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THREE-FREQUENCY, POLARIMETRIC, AIRBORNE SAR OBSERVATIONS 
OF THE GREENLAND ICE SHEET 
E. Rignot, J. J. van Zyl, K. Jezekf. 
Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA 91109 
fOhio State University, Byrd Polar Research Center, Columbus, OH 43210 
ABSTRACT: Synthetic-aperture radar images of the Greenland Ice Sheet collected by an airborne 
system clearly reveal the four melting facies of the Greenland Ice Sheet defined 30 years ago from 
snow stratigraphy studies by glaciologists. In particular, the radar echoes from the percolation facies 
have radiometric and polarimetric characteristics that are unique among terrestrial surfaces, but 
that resemble the exotic radar echoes recorded from the icy Galilean satellites. There, the radar 
signals interact with subsurface, massive, ice features created in the cold, dry snow by seasonal 
melting and refreezing events. The subsurface features act as efficient reflectors of the incident 
radiation most likely via internal reflections. In the soaked-snow facies, the radar reflectivity is 
much lower because radar signals are attenuated by the wetter snow before they can interact with 
subsurface structures. Inversion algorithms to derive geophysical information from the SAR data 
are developed in both cases to estimate snow wetness in the soaked-snow facies and the mass of 
ice-water retained in the percolation facies. 
KEY WORDS: Synthetic-aperture radar, glaciology, polarimetry, inversion. 
1- INTRODUCTION 
In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR) 
instrument [1] collected the first calibrated, three-frequency, polarimetric, radar observations of the 
Greenland ice sheet [2]. At the same time, ground teams recorded the snow and firn (old snow) 
stratigraphy, grain size, density and temperature [3] at ice camps in three of the four snow facies 
identified by glaciologists to characterize four different degrees of summer melting of the Greenland 
ice sheet [4]. The four snow facies are: 1) the dry-snow facies, at high elevation, where melting 
rarely occurs; 2) the percolation facies, where summer melting generates water that percolates 
down through the cold, porous, dry snow and then refreezes in place to form massive layers and 
pipes of solid ice; 3) the soaked-snow facies where melting saturates the snow with liquid water and 
forms standing lakes; and 4) the ablation facies, at the lowest elevations, where melting is vigorous 
enough to remove the seasonal snow cover and ablate the glacier ice (Fig. 1). 
Mapping the spatial extent and temporal variability of these different snow facies repeatedly by 
using remote sensing techniques is of interest to climate change studies in the polar regions. Passive 
microwave sensors cannot separate the different snow facies due to their poor spatial resolution; 
while optical sensors are shrouded by clouds and unsensitive to differences in subsurface configura 
tions between the dry-snow facies and the percolation facies. 
The objectives of the 1991 experiment were to study quantitatively the radar scattering properties 
of the four snow facies of the Greenland Ice Sheet and establish a relationship between radar 
echoes and snow physical and electrical properties. Based on the results, inversion techniques are 
developed to extract geophysical information from the airborne SAR data, in anticipation for more 
extensive applications using spaceborne SAR data collected by Earth-orbiting platforms.