A mixed-mode GPS network processing approach for deformation monitoring applications

The Global Positioning System (GPS) can be utilised in a wide range of deformation monitoring applications, such as the monitoring of active volcanoes, tectonic fault lines, landslides, ground subsidence, bridges, dams, high-rise buildings, etc. 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 data processing for the former is carried out using a modified version of the Bernese software, to generate a file of ‘corrections’ (analogous to Wide Area DGPS correction models for the distance dependent biases - primarily due to atmospheric refraction). These ‘corrections’ are then applied to the double-differenced phase observations from the inner receivers to improve the single-frequency baseline accuracies (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected).

This configuration has been tested by simulating such a two-stage network using data collected by part of the Southern California Integrated GPS Network (SCIGN). Six sites forming an inner network of three GPS receivers surrounded by three ‘fiducial’ GPS receivers are used in this study. All sites are equipped with dual-frequency instrumentation, however, for most of the analysis the inner sites are treated as single-frequency stations by ignoring the observations made on L2. A description of the processing strategy, together with a discussion of the results is presented.