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Microwave/RADAR Data

Remote sensing for Earth surface information increasingly uses varied components of the electromagnetic spectrum, such as microwave energy in wavelengths between 1 mm (millimeter) and 1 m (meter). Microwave can be passive or active. Passive sensors record the energy received. There is limited passive microwave sensing primarily at coarse spatial resolution for atmospheric studies. Active microwave sends and receives energy and is much more widely used.

Active microwave is RADAR, an acronym for RAdio Detection and Ranging. RADAR systems emit energy, and the returning waves are recorded in intensity and time of travel. The time of travel can be converted to distance from the sensor to the surface and used to determine with considerable accuracy surface distance and elevations by sensors known as Interferometric RADAR (IFSAR or ISAR). RADAR has extensive use in navigation and meteorology, but the focus here is on imaging radar for feature analysis. RADAR advantages over optical sensors include the ability to collect imagery at night, to penetrate clouds, and, under some conditions, to penetrate into vegetation and hyperarid soils.

Operational Characteristics

RADAR images to the side of the platform at a variable angle are called Side Looking Airborne Radar (SLAR or SAR). RADAR uses different specific wavelengths, such as X, L, C, P, or K band. RADAR energy can also be sent and received in different polarizations, vertical (V) or horizontal (H), for four possible images from one wavelength; HH, HV, VH, and VV (quad polarization). A historic RADAR limitation was that the quality of the data was limited by the size of the antenna. This problem was ingeniously resolved by incorporation of the Doppler effect in processing, caused by slight frequency shifts (caused by the movement of the sensor) between sending and receiving energy known as Synthetic Aperture Radar (SAR).

Variations in RADAR return result in high or low image values, bright to dark tones called backscatter. The backscatter will vary based on wavelength and polarization but is generally determined by surface aspect or geometry, dielectric constant, and texture or roughness.

Applications

There are multiple applications of RADAR. Foremost among these are information in cloudy areas, humid tropics and high latitudes, and areas of limited sunlight and high latitudes in winter. SLAR provides a shaded-relief appearance excellent for delineation of geologic and geomorphic features (Figure 1).

RADAR successfully maps snow and ice in lake or ocean flows and monitors shipping lanes for icebergs. Because oil dampens waves, RADAR can locate oil spills and natural seeps. Other applications include urban analysis, deforestation, biomass estimations in a variety of vegetation communities, flood mapping, agriculture, and generalized land cover mapping (Figures 2 and 3).

RADAR has been acquired from aircraft for more than 50 years. It has been collected from space since 1978, but systematic space-borne acquisition has only occurred since about 1995. Recently, space-borne RADAR has been obtained by the Canadian, Russian, and European space agencies and through German and Japanese platforms, among others.

Figure 1 Volcanic structures in Indonesia via RADAR

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Source: Canadian Space Agency.

Figure 2 Sandia Labs RADAR image of rice fields in southeast Asia

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Source: Sandia National Laboratories.

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