A New Approach to the Optimal Filtering of Differential Phase Measurements of GPS Signal in the Precision Survey
The Global Positioning System (GPS) has become popular research and application interests in surveying and many other areas. Nowadays, the accuracy of the Differential GPS can easily reach the order of a few meters. Yet, there are still many ways to exploit the GPS system signal carrier to improve the accuracy to 2 less than meter level. In this thesis, a new approach to improve the accuracy to less than meter level is presented while the observer is in the dynamic situation. In order to reach
... he sub-meter accuracy, we measure on the carrier phase difference (The L1 carrier frequency is 1575.42 Mhz, 1=19 em) between a reference point A and a primary point B. This actually means we work on the accuracy of centi-meter. In this proposed method of the precision survey, first the Differential GPS is used to flx the position in the accuracy of meter level, and then by measuring the signal carrier relative difference we can work on the accuracy in the accuracy level of the wavelength (19 em). The measuring on the relative carrier phase will introduce the problem of initial modulo 21r phase (integer wavelength) ambiguity. To solve the initial integer ambiguity, A Multiple Model Estimation Algorithm (MMEA) which was developed by D.T. Magill in 1965 is applied. The MMEA is composed of a bank of parallel Kalman filters, all operating on the input measurement sequence simultaneously. Each filter in the bank of filters is modeled around a different hypothesis. The number of the required parallel filters is the number of hypothesis of integer ambiguity which is determined by the error range of the differential phase measurement. And the error range of the differential phase measurement is related to the accuracy of the Differential GPS. The precision positioning by MMEA method has some advantage compares with other methods now being used . . It does not require continuous observation of the satellites initially. . Kalman filter is recursive technique. So it has the potential of on-line . . Kalman filter is widely used in navigation and approved to be very efficient and versatile. 3 Computer simulation results are given for a hypothetical GPS system. They demonstrate that the MMEA can effectively solve the integer wavelength ambiguity problem in dynamic situation. The simulation results presented are especially encouraging with regard to the flexibility and efficiency in precision survey. A further improvement of precision surveying by GPS is also discussed in the last Chapter. By using Markov Model and Verterbi Algorithm, a more fleXIble and reliable precision surveying method could be available.