Ambiguity Resolution Constraints In The Estimation Of Very Long Baselines By GPS In The Global Network Solution

A new global network solution was adopted in the estimation of very long baseline lengths from Hyderabad IGS Permanent GPS Station to other select IGS stations in and around Indian Plate. As part of on-going study on Indian Plate Kinematics, very long baseline lengths were estimated using the data from IGS select stations, including Hyderabad, for the time spanning 9 years from September 1995 to May 2004. Since some of the estimated baseline lengths are more than 6000km, it is a common knowledge that the ambiguity parameters have to be estimated as real integer values, instead of being `fixed' to integer values. Use of ionosphere-free linear combination of two frequencies has helped to remove the first order ionospheric noise. But this results in an accentuation of the non-dispersive noise. The lengths of the baselines of our network, that are between 2000 and more than 6500 km, make ambiguity fixing impossible. All the solutions presented in this study leave the ambiguity non-fixed. In the data analysis for long-term precision of continental scale interstation vectors measured with GPS, the L3 ionosphere-free combination was used with wide-lane ambiguity fixing for baseline lengths up to 2000km.When it is extended to the baseline lengths beyond this and more than 6500km, evidently the level of accuracy deteriorates, as the ambiguity parameters have large aposteriori rms errors. The strategy adopted, when measuring very long baseline lengths to pre-eliminate the ambiguity parameters estimated as real numbers and the normal equation matrix inversion, are discussed in this presentation. The length dependent accuracy deterioration is also discussed and shown for the entire data set.