Experiences with a mixed-mode GPS-based volcano monitoring system at Mt. Papandayan, Indonesia

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 geodynamic applications such as volcano deformation monitoring, 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. This configuration has recently been tested at the Mt. Papandayan volcano in West Java, Indonesia. The two-stage network design consists of an inner network of four single-frequency Canadian Marconi (CM) GPS receivers surrounded by three dual-frequency Leica CRS1000 GPS receivers. The inner network logged and transmitted GPS data from the 'slave' stations located on the volcano, to a base station. The combined processing of the CM and Leica receiver data was performed offline so as to investigate the performance of such a mixed-mode system. The basis of the processing methodology is to separate the dual-frequency, 'fiducial' station data processing from the baseline processing involving the single-frequency receivers on the volcano. The data processing for the former was 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' will then be 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). A description of the field testing (and its challenges) during February-March 2000, together with a discussion of the results are presented.