416 REMOTE SENSING/Active Sensors of the millimetre wave part of the spectrum will add considerably to these capabilities The motion of a plane or a satellite relative to the Earth’s surface can be used to build up a 2D image of the radiation returning from its reflections, or backscatter Such imaging radars are usually pointed to one side of the platform (Figure 1) This removes the ambiguity of signals returning from equal distances to the left and right of a downward pointing instrument Most imaging radars are Synthetic Aperture Radars (SARs) In these instruments, the Doppler frequency information from radar returns received ahead of the moving radar (positive shift) relative to those behind (negative shift), are used to ‘synthesize’ electronically a larger antenna size This is needed to improve the resolving power of the radar in the along-track or azimuth direction The details of radar image formation and processing are complicated, but the resulting images essentially contain data on both the magnitude or amplitude and the phase (f) of the returned signal reflected from the ground Three main factors control these quantities in a radar image: the wavelength of the radar, the slope, orientation and roughness of the reflecting surface, and the dielectric properties of the surface The radar image amplitude is generally greater if the surface slope is close to perpendicular to the radar line-of-sight (LOS), which is often several tens of degrees from the vertical It is also greater if the surface is ‘rough’ and if it is ‘wet’ (water content is the main determinant of dielectric behaviour) Within the instantaneous field of view of the radar there may be several individual surfaces or edges (e.g., rock facets) that return much stronger signals than intervening areas Because the radiation is coherent, these strong signals may reinforce or destroy each other, depending on their relative positions Depending on the arbitrary location of these strong reflectors on the surface then one radar pixel may have a high amplitude whilst its neighbour, comprising very similar surface material, has a low amplitude This gives radar amplitude images their ‘speckle’ character It also means that, in general, a meaningful measure cannot be retrieved of the surface properties from a single pixel of an amplitude image (unlike a passively acquired optical image) Instead a spatially averaged measure of backscatter amplitude is needed A similar conclusion applies to the phase information measured The cumulative sum of phase values from an individual pixel gives a cyclic angular measure (0 < f < 2p) that has no intrinsic significance However, these values are not noise and the radar should retrieve exactly the same values on re-measurement These phase data are used by taking temporal differences of phase in the technique of radar interferometry Radar Applications Structural/Geomorphological Mapping The ability to image large contiguous areas through cloud from airborne radars was first demonstrated in the 1960s It enabled the first hundred kilometre scale reconnaissance structural mapping of cloud covered areas in the humid tropics Slopes nearly perpendicular to the radar LOS appear bright in amplitude images and slopes away from this appear dark This light and shade effect is the same as that seen in optical imagery However, radar amplitude images suffer from distorting effects that complicate qualitative interpretation Radar-facing slopes are foreshortened, whilst those facing away are extended At very steep slopes in alpine-type terrane the foreshortening becomes ‘layover’ when the tops of peaks appear to overhang their valleys Most of these distortions can be corrected quantitatively if a DEM is available Suitably detailed surveys of several areas, particularly young fold and thrust belts in cloudy regions, have proven genuine exploration aids The sensitivity of a radar amplitude image to a fault-controlled topography or any other geomorphological features is always a function of the angular relationship between the radar LOS and the feature on the surface Features parallel to the LOS are hard to see Airborne surveys can be designed with this in mind The erosional morphology of different lithological terrains is also well expressed in radar amplitude imagery, permitting lithological discrimination (Figure 2) This is, of course, scale dependent, particularly where there is tree cover obscuring ground features less than about 100 m in size With the increasing availability of high resolution DEMs for much of the globe then the value of imaging SAR at this reconnaissance scale is being superceded The most remarkable radar mapping survey yet undertaken was that performed by NASA’s Magellan mission to Venus (see Solar System: Venus) from 1990 to 1992 Radar had to be used to penetrate the thick Venusian atmosphere In addition to the huge scope of the global mapping undertaken, a lot of the geomorphological features seen had no obvious terrestrial analogues and hence stretched the interpretational skills of the Magellan science team Roughness Mapping Radar backscatter is sensitive to the roughness of surfaces at length scales much less than the wavelength of the radar For example, the C-band (wavelength ¼ 5.8 cm) radar of the ERS/ENVISAT satellites are sensitive to surface roughness at the scale of millimetres Thus, any changes in roughness in space or time that are relevant to geology can be utilized