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The shuttle imaging radar (SIR-A) acquired images of a variety of the earth's geologic areas covering about 10 million square kilometers. Structural and geomorphic features such as faults, folds, outcrops, and dunes are clearly visible in both tropical and arid regions. The combination of SIR-A and Seasat images provides additional information about the surface physical properties: topography and roughness. Ocean features were also observed, including large internal waves in the Andaman Sea.

On October 5, 1984, the second Shuttle Imaging Radar (SIR-B) was launched into orbit aboard the space shuttle Challenger. SIR-B is part of an evolutionary radar program designed to progressively develop a multifrequency, multipolarization synthetic aperture radar with a variable Earth-imaging geometry. The SIR-B instrument is an upgraded version of SIR-A, with the additional capability of tilting the antenna mechanically to acquire imagery at variable incidence angles ranging from 15°to 60°. The variable look angle capability provided a means of acquiring multiple incidence angle imagery over specific targets on successive days of the mission. These data are being used to classify surface features by their backscatter signatures as a function of incidence angle and for topographic mapping. In addition to the antenna tilt capability, a digital data-handling system was added to increase the dynamic range, the resolution was improved by a factor of two over SIR-A, and a calibration subsystem was added to improve the radiometric accuracy of the data. The mission had a number of problems, including loss of the primary digital data path between the shuttle and the ground. In spite of these problems, approximately 20 percent of the planned digital data were collected over the 8-day shuttle mission corresponding to an areal coverage of about 6.4 million km2. I. INTRODUCTION JUST BEFORE sunrise on October 5, 1984, the second Shuttle Imaging Radar, SIR-B, was launched into orbit aboard Challenger on mission 41-G. SIR-B was an upgraded version of SIR-A, which flew aboard Columbia on its second flight in November 1981. Both SIR-A and SIR-B were L-band (23-cm wavelength) synthetic aperture radars with horizontal (HH)-polarization. SIR-A imaged the Earth at a constant look angle of 470, and data were recorded and processed optically at a resolution of 40 m over a 50-km swath. Improved capabilities added to the SIR-B system included a mechanically tilting antenna, a digital data-handling system, and a doubled bandwidth. The tilting antenna allowed imaging at selectable look angles between 150 and 60°. This new capability provided a means of acquiring multiple-incidence angle imagery over selected targets on successive days of the mission. These data are being used to classify surface features by their backscatter signatures as a function of incidence angles and for stereotopographic mapping.

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The Shuttle Imaging Radar-B experiment consisted of a large number of scientific investigations in the earth sciences. Nine oceanographic experiments were conducted to study the generation and propagation of surface waves, the dynamics of internal waves, oil slick detection, and the properties of southern polar ice. Stereo imaging from space allowed three-dimensional viewing of surface features. Geologic experiments were conducted to study subsurface penetration, structural mapping, and lithologic classification. Imaging radar angular scatterometry was used in the vegetation cover, forest type, and urban areas classification experiments. This article provides an overview of the scientific results, some of which are also presented in this issue.