Mixed-mode GPS network processing for deformation monitoring applications in the equatorial region

The Global Positioning System (GPS) can be utilised in a wide range of deformation monitoring applications. During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent, 'fiducial' GPS stations with dual-frequency, geodetic-grade receivers surrounding an 'inner' network of low-cost, single-frequency GPS receivers. Such a configuration offers considerable flexibility and cost savings for deformation monitoring applications, which require a dense spatial coverage of GPS stations, and where it is not possible, nor appropriate, to establish permanent GPS networks using dual-frequency instrumentation. The basis of the processing methodology is to separate the dual-frequency, 'fiducial' station data processing from the baseline processing involving the inner (single-frequency) receivers located in the deformation zone. The dual-frequency GPS network is used to generate a file of 'corrections', analogous to Wide Area DGPS correction models for the distance dependent biases. These 'corrections' are then applied to the double-differenced phase observations from the inner receivers to improve the baseline accuracies (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected). The performance of this configuration under severe ionospheric conditions in the equatorial region has been investigated by simulating such a two-stage network using data collected in the Hong Kong GPS Active Network. A description of the processing strategy, together with a discussion of the results, is presented.