2. EXPERIMENTAL RESULTS 
The AIRSAR flight path during the 1991 campaign is shown in Fig. 1. Here, we will limit the 
discussion to the radar scattering characteristics of the soaked-snow facies and of the percolation 
facies. 
In the percolation facies, radar reflectivities at C- (5.6 cm or 5.3 GHz) and L-band (24 cm or 
1.2 GHz) are extremely high (Fig. 2) and significantly lower at P-band (68 cm or 0.4 GHz). Rayleigh 
scattering from small snow grains cannot explain these properties because the backscatter contrast 
between C- and L-band is too small. Similarly, scattering from an extremely rough air-snow or 
snow-ice interface cannot generate such high backscatter values at both C- and L-band. This result 
is confirmed by radar backscatter model simulations using the Integral Equation Method (IEM) as 
well as on numerous radar observations of some of the roughest lava flows of the California Desert 
by the AIRSAR instrument. In addition to high radar reflectivity, the percolation facies preserve 
the helicity of incident polarization as the polarization ratio of echo power in orthogonal senses 
at circular polarization (denoted /r q ) is greater than unity at C- and L-band, but not at P-band 
(Fig. 2). Similarly the linear polarization ratio is unusually high at C- and L-band. 
To our knowledge, no other natural terrestrial surface has such properties. The circular polarization 
ratio is generally much lower than one (it reaches one for tropical rain forest and is close to zero 
for bare soils) and radar reflectivities are several decibel lower. However, similar exotic radar 
properties have been recorded from the icy Galilean satellites for almost 20 years by EArth-based 
radar telescopes. The disk-integrated radar reflectivity and polarization ratio of Europa, Ganymede 
and Callisto are shown in Fig. 2 at 3.5 and 13 cm [5]. Disk-integrated values for Greenland 
are comparable in magnitude and also dwarf the values recorded for terrestrial surfaces, outer 
planets, the moon and asteroids. At the same time, the frequency dependence of these properties 
is much more pronounced for Greenland than for EGC; the radar reflectivity of Greenland is a 
much stronger function of the incidence angle and nc is lower than one at small incidence whereas 
no such dependence has been observed in disk-resolved echo spectra of the icy satellites. These 
differences suggest the distribution in size of the scatterers for the EGC is much broader than that 
for Greenland, and there is a strong specular component in the radar retuns from the Greenland 
percolation facies that is not observed from the EGC. 
Years ago, it was suggested that ice inclusions could explain low emissivities measured for the per 
colation facies by spaceborne microwave radiometers [ 6 ]. Since then, surface-based radio sounding 
experiments, and airborne active and passive microwave measurements [ 7] , have supported the 
hypothesis that volume scattering from subsurface ice layers and ice pipes is the major influence 
on the radar returns. Yet, recent surface-based radar observations conducted at Crawford Point [3] 
at 5.4 and 2.2 cm actually proved that, at incidence angles between 10° and 70°, radar backscat- 
tering takes place in the most recent annual layer of buried ice bodies, which was 1.8 m below the 
surface at the time of imaging (June 1991, early beginning of the summer season), from a previous 
summer melt. Ice layers and ice lenses, a millimeter to a few centimeters thick, extend at least 
several tens of centimeters across, parallel to the firn strata. Ice pipes, several centimeters thick 
and several tens of centimeters long, are vertically extended masses reminiscent of the percolation 
channels that conduct meltwater down through the snow during summer, feeding ice layers. The 
fact that radar returns measured at 68 cm are significantly weaker and have lower polarization 
ratios than those at 5.6 and 24 cm suggests the discrete scatterers responsible for the radar echoes 
are of typical dimension less than a few tens of centimeters, similar to the scales of the solid-ice 
inclusions. The relatively sharp decrease in /ic for 6 less than 40° reveals the presence of a strong, 
specular reflection from the ice layers at small incidence, also suggested by the strong dependence 
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