Permanent GPS networks have been established since the 1980s to support a variety 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 their comparatively high cost, the density of such GPS stations is rarely high enough to support all geodetic applications. For example, although the Geographical Survey Institute has established nearly 1000 permanent GPS stations 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 density of typical CGPS networks. In this way it is possible to increase the spatial resolution of the measured ground deformation, while still maintaining the same level of precision as a CGPS network comprised entirely of dual-frequency GPS receivers. In order to reduce the system biases associated with single-frequency data processing, an innovative medium-range GPS positioning technique that combines the processing of single-frequency and dual-frequency data is proposed. Several data sets are analysed in order to address critical issues such as: 'Can the technique work equally well for different geographic locations across Asia, traversing large elevation changes, in various seasons?', 'Can the sub-network incorporate single-frequency receivers of different brands while maintaining similar levels of precision?', and 'Can the sub-network yield generally uniform high precision results for different baseline lengths?' The analyses undertaken by the authors confirm that the proposed technique can achieve relative accuracies similar to those obtained from dual-frequency, static positioning, over inter-station distances up to a few tens of kilometres, under a variety of operational environments. These investigations are a necessary first step in making this technique an operational reality.
