Advantages and problems of combining GPS with GLONASS

The Global Positioning System (GPS) has been an undoubted success and a great many applications have benefited from it. It does however have limitations, which make its use in certain environments, and for certain tasks, difficult or indeed impossible.In recent years a second satellite based navigation system, the Global'naya Navigatsionnaya Sputnikov Sistema (GLONASS) has become increasingly available. A great deal of interest has been expressed in combining both these systems, in the hope that combined GPS/GLONASS technology will present significant benefits under conditions where GPS alone has struggled. The research described in this thesis was undertaken to examine the potential benefits and problems of such a combination. This has been primarily achieved through the modification of the existing GPS processing software of the Institute of Engineering Surveying and Space Geodesy (IESSG) to accept GLONASS observations. The analysis of data collected under controlled conditions and processed through this software has highlighted biases in the pseudorange measurements from the GLONASS satellites. This is due to the fact that each GLONASS satellite broadcasts on a different frequency, which is then delayed by slightly different amounts through the Radio Frequency (R/F) section of the receiver. If these R/F sections were identical in each receiver, this error source would cancel, but this has not been found to be the case with the receivers used in this research. Interestingly, no such biases have found to be present in the GLONASS carrier phase observations. Various tests have been performed and the data processed through both IESSG and commercially available software. These have highlighted that there are undoubted potential benefits of using combined GPS/GLONASS receivers in environments where visibility is restricted. Under ideal conditions however, the effect of any benefit is reduced, and indeed the biases present in the GLONASS pseudoranges may slightly degrade the accuracy of differential positioning. The software developed has already been used in other research projects within the IESSG. Although the future of the GLONASS system is somewhat uncertain, any future changes to it should be easily accounted for within the code. There is however a real need to further develop and incorporate cycle slip detection software, especially for GLONASS observations, and to investigate the possibility of solving for the biases in the GLONASS pseudoranges.