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SECTION 8

RADAR AND MICROWAVE REMOTE SENSING*

Radar Defined

Radar is an acronym for Radio Detection and Ranging. Radar operates in part of the microwave region of the electromagnetic spectrum, specifically over the frequency interval from 40000 to 300 megahertz (MHz), the latter extending just into the higher frequency end of the radio (broadcast) region. Commonly used frequencies and their corresponding wavelengths are specified by a band nomenclature, as follows: K-alpha Band: 40000-26000 MHz (0.8 - 1.1 cm); K Band: 26500-18500 MHz (1.1 - 1.7 cm); X Band: 12500-8000 MHz (2.4 - 3.8 cm); C Band: 8000-4000 (3.8 - 7.5 cm); L Band: 2000-1000 MHz (15.0 - 30.0 cm), and P Band: 1000- 300 Mhz (30.0 - 100.0 cm). Unlike other sensors which passively sense radiation from targets illuminated by the Sun or thermal sources, radar generates its own illumination (hence, it is active; another example is the flash camera) as bursts or pulses of energy directed to the target and then sensed as fractions of the energy returned. Thus, a radar system is a ranging device that measures distances as a function of round trip travel times (at light speed) of a directed beam of pulses (the signal, whose strength is measured in decibels, dB) spread out over specific distances. In this way radar establishes the directional location and separation distances from the (either fixed or moving) instrument to a scattering target (at any instant, a field of view corresponding to an area on the ground surface determined by system resolution). Information about target shapes and certain diagnostic physical properties of materials at and just below the surface is also derived from analysis of signal modifications.

By supplying its own illumination, radar can function during both day and night and, for some wavelengths, without significant interference from blocking atmospheric conditions (e.g., clouds). These characteristics prompted development of radar in World War II as a dynamic range finder for tracking aircraft and ships; both ground (fixed) and airborne (mobile) radar systems are used extensively today for marine navigation and air traffic control. Imaging radar mounted on air or space platforms has proven especially useful in mapping cloud-shrouded land surfaces (a landforms map of Panama used this approach); this also permits expression of the surface shapes in regions heavily covered by vegetation ("penetrated" by some bands).

This ability to "mirror" ground surfaces to display topography is a prime use of radar operating on moving platforms. This is strikingly confirmed in the radar image strip (SIR-A system on the Shuttle; see below) shown here that extends 200 miles (300 km) to the northeast (right side) across the folded and dissected South American Andes in Bolivia from the high plains (Altiplano) on the west to the lowlands (Amazon Basin)on the east. (Scroll to see this right end.)

Radar (as well as passive sensors operating at microwave wavelengths straddling or outside those used by radar) is also effective in detecting soil moisture and sea states.

* This unit is adapted and expanded from the section on Radar Systems (pp. 367- 374) in the Landsat Tutorial Workbook, NASA RP 1078, 1982. For an excellent, up-to-date review of Imaging Radar Technology, covering both air and space systems, click on the NASA JPL Radar Page
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Code 935, Goddard Space Flight Center, NASA
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