Ambiguity Processing Approach in Combined GPS/GLONASS Positioning

The construction and status of GPS and GLONASS are introduced precisely, then the iterative detecting and repairing method of cycle slips based on triple-difference residue on combined GPS/GLONASS positioning are discussed systematically. The basic principle is taking iterations of triple-difference observations, and repairing some cycle slips in every iteration until all cycle slips are repaired or introducing new ambiguities while the cycle slips can not be repaired. Then an iterative ambiguity resolution approach suitable for combined data processing of GPS/GLONASS observations and the corresponding ambiguity fixing strategy based on FARA(fast ambiguity resolution algorithm) method are discussed. Because of the FDMA(frequency difference multiple address) technology used by GLONASS, the frequency and corresponding wavelength are not equal among different GLONASS satellites,and when making double-difference between GLONASS or GPS/GLONASS satellite pairs, the ambiguities will not maintain integrity-character as GPS satellite pairs. So the ambiguity can not be fixed to integer as common GPS only positioning. A transformation of the double-difference observation is taken, then every double-difference equation contains an integrity-characterized double-difference ambiguity and a single-difference ambiguity related part of the reference satellite. Then the double-difference and single-difference ambiguities can be fixed using the SEARCH strategy based on statistics. For dual-frequency data, geometry-free L₄ observations can be made to fix ambiguities. Based on the forenamed theory and BERNESE Ver 4.0 GPS only processing software, several modules are added and improved. Thus, high precision data processing software on combined GPS/GLONASS relative positioning(we call it improved BERNESE) is accomplished, then some experiments are taken and the observations are processed to verify the precision of the data processing software. Precise GPS/GLONASS ephemeris of International GLONASS experiment, sampling rate of 15 seconds,cutting off angle of 15 degree, troposphere model of Saastamoinen and standard meteorological parameters are adopted. The single-frequency GPS/GLONASS data collected by GG24 receivers are divided into GPS only and GPS/GLONASS combined.The GPS only data are processed using BERNESE Ver 4.0 GPS only processing software, and the improved BERNESE software processes GPS/GLONASS observations. The data processing results indicate that the result precision of several hours'single-frequency data is bellow several millimeters. In order to check the exterior precision of the enhanced BERNESE software thoroughly, another experiment is taken in November 1999 with JAVAD GPS/GLONASS dual-frequency receivers. 40 hours' data are cut into 20 segments of 2 hours' data, then the same processing strategy is used, and the results of improved BERNESE software are compared with the results of Pinnacle software developed by JAVAD company. The comparison shows that the result precision of 2 hours' dual-frequency observations is only a few millimeters. It is obvious that the result precision of dual-frequency observations is higher than that of single-frequency observations. The results of these experiments prove the correctness of the iterative detecting and repairing method of cycle slips based on triple-difference residues and the iterative ambiguity processing approach suitable for combined positioning. The precision of the software is at the order of millimeter on short baselines, which prove the correctness and feasibility of the high precision relative positioning theory, method and the data processing software itself, and open a new approach of broad application in the future.