consuming and incomplete. Moreover, most of the volcanoes of the world are not monitored in any way; it would be possible to monitor them all with a constellation of InSAR satellites.
Despite the advances made possible by InSAR, these observations have even greater potential for the future. For example, opportunities to observe the deformation associated with earthquakes have been missed because of insufficient space-based instrumentation. Although Global Positioning System (GPS) measurements provide high-accuracy determinations of displacements, these measurements are available only at a limited number of points. InSAR provides the capability for more complete spatial coverage, but available data have been limited in coverage (in space, time, and direction) and are limited to the shorter-wavelength C-band, where phase decorrelation may be more problematic. Moreover, these satellites have reached the end of their useful lifetimes.
The InSAR missions recommended in the SESWG report would provide observations superior to any that have previously been available. An InSAR satellite with orbital parameters and observation scheduling responsive to the needs of solid-earth scientists would provide global coverage at improved spatial and temporal resolution, and would potentially be poised to observe specific areas of interest. Because the earth deforms slowly and large earthquakes occur in any given region infrequently, the chance of obtaining the observations needed to understand the processes involved is substantially improved by global coverage. The proposed missions are L-band (~ 24-centimeter wavelength), which allows more robust phase correlation than previous C-band (~ 5.6-centimeter wavelength). Taking observations on both ascending and descending passes allows determination of multiple components of (vector) surface deformation. No active components are needed on the ground, making it possible to obtain observations quickly in remote and dangerous regions and without interfering with disaster response work.
Measuring surface deformation in regions of active fault systems, volcanoes, and landslides provides data for assessing the risk associated with these phenomena. For example, because volcanic eruptions are typically preceded by surface deformation, InSAR measurements could be used to make short-term predictions of volcanic eruptions. Measurements of surface motions associated with changes in pore fluids are relevant to hydrocarbon exploitation, water usage, and geothermal power production.
InSAR data are useful to agencies and organizations concerned with the deformation of the earth and associated natural hazards (see Appendix A and references therein). A National Research Council (NRC) review of the multiagency EarthScope initiative strongly endorsed the integrated approach proposed, including all four components: (1) the United States Seismic Array (USArray), (2) the Plate Boundary Observatory (PBO), (3) the San Andreas Fault Observatory at Depth (SAFOD), and (4) InSAR.3 The report concluded that the scientific rationale for EarthScope is sound, that the scientific questions to be addressed are of significant importance, and that no necessary components have been omitted. It recommended that that all four components be implemented as rapidly as possible and that the National Science Foundation (NSF) and NASA collaborate to realize the InSAR goal at the earliest opportunity.
InSAR is of clear relevance to the NASA Earth Science Enterprise mission of predicting and mitigating natural hazards and also contributes to answering earth-science questions related to changes in ice cover and changes in the earth’s surface, and to understanding the coupling