Remote Sensing of Ice with GPS Scattered Signals

The possibility of using the GPS signal scattered off ice is investigated here, as a means of providing additional information over existing remote sensing instruments. The first important thing about ice scattering is that a high incidence of high level, coherent scattering has been detected with conventional altimeters for first year ice. Snow-free sea ice can give rise to such responses because of very smooth surfaces (particularly at L-band). When new ice is forming there tends to be pools of ocean water surrounded by thin sheets of highly saline ice. Due to the sizes of the pools, this water is generally calm, unaffected by winds. This situation would give rise to coherent scattering and we should expect high SNR and the likely ability to track phase with an improvement in resolution. Then we could monitor the extent of new ice formation by mapping SNR levels to concentration of ice versus water. For ice sheets strong coherent scattering is expected where melting occurs since the surface is smooth. GPS could allow us to monitor the seasonal variation of the ice coverage of land, which is currently done with scatterometers. Incidentally, scatterometers monitor the backscattering return from the melting area, which decreases with progressive melting, so the instrument is not very sensitive to the effect it is monitoring. Our instrument monitors forward scattering, which increases with melting and should have good sensitivity to the process. The impact of phase tracking is significant since it will allow for a more accurate determination of height at an increased spatial resolution. Specifically, a coherent return from a "layered medium" representing ice in combination with snow will produce phase variations at the discontinuities between the different layers which will tell us information about the layer thickness and composition. A simple example is the determination of the location of true ice under a layer of snow. Additionally, polarimetric data is very useful for determining variation in material composition of the layers. This paper discusses the relevant features of the GPS signal scattered off surfaces covered with ice, and illustrates under what conditions these measurements are feasible. We examine properties of the GPS coherent component as a function of both the ice thickness and surface statistics and of the elevation angle as well. A comparison with corresponding characteristics for signals in the case of the wind-driven sea surface is discussed. More attention is paid to the polarization behavior of the reflected signal and its dependence on ice parameters.