From simulation to implementation: low-cost densification of permanent GPSnetworks in support of geodetic applications

Permanent GPS networks have been established since the 1980s to support a v ariety of geodetic applications, ranging from local deformation monitoring to large-scale crustal motion measurement. Continuously operating GPS (CGPS ) networks, consisting of geodetic-grade, dual-frequency receiver systems, generally support relative positioning to sub-centimetre accuracy, even for baselines up to several thousand kilometres in length. However, due to the ir comparatively high cost, the density of such GPS stations is rarely high enough to support all geodetic applications. For example, although the Geo graphical Survey Institute has established nearly 1000 permanent GPS statio ns across Japan, the average inter-station spacing is of the order of 30 km . This paper describes a method by which a sub-network of comparatively low -cost, single-frequency GPS receivers can be deployed to increase the densi ty of typical CGPS networks. In this way it is possible to increase the spa tial resolution of the measured ground deformation, while still maintaining the same level of precision as a CGPS network comprised entirely of dual-f requency GPS receivers. In order to reduce the system biases associated wit h single-frequency data processing, an innovative medium-range GPS position ing technique that combines the processing of single-frequency and dual-fre quency data is proposed. Several data sets are analysed in order to address critical issues such as: 'Can the technique work equally well for differen t geographic locations across Asia, traversing large elevation changes, in various seasons?', 'Can the sub-network incorporate single-frequency receiv ers of different brands while maintaining similar levels of precision?', an d 'Can the sub-network yield generally uniform high precision results for d ifferent baseline lengths?' The analyses undertaken by the authors confirm that the proposed technique can achieve relative accuracies similar to thos e obtained from dual-frequency, static positioning, over inter-station dist ances up to a few tens of kilometres, under a variety of operational enviro nments. These investigations are a necessary first step in making this tech nique an operational reality.