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The Apollo Lunar Sounder Equipment (ALSE) was the first geoscience 
imaging radar designed for operation from spacecraft. It was a syn- 
thetic aperture system which operated 5, 15, and 150 MHz (60, 20, and 
2 m) and produced subsurface profiles as well as imagery of the lunar 
surface. 
A very significant advance in the use of microwave systems from 
space will be the Seasat Program which is expected to have a sensor 
package of four microwave systems.  Seasat-A is planned for an ocean- 
ographic mission; imaging radar will record global wave spectra, ocean 
ice, and coastal features. Some imagery may be obtained over land for 
research. The synthetic aperture, L-band (22.2 cm) radar system will 
image a 90-km swath with resolution of approximately 25 m. Intended 
launch date is the second quarter, 1978. 
The availability of side-looking radar systems to supply data for 
earth resources studies in Europe has been limited. However, the Royal 
Radar Establishment (RRE) in Malvern, England, and several Soviet 
agencies have acquired imagery for many years. The RRE flies the P391 
radar system built by EMI Ltd. This is a real aperture, Ka4-band 
(0.86 cm) device that images a maximum ground swath of 28 km. The 
Dutch have used the RRE system and have cooperated in a Swedish ice- 
surveillance experiment (Eckhart and Geerders, 1975). The U.S.S.R. has 
published reports of the use of the Toros side-scanning radar for 
geologic research (Komarov and others, 1973, 1974; Komarov and Starosin, 
1975). The system is a real aperture, K,-band (approximately 2 cm) 
imager with resolution in the order of 40-50 m. 
Platform status 
  
Side-looking airborne radar systems operate from aircraft at 
different altitudes depending upon design and mission. Most radars work 
most efficiently at jet altitudes, but flights at lower levels sometimes 
aid in revealing subtle terrain features. Doppler and/or inertial navi- 
gation systems onboard survey aircraft are desirable to improve the 
geometric fidelity of the radar imagery.  Seasat-A will have the first 
United States radar to operate from Earth orbit. 
Interpretation status 
  
Side-looking radar imagery is usually studied by conventional 
photointerpretation techniques.  Semi-controlled mosaics for multipurpose 
mapping compiled for Projecto RADAM (1973) are accurate to within + 200- 
300 m when tied to adequate ground control (Roessel and deGodoy, 1974). 
Multispectral possibilities may be provided by proposed Microwave 
Hologram Radar (Larson and others, 1974) which will provide the same 
field of view as photographic and infrared sensors. A more immediate 
interpretive aid is the combination of radar imagery with two bands of 
Landsat imagery to provide a color composite with the best features of 
both. 
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