Generation And Distribution Technique Of Precise Differential Corrections For GPS/VRS Network

Current carrier phase-based Real-Time. Kinematic (RTK) positioning system at the centimetre accuracy level is practically constrained by two aspects: one is that the user must be located within the vicinity of the reference station, say no more than 20 kilometers, the other is that the differential correction is broadcast by the out-of-date radio communication mode (e.g., LF, MF, HF, UHF) with much cost and low performance. This thesis deals with the new generation technique of RTK, namely the network RTK technique, which integrates multiple continuously operating reference stations into one combined network to support multiple functional navigation and positioning services, especially for the medium range centimetre level carrier phase-based applications. Based on systemic research about the data process theory of network RTK, the thesis put its focuses on superior methods to fulfill the network correction computation, correction Interpolation and the correction distribution. The network correction computation uses the network stations to precisely estimate the differential correlated errors between the baselines covering the region using carrier phase observations with fixed ambiguities. The second process interpolates these network corrections to determine the effects of the correlated errors at the rover's position. Then the Virtual Reference Station (VRS) technique is used to relay the corrections to the rover receiver with standard RTK device. An advanced technique-"Networked Transport of RTCM via Internet Protocol (NTRIP)" using the Internet for streaming and sharing differential GPS corrections to allow precise positioning and navigation is introduced. Finally detailed systemic design of the configuration and architecture about the auto-developed Internet based precise VRS differential correction service system (Venus software) is under discussion. Many numerical tests presented in this study based on the Sichuan GPS Reference Station Network (SGRSN) show the better availability and reliability for such centimeter-level level precise network RTK system.The main contributions of the paper include:1. Mathematic model of the universal VRS observationAfter briefly discussion and comparison of the three possible network RTK architecture, including Virtual Reference Station (VRS), Correction Broadcasting (FKP) and Master-Auxiliary Concept (Mac), new VRS (UVRS) observation calculation model universal to both DGPS and RTK positioning technique is developed. The UVRS is composed of the un-differenced GPS observation from the master reference, the geometrical correction term for the VRS installation and the double-differenced distance-dependent error corrections. Theoretic deduction proves that UVRS can be generated in standard DGPS/RTK format, e.g. RTCM V2.0 to V2.3. Furthermore, it can support the last RTCM V3.0 to 3.1 formats, which are designed for network RTK correction application, but with some loss of positioning integrity information. There is no further request for the standard rover user to use the new UVRS technique in which point the UVRS is by now superior to the FKP and MAC in practical application. More over, to facilitate the application of the VRS network, some auto-networking procedure for the tri-angular network, independent baseline network and centered network are introduce.2. Classification and extraction of the differential distance -dependent errors on the baselines within regional reference station network using carrier phase observations with carrier phase ambiguities fixed.There are many kinds of bias sources in the VRS network due to the effect of ionosphere, troposphere and orbital bias, and so on. To cancel the overall effects of the systemic errors more accurately, it is pivotal to classify them with their spatial or environmental correlated characteristics and extracted the errors like DD ionospheric delay, tropospheric delay individually before estimate the accurate spatial error distribution model covering the network region. The zenith tropospheric delay (ZTD) is treated as unknown parameter with the carrier phase ambiguities when perform network filtering calculation. After the ambiguities being fixed, the DD tropospheric errors can be calculated. Then the DD ionospheric errors are estimated with the double-frequency or triple-frequency ionospheric model with carrier phase observations. Finally some other algorithms to estimate the multipath effect for the pseudo-range and carrier phase observation are proposed.3. Distance-dependent error modeling and corrections interpolating for rover receiver at arbitrary position within the valid network coverage regionAfter the DD residuals on the baselines covering the network region being estimated, the spatial correlated errors like ionospheric and tropospheric delay, satellite error and even multipath effect of the pseudo-range and carrier phase observation for VRS should be Interpolated or generated in other way both for L1 and L2 frequencies. The method applying the ionospheric prediction corrections to improve the accuracy of network Interpolation model (e.g. the linear Interpolation model, LIM) is introduced. For long range network with station spacing up to 100km, the ionospheric prediction corrections based on the temporal correlation for the ionospheric delay can effectively reduce small and medium scale(generally within 100km distance)active ionospheric disturbance. In the research, for the medium latitude area, the accuracy of the LIM can be 1 to 2cm. With the help of ionospheric prediction corrections, it can remain 2cm even under solar maximum conditions.The unmodelled residuals of tropospheric delay increase in mountainous areas due to large height variations between the reference stations and rovers, especially for sparse networks composed of only three or four reference stations with an average reference station spacing of more than 100km. To reduce the influence from the height differences, a network-based error reduction approach called "Modified Height and Distance-based Linear Interpolation Method (MHDIM)" is proposed. The 'apriori' tropospheric models are applied in this network approach to recover the homogeneous spatial correlation characteristic of the atmospheric delay, and then the ionospheric and tropospheric delays are interpolated respectively. It was shown that 2 to 4 centimetre accuracy is achieved in the reduction of the atmospheric errors over sparse networks.A precise real-time method using the IGS ultra rapid products (IGU) and the GPS broadcast ephemeris to calculate the VRS orbit corrections is presented here for VRS /RTK technique. The newly proposed method is proven to be more precise and reliable compared with the existing conventional network-based orbit error interpolation method. It is shown that 0.004ppm relative accuracy was reached, namely the influence from the orbit bias for the RTK positioning within 100km area can be of sub-millimeter level.SNR-based interpolation model (SNRIM) is developed to calculate the precise real-time multipath corrections for the RTK rover, based on the strong-correlative relationship between the multipath effect and signal-to-noise ratio (SNR). The technique for multipath mitigation is verified feasible and reliable in reference station network based real-time kinematic differential GPS positioning at cm-level. 4. Research on the Networked Transport of RTCM via Internet Protocol (NTRIP) "Networked Transport of RTCM via Internet Protocol, NTRIP" enables the streaming of DGPS or RTK correction data via the Internet using GPRS or other future technologies. The main intension of the technique is using the Internet as an alternative from the current existing real-time correction services provided via radio transmission (LF, MF, HF, UHF) or mobile communication networks like GSM. Both the theoretical and technical basics of Ntrip are discunssed and the practiceal application of the technique is introduced using autodeveloped Ntrip Server/Caster/Client supported software. Thus the Precise differential VRS corrections service can be accessed by a variety of clients/user through the wireless internet media.5. Development and test of internet and VRS reference station network based navigation and positioning service systemOn the base of the above techniques, the so-called "VRS Enhanced Network Utility Solution, Venus" software is design and developed. The software is aimed to support precise VRS Reference Station network based Navigation and Positioning Service form from cm to dm level. Detailed systemic tests about the accuracy, reliability and the availability of the network positioning service are performed based on the SGRSN. Static and real-time results show that the work dune in this dissertation attains a well effect and the Venus is suited for regional reference station network based long range (up to 70km) real-time kinematic differential GPS positioning at cm-level.