433 
of radar reflectivity on incidence angle. Ice layers and pipes also form in the soaked zone, but 
the snow there is so saturated with liquid water that the radar signals are strongly attenuated, 
cannot interact with the buried ice formations, and hence yield echoes with low reflectivities and 
polarization ratios close to zero. In the dry-snow zone, the snow is dry, cold, porous, clean, and 
therefore very transparent at microwave frequencies, but does not contain solid-ice scatterers that 
could interact with the radar signals. 
For the satellites, no in-situ measurements exist, but theoretical interpretations favor subsurface 
coherent volume scattering as the source of the radar signatures [8], a phenomenon also known 
as weak localization [9 ]. Coherent backscattering can theoretically produce strong echoes with 
[i c > 1 provided (i) the scattering heterogeneities are comparable to or larger than the wavelength 
[10], and (ii) the relative refractive index of the discrete, wavelength-sized scatterers is smaller 
than about 1.6 [11]. For EGC, prolonged impact cratering of the satellites probably has led to the 
development of regoliths similar in structure and particle-size distribution to the lunar regolith, 
but the high radar transparency of ice compared with that of silicates permits longer photon path 
lengths, and higher-order scattering ( 12 ]. Hence coherent backscatter can dominate the echoes from 
EGC, but contributes negligibly to lunar echoes. Similarly, the upper few meters of the Greenland 
percolation zone are relatively transparent (unlike the soaked zone) and, unlike the dry-snow zone, 
contain an abundance of solid-ice scatterers at least as large as the radar wavelength, with a relative 
refractive index of about 1.3, so coherent backscatter also can dominate the echoes there. However, 
although similar scattering mechanisms may take place, the detailed subsurface configurations of the 
satellite regoliths, where heterogeneities are the product of meteoroid bombardment, are unlikely 
to resemble that within the Greenland percolation zone, where heterogeneities are the product of 
seasonal melting and freezing. 
Coherent backscatter is however not the only possible explanation for the unusual radar echoes from 
Greenland. Using an exact solution for the scattered field from discrete, dielectric, cylinders, and 
representing the ice pipes and ice layers by vertically oriented and horizontally oriended cylinders, 
respectively, we have been able to match the radar observations at all frequencies, polarizations, 
and incidence angles. Large values of fic and of radar reflectivity result from internal reflections in 
the cylinders (glory rays). In the coherent backscatter theory, large reflectivities and polarization 
ratios results from constructive interferences between rays travelling along time-reversed paths. In 
this new model, the explanation for the radar echoes is simpler and also is more consistent with 
the sub-surface configuration of the ice sheet. Also, the cylinder model can be used to estimate the 
size (diameter) and number density of those objects. The values obtained (see poster session) are 
consistent with those recorded at the time of the SAR overflight near Crawford Point. 
The radar echoes from the soaked-snow facies do not show exotic behavior, and in particular are 
characterized by pronounced spatial variations correlated with variations in snow albedo as seen 
from on-board video camera equipment; and anticorrelated with spatial variations in snow water 
content induced by melt-water re-distribution on a meter-scale topography. 
For wet snow, it is expected that surface scattering dominates the radar returns from the ice sheet 
due to the strong attenuation of the radar signals by the wet snow layer. We therefore developed an 
inversion technique for estimating the dielectric constant and surface roughness (at the wavelength 
scale) of a bare surface radar-equivalent to the snow surface. To estimate these two unknowns, we 
use the radar reflectivity at two different polarizations at a wavelength large enough compared to 
the roughness scale of the surface to minimize its effect on the retrieval of the dielectric constant. 
The results (see in the poster session) indicate that the inversion technique performs well, and 
significantly better than the small perturbation theory that ignores the role of surface roughness in