decimeter positioning can be achieved by using EWL observations. Using WL observations, sub-meter accuracy and sub-meter to decimeter accuracy can be achieved for horizontal and vertical directions respectively for the baselines under 41.5km. For the longer baselines, decimeter-level accuracy can be achieved for both horizontal and vertical directions. Compared with pseudorange, the positioning accuracy can be significantly improved by using EWL and WL observations. For medium-long baselines within 80 km, single-epoch decimeter-level or even sub-meter level positioning accuracy can be achieved.
(2) In Ionosphere-float mode, the single-epoch positioning accuracy with EWL/WL observation is decimeter level for the horizontal direction, and decimeter to sub-meter level for the vertical direction. While for pseudorange observations, the horizontal and vertical accuracies are both meter-level. In multi-epoch mode, the effect of noises and carrier multipath effects can be effectively reduced by introducing NL observations. Adding GPS dual-frequency observations can further speed up the convergence. However, it is difficult to converge to the sub-decimeter or centimeter accuracy in short time due to the existence of amplified carrier multipath effect. It is also hard to improve the convergence speed and positioning accuracy by decreasing the sampling interval.