Development and Application of Small Spaceborne Synthetic Aperture Radars (1998)

It is apparent that synthetic aperture radar will assume a position 20003214ddd000048 of greater importance in Earth science than heretofore. In the past decade, aircraft and satellite missions have provided a growing body of SAR observations of a wide variety of phenomena in the Earth system. These observations, when coupled with the thorough in situ measurements that are an integral part of recent SAR campaigns, are moving far beyond the phenomenological and descriptive approaches of earlier SAR missions. However, much remains to be done.

Past SAR missions have generally been large and expensive. Radarsat, for example, cost approximately $500 million; ERS-1 came in at about $750 million; and Envisat, one of the most expensive, is projected to cost about $1.2 billion. On the other hand, Magellan, at approximately $300 million, has proven that all data collected (i.e., the entire planet of Venus several times over, at 75-m resolution and about seven looks) can be processed and turned into (map) products at a real rate without a massive processing infrastructure. The notion that SARs are extraordinary resource users (both financial and spacecraft), as well as the perception that SAR signals have not been linked to Earth system phenomena in a quantitative manner, have impeded the development of a free-flying SAR capability in the United States.

The committee recognizes that SAR will become increasingly important in many areas of the NASA-ESE science strategy and its five themes, i.e., the five major components of the Earth system: hydrological, biogeochemical, atmospheric, ecological, and geophysical processes. In this light, the committee recommends the following strategy for a small SAR program.

1. Develop a well-defined focus for any small SAR minion.

It is important for NASA to consider what objectives are to be served by a potential spaceborne SAR system, in a broad sense, and their relative priorities. Three general areas are recognized: (1) providing scientific data (e.g., of the type required by ESE), (2) providing information in support of the general public good (e.g., environmental monitoring and hazard assessment), and (3) providing data for commercial interests (e.g., digital elevation models for cartographic applications, mineral exploration, or forest management). The committee recommends that the relative priorities and interests of all three use categories be weighed at the outset of the mission design process. End-to-end system engineering can then be optimized to serve the prioritized suite of information needs.

A well-defined focus will guide the various technical trade-offs in terms of wavelength, spatial resolution, incident angle, polarization, swath width, image quality, and data rate that must be made to constrain small SAR costs. Rather than defining a specific mission, the committee has outlined some candidate missions and their associated technical trade-offs to illustrate a suitable process.

The committee notes that of the several proposed operating frequencies associated with SARs, the L-band is especially useful in forest and desert ecology applications, but other applications such as agriculture may lead to a small SAR design based on C-, X-, or K_u-band frequencies. These frequencies require smaller antennas than does the L-band, which may simplify deployment from a small spacecraft. In the committee's view, design parameters such as frequency, polarization, resolution, and swath width should be chosen to match the mission focus, while the results of all available research, including that from the 1960s and 1970s, are considered.

2. Adopt new technologies to reduce SAR costs.

In the committee's view, many new technologies may come from outside NASA. In addition, new technologies currently available for data capture and processing can be used to lower overall SAR system costs. Many of these technologies can be evaluated without resorting to costly spacecraft missions. According to JPL's LightSAR point design report (JPL, 1997), the estimated end-to-end mission cost of the baseline design is $125 million, which is only a fraction of the estimated mission costs of the single-frequency, single-polarization SARs of ERS-1 ($750 million) and Radarsat ($640 million). The significant reduction in cost is attributed to the incorporation of new technologies. As examples of cost-reducing technologies, L-band antennas are seven times lighter and require only half as much power as SIR-C's antenna. Small SAR synthesizers are seven times lighter than SIR-C's and require one-tenth the power.

3. Continue support of a vigorous research and analysis program in radar remote sensing.

Although considerable progress has been made in recent years, there are many areas in which understanding of the physical link between the SAR signal and the geophysical phenomenon is weak. For example, there is a strong soil-moisture signal present in certain SAR imagery, but more research is necessary to learn how to quantify the effects of surface roughness and vegetation on soil-moisture signals. NASA should continue appropriate studies to strengthen these links. These studies may involve theory, ground-based scatterometers, and aircraft SARs. Such calibration and validation studies should also be an integral part of any small SAR mission.

4. Establish a clearly defined small SAR data policy that will protect commercial interests while ensuring free and open access by the public and research communities.

SAR imagery has potentially important applications for research, the public sector, and commercial users. If NASA continues to seek commercial partners for a small SAR mission (to reduce costs), it must define data policies clearly to protect the proprietary interests of commercial entities while ensuring open access to other user communities. In addition, widely distributed data processing and dissemination may both lower costs and increase access to small SAR data. Such a “federated” approach is consistent with the strategy being pursued for the Earth Observing System Data and Information System (EOSDIS).

It is expected that much data will be dual-use in nature and can serve multiple interests (science, public use, and commerce). Innovative data access policies could protect both research and commercial communities. For example, most commercial applications may have a relatively

short shelf life. When they no longer have commercial value, these older data sets may still be valuable to the research community. However, given the longer time scales for many terrestrial applications (e.g., glacial recessions), the commercial shelf life may be many years for some data sets.

Consideration of public use and commercial interests complicates issues of data access and distribution because of conflicting needs. At the same time, public use of data should also be expected and encouraged, but many such users may not be able to afford commercial rates for access to data.

The committee recognizes that flexibility must be maintained in the data acquisition and dissemination system. The concept of life-cycle mission design should be applied to minimize conflicts in scheduling SAR operating modes. The various scientific objectives of the LightSAR science plan imply that conflicts will arise and require dissimilar data types over common areas. such conflicts will be exacerbated by the differing needs of public and commercial users. Mission life-cycle planning can be used to balance conflicting needs weighted by relative priority.

5. Consider an enhanced multifrequency small SAR configuration.

There is sufficient evidence to warrant consideration of a multifrequency small SAR (Evans et al., 1995; Dobson et al., 1997). Single-frequency, single-polarization spaceborne SAR systems cannot meet all of the scientific objectives outlined in the JPL LightSAR science plan. These needs might be met at some level of accuracy by a single-frequency, polarimetric small SAR as defined in the baseline plan. For some applications, the accuracy level is known to increase markedly with the addition of higher-frequency SAR data. This may be very compelling to industrial partners seeking to satisfy commercial demands. Industry teams can be expected to pay close attention to the end-to-end costs of any system enhancements relative to the expected commercial value of such a system. The commercial value of LightSAR enhancements is currently being evaluated by the marketplace via LightSAR business development and system design definition studies (JPL, 1997). It may be prudent for NASA to conduct a parallel evaluation of the relative scientific value of potential enhancements (C-band dual polarization and X-band single polarization).

6. Continue coordinating small SAR with other international SAR missions in an Integrated Global Observing Strategy framework.

The committee acknowledges that there are many difficult issues associated with international coordination of radar missions, but also recognizes that essentially all major remote sensing efforts in recent years have involved international cooperation. The constellation of SARs that is required to exploit this technology fully cannot be deployed by a single nation. As part of its activities with the Committee on Earth Observing Satellites and its Integrated Global Observing Strategy (IGOS), NASA should continue to coordinate small SAR with other international SAR missions within the IGOS framework. International cooperation provides a real opportunity to test the advantages and disadvantages of combinations of data from different single-channel and multiple-channel sensors. International coordination should help ensure that the systems launched by different countries complement rather than duplicate each other. In addition, international cooperation in SAR systems requires close attention to details that could

interfere with the international use of scientific data. For example, data availability agreements must be clear and must be honored, and calibration and validation plans must be negotiated formally, not left to the good will of participating space programs and scientists.

In addition to the coordination of missions launched by different countries, it is worth exploring cooperation in international joint missions, either by combining assets in a single satellite or by flying a formation of separate satellites designed for this purpose. Such programs will have to be approached on the basis of technology exchange among equals and with integrated science teams.

The foregoing strategic elements support consideration of a small SAR program. They result from the committee's assessment of the value added by a multifrequency small SAR, as opposed to a single-frequency system, and other issues raised in this report. Recent technological advances can significantly reduce the size and cost of spaceborne SAR. Interferometric data from a small SAR would be of high value to ESE, but a vigorous research and analysis program is necessary to nurture nascent applications. Carefully constraining mission focus can reduce end-to-end system complexity and cost. The commercial interests of potential small SAR industrial partners may require a multifrequency enhancement of the LightSAR baseline design. The added scientific value of such an enhanced capability should be ascertained by NASA. Policies must be clearly defined that preserve the interests of all partners in a small SAR mission. The discussions in this report of new technologies, advances in data capture and processing, the advantages of SAR over electro-optical imaging, and potential trade-offs to reduce weight led the committee to conclude that focused applications of a multifrequency small SAR mission could provide more and better information and understanding of earth, ocean, and atmospheric processes at lower costs than heretofore possible.