Permanent Automatic GPS Deformation Monitoring Systems: A Review of System Architecture and Data Processing Strategies

Ground deformation due to volcanic magma intrusion, crustal motion, ground subsidence, etc., are phenomenon ideally suited for study using GPS. The change in length, height difference and orientation of baselines connecting GPS receivers in a carefully monumented ground network can be monitored. This is done by repeatedly measuring the same baseline components to an accuracy commensurate to, but preferably much higher than, the expected baseline component changes. Such GPS techniques are based on the “campaign” principle: the periodic (often annual) re-survey of a network of control points. However, over the last half decade or so, there has been a growing interest in the deployment of permanent, continuous GPS monitoring networks. The factors responsible for this trend include the declaration a few years ago of Full Operational Capability of the GPS system, and the steady decrease in price, size and power requirements of GPS receivers. Important additional factors have been the high cost of annual GPS surveys (manpower, travel, logistics, etc.), as well as the fact that geo-scientific research can be fürthered because of the continuous measurement of a deformation phenomenon, rather than its periodic measurement.The GSI network in Japan, the SCIGN network in California, and the SWEPOS network in Sweden are examples of large scale, continuous GPS networks for near-real-time crustal motion monitoring. However, smaller scale GPS arrays such as those on the Augustine volcano (Alaska), the Popacatepetl volcano (Mexico), the Kilauea volcano (Hawaii), and the Rabaul volcano (Papua New Guinea) reflect a growing interest in local continuous GPS volcano monitoring systems. GPS is also increasingly used to monitor engineering structures such as dams, bridges, offshore drill platforms, etc.Developing an automatic GPS array system for such small scale monitoring applications is an engineering and software challenge. A network of permanent GPS receivers need to be deployed, often in an inhospitable environment in which they must operate reliably on a continuous basis. The GPS observations must be telemetered to a central computing facility where data processing occurs with minimum delay. Analysis of the time series of baseline results then takes place in order to detect any baseline component change between successive solutions which may be a precursor to failure or eruption. What are the bases of small scale monitoring systems? Often the solution has been simply to purchase commercial “off-the-shelf” real-time-kinematic (RTK) GPS systems. This is the high cost option, yet there are several hundred active volcanoes in the world, many located in the less developed countries, and the cost of GPS monitoring systems must be significantly reduced if the technology is to contribute to volcano hazard mitigation. An alternative approach is to develop a system based on single-frequency GPS receivers, integrated with communications and in-field computer sub-systems.This paper discusses the characteristics of GPS monitoring networks and considers such issues as monument design, network design, GPS hardware, communication links, power supply, and data processing strategies.