Remote Sensing - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 221

Fig. 18.19 A near-global low-resolution radar backscatter (Ku
band—2 cm) map of the Earth's land surface. Ice-covered terrain is
exceptionally bright, while forested areas (Brazil, Equatorial Africa
and S.E. Asia) are somewhat bright and uniform. The darkest areas are

the Sahara, Arabian and Gobi deserts, compare with the optical
appearance of Earth in Fig. 11.2 . Map courtesy David Long, Brigham
Young University

Shorter wavelengths allow more compact antennas to be
used and generally achieve higher spatial resolution. The
Venus atmosphere is in fact thick and absorbing enough that
Ku-band radiation does not penetrate (Cassini did not
receive an echo when it tested its radar during a Venus flyby
en route to Saturn and Titan); for this reason, Magellan at
Venus used a 12.6 cm wavelength. The information from
different wavelengths can be combined as different primary
colors (e.g., Fig. 18.21 ).
Beyond the choice of wavelength, there are other
important aspects (of a radar) that influence the appearance
of surfaces in an image (to the point of making some bright
surfaces dark and vice versa). These include the incidence
angle, and the polarization (e.g., Blumberg 1998). Further,
because the energy returned from a scene depends so much
on the orientation of surface facets at the instant of the
observation, a radar image can often look 'speckled', which

can appear noisy (even though the formal signal-to-noise
can be high). Yet a fraction of a second later, when the
imaging spacecraft has moved on some hundreds of meters
and the geometry has changed, a different (but still speck-
led) image would result. A more interpretable image often
results when many of these instantaneous images ('looks')
are combined together. Thus the number of looks in an
image is often an important quality metric. There are other,
geometric, aspects of radar imaging that can be challenging
to those unfamiliar with them, like 'layover' (the apparent
tilt of mountains towards the observer due to the way the
range to the target is mapped onto the image). Perhaps due
to the complexity of these factors which may seem daunting
to people other than electronics engineers, and perhaps due
to the poorer accessibility of data in the pre-world-wide-
web days when imaging radar first became available, the
full potential for radar in dune studies has not yet been