Mixed-mode GPS deformation monitoring - a cost-effective and accurate alternative?

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 GPS stations with dual-frequency, geodetic-quality receivers surrounding an ‘inner’ deformation monitoring network of low-cost, singlefrequency GPS receivers. 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 enhance baseline accuracy, primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected. Moreover, epoch-by-epoch baseline solutions are preferred in order to detect deformational signals in (near) real-time. Data from two continuous GPS networks have been used to investigate the performance of this configuration under severe ionospheric conditions and in different geographical regions. In the mid-latitude region the L1 baseline repeatability has clearly been improved by 40-50%, while an improvement of about 20% has been achieved in the equatorial region. The findings also indicate that the proposed procedure is sensitive to extreme ionospheric conditions, such as those experienced in close proximity to the geomagnetic equator during solar cycle maximum periods.