From Air to Space
For nearly 30 years, airborne side-looking radar has been applied with great success to a variety of commercial and environmental endeavors including exploration geology investigations that have led to the discovery of petroleum and minerals in the United States, South America, Africa, and East Asia. More recent research has extended the applications of airborne radar to the examination of the processes of desertification, the circulation of oceans, the dynamics of glaciers, and a host of other environmental problems.
      Therefore, it is appropriate, now that the airborne systems have proved themselves, that advanced radars have moved up out of the troposphere into space. For example, imagery from the Russian satellite ALMAZ-1 was offered for sale to the international remote sensing community in 1992. Additionally, this year the Canadian RADARSAT will be launched into a sun-synchronous orbit from which it will transmit radar data to commercial and environmental users on Earth. In the meantime, the Japanese and the European Space Commission have each launched satellite-borne radar systems - JERS 1 and ERS 1, respectively - whose imagery is being used in a variety of scientific and environmental applications by a large but limited number of investigators. Eventually, however, this imagery will also be made available to the world-wide remote sensing community. This unparalleled proliferation of high-quality, low-cost, quick-response data can confidently be expected to usher in a period in which side-looking radar imagery will achieve a level of acceptance and use that will rival that of the multispectral sensors carried on the Landsat and SPOT satellites. But there should not be any rivalry here because, for maximum scientific results, the textural information supplied by the radar can and should be used in synergistic combination with the data from the multispectral world. For instance, an investigation conducted for the U.S. Geological Survey by Autometric Inc. demonstrated an unexpectedly large degree of synergy in the combined exploitation of airborne radar and the Landsat Multispectral Scanner. The results of the investigation, which involved the geologic analysis of lineaments related to petroleum and gas exploration, demonstrated both the synergistic and additive contributions of radar when used in conjunction with a multispectral system.

Characteristics of Radar Sensors
| In operation, radar imagery is obtained by transmitting a high-energy microwave pulse to the ground from the radar antenna and electronically processing and recording the reflected signal. The proportion of the signal that is reflected back to the antenna is a function of the slope of the terrain, the "roughness" of the terrain at the centimeter level, and the dielectric constant of the surface materials, including rock, soil, and vegetation. Thus, the radar sensors record a set of data that is entirely different from the data set recorded by electro-optical multispectral sensors.
      One of the most important principal characteristics of the current and prospective satellite radar systems is wavelength, which varies between five and 25 centimeters. Radiation within this range penetrates clouds, enabling the sensors to acquire data regardless of weather. Another important characteristic of the radar sensors is that they are active; i.e., they supply their own illumination, unlike the multispectral sensors, which detect reflected or emitted solar illumination. Being active means that they can acquire data throughout their orbits in either daylight or darkness. It also means that, in the case of ALMAZ-1 and RADARSAT, the incidence angle of illumination can be varied from steep (so as to avoid casting shadows in rugged terrain) to shallow (so as to emphasize shading and thereby detect topographic variance, geologic structure, and stratigraphy in flat terrain). Also of note is the radar's combination of synoptic overview - highly important in most commercial and environmental applications - and high resolution, superior to that of the Thematic Mapper and about equal to that of SPOT.

Applications of the Satellite-Borne Radar Systems
Although there is a wide variety of commercial and environmental applications of satellite-borne radar systems, and although the excellent results obtained from side-looking radar are well documented in the literature, radar remains relatively little used outside of a large and active community of researchers, about half of whom receive their radar data on a reimbursable basis from the NASA/JPL AIRSAR Program. The size and variety of this community of users was revealed in a survey of research papers published between January 1989 and June 1992. During this three-and-a-half-year period, 132 unclassified papers, dealing with 14 categories of SAR research, were published by 219 investigators working in 86 research centers located in 14 countries in North America, South America, Western Europe, and Asia, including Australia, and New Zealand. Since these users are principally in research organizations, they represent only the tip of the iceberg of the potential market of commercial SAR users.
      At the present time, however, these commercial users can obtain their imagery only by contracting with one of several vendors who will mobilize an aircraft and crew to the user's area of interest. This procedure is simply too slow, cumbersome and expensive for most potential users, which highlights the fact that the use of radar has heretofore been limited by the cost and availability of the data, not by the capabilities of the sensor. With satellite-borne radar, however, the cost of launch vehicle, satellite, and sensor have been paid for by a national government or by a government/industry partnership, which expects to recoup costs by selling data to a large population of users at prices commensurate with those charged for Thematic Mapper and SPOT data.
      To a satellite system, all areas on Earth are equally accessable, regardless of distance or political climate, and, with radar, they are accessable at any time, regardless of darkness, weather, or season. Thus, it appears to be inevitable that, with the upcoming launching of RADARSAT and ALMAZ-2, the use of radar data will rapidly increase to a level approximating that of the Thematic Mapper and SPOT. This will be especially so when users come to realize that the combination of radar and multispectral data is nearly always not simply additive but synergistic, and that those who have hitherto been restricted to using multispectral data alone will benefit greatly by integrating radar data into their analyses.

About the Author:
Richard Pascucci is a remote sensing geologist who has spent the past 35 years investigating the applicability of radar and multispectral sensors to the solution of geologic problems. He works for Autometric Inc. in Alexandria, Va.

Back