Remote Sensing - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 220

Fig. 18.18 One of the first
spaceborne radar observations of
dunes. The Algodones Field.
Mountains are seen at right,
while the checkerboard pattern at
left shows the different
reflectivity of different crops,
ploughing and irrigation on
fields. The dunes in this image
(center band of irregular strips—
compare with optical image in
Fig. 7.14 ) are bright due to
topographic shading on a rela-
tively deep (and thus dark) sand
substrate

radar studies (altimetry and scatterometry), and NASA has
flown a number of scatterometers since Seasat—e.g.,
NSCAT and Quickscat—and an NSCAT view of the Earth
showing the major sand seas as 'stealth' areas with poor
radar reflection is shown in Fig. 18.19 .
The spatial resolution achieved by Earth-orbiting SARs
is usually a few tens of meters; that of Magellan was about
100 m, while Cassini achieves about 300 m. The footprint
of scatterometer measurements is often several tens of km.
The first dune observations from airborne SAR as well as
Seasat and the shuttle experiments were reported by Blom
and Elachi (1981, 1984, 1987).
The interpretation of a radar image requires some
familiarity with how radar is reflected from the scene.
Crudely, radar is a radio equivalent of a flash photograph—
the source of illumination and reception of the reflected
energy are co-located. This means that any facets of a
reflective surface that are normal to the illumination will
reflect strongly, whereas more generally, very smooth sur-
faces that are not normal to the illumination will look dark.
For

incidence angles well away from vertical, all else being
equal, radar brightness is often a measure of roughness—
rougher surfaces tending to look brighter (see Fig. 18.20 ). It
should be understood also that here 'roughness' is defined
on a scale comparable with the wavelength of the radar:
thus a surface that is 'smooth' at a long wavelength can be
'rough' at a shorter wavelength, and will look dark in one
image and bright in the other. There is a correlation between
the roughness one might infer from a radar image, and the
'aerodynamic roughness' (see Chap. 3 ) related to the wind
drag on the ground (e.g., Greeley et al. 1997).
For historical reasons, radar wavelengths are often
referred to by bands designated by letters, associated with
the introduction of different kinds of radar in World War 2.
Specifically, most terrestrial radar observations have been at
L-band (23.5 cm: Seasat, SIR-A, B, C, JERS-1) and C-band
(5.8 cm: SIR-C, Radarsat, ERS-1) with a few shorter-
wavelength observations at X-band (*3 cm, XSAR on
SIR-C). Ku-band (2 cm) is often used in nonimaging radars
such as altimeters and scatterometers, but is also used by the
Cassini radar system used to observe Titan. typical scenes of planetary surfaces observed at