Acting upon the assessments and recommendations provided by the science community and documented in a report submitted to the Committee on Earth Sciences of the Space Sciences Board of the National Research Council entitled "Spaceborne Synthetic Aperture Radar: Current Status and Future Directions," NASA has constructed a major SAR program plan named "SAR-2000, Spaceborne Imaging Radars for Earth Observations." This program plan is shaped by the science community conclusions that active microwave sensors will become increasingly important in the future and, while playing a secondary role to electro-optical sensing systems today, will likely reverse that role in the future. The report also observes that one of the most compelling uses of SAR for solid Earth studies involves interferometric SAR (IFSAR), citing the ability of IFSAR data to permit the construction of a global digital elevation model, to detect surface deformation at the millimeter level associated with natural hazards such as earthquakes and volcanic eruptions, to measure coastal ocean currents, and to measure the velocity and topography of glaciers and ice sheets. The NASA SAR-2000 Plan incorporates IFSAR as a performance centerpiece, but also incorporates multiparameter SAR systems to acquire data on such important environmental parameters as vegetation biomass estimates and classification, the detection of flooding and flooding under vegetation, measurement of snow pack depth, and detecting ocean surface slicks and distinguishing between oil spills and natural surfactants. The NASA SAR-2000 characteristics emphasize small, focused, affordable missions conducted within a coordinated international SAR program, the utilization of high technology, and lightweight SARs which capitalize upon advancements in NASA's New Millennium Program.[1] This initiative provides an expansion of a core radar technology component, provides for an aggressive data exploitation and outreach program, and provides for a long-term series of vital scientific measurements.
How this program affords opportunities for users to acquire and utilize data to support operations remains to be defined, and presents a challenge to the operational SAR community to shape this and companion programs to meet both scientific and operational needs.
The concepts of a coordinated international SAR program in the early 2000s is being pursued by NASA, and, at an April 1995 Space Studies Board meeting, discussions between representatives from the U.S., Canada, Japan, ESA, Italy, and Germany, were held to explore the concept of an international SAR constellation. This international concept grows out of a realization that within the next decade, no one country is likely to afford to undertake the exploitation of the emerging scientific and operational potential of space-based SAR, given the declining space agency budgets worldwide. Cooperative partnerships are now viewed as essential to pursuing a robust space-based civilian SAR program. The envisioned concept of an international SAR constellation would be comprised of multiple small satellites, developed, funded, and operated by different world agencies, with each satellite providing SAR measurements at generally one frequency. The satellites would fly in formation to enable interferometric and multiparameter applications of SAR data, allow the pursuit of high priority objectives including high- and variable-resolution global mapping, observations for hazard monitoring and mitigation, coastal ocean feature measurements, and studies/applications supporting ecology, hydrology, and geology. An enormous amount of work remains in order to design the political, economic, commercial, and technical dimensions of such international SAR partnerships. However, the concept appears to have considerable merit, and the operational SAR community must be actively involved in the discussion and planning process to ensure that the emerging concepts embrace operational requirements and considerations. It is unlikely that U.S. resources will be adequate to permit separate scientific and operationally-oriented civilian SAR satellite systems anytime in the foreseeable future.
Based upon the successes of the 1994 SIR-C/X-SAR SRL-1 and SRL-2 missions, and the desire to increase the momentum of the U.S. civilian space-based SAR program, serious consideration is being given to flying an SRL-3 mission in the 1997 time frame. Strong recommendations by both science and operational users to use SAR interferometry to conduct global topographic mapping have driven the objectives for an SRL-3 flight, possibly serving as a precursor to a dedicated topography SAR (tentatively being called the Global Topography Mission or GTM) in the 2000 to 2002 time period. The SRL-3 mission would encompass a Shuttle Radar Topography Mapper (SRTM) (Figure 8-1), devoted to performing a medium resolution topography mission from the Shuttle. An 11-day mission, which could be launched in the 1999-2000 time frame, would acquire quasi-global topography (coverage from 60 degrees North to 60 degrees South latitude) with 30-m spatial resolution and 9- to 12-m vertical accuracy (DTED-2 level). The SAR sensor would be a SIR-C/X-SAR, augmented with a C-band receive-only antenna attached to a 62-m boom for topographic mapping. Additional features would include an 8-m receiving SCANSAR C-band antenna, a metrology system for baseline orientation/length determination, and the provision of a ground SAR processor to permit all data to be processed into map products by one year following the completion of the mission.
While the IFSAR topography component of the mission would occupy a considerable portion of the mission timeline, NASA could provide opportunities to obtain SAR data in support of demonstration projects designed to evaluate the utility of the multiparameter, interferometric SAR in a variety of operational applications. In anticipation of the possibility of this SRL-3 mission, several agencies have begun to define candidate demonstration projects so that current mission activities and resource allocations can be tailored to support a selected set of demonstrations.