Wideband Autocorrelation Radiometric Sensing of Microwave Travel Time in Snowpacks and Planetary Ice Layers

Wideband autocorrelation radiometry (wideband AR) offers a deterministic method of remotely sensing microwave travel time τ
s
in planetary surface layers that are quasi-transparent to microwaves. Combining τ
s
with an independent estimate of the layer's average microwave index of refraction
ns
yields a measure of layer thickness whose accuracy is primarily limited by the accuracy of
ns
. The technique requires that four conditions be met: 1) The correlation time of the radiometric signal must be less than the time difference at the radiometer between an upwelling ray that traverses the quasi-transparent layer once and a multiply reflected ray that traverses the quasi-transparent layer three times; 2) interfaces at the top and bottom of the layer must be effectively specular at the frequency of the radiometer; 3) dielectric transitions at the top and bottom of the layer must be distinct; and 4) rays transiting the layer must not be significantly absorbed or scattered. The performance of wideband AR for sensing dry snowpacks is governed by the relationship between system bandwidth and minimum snowpack thicknesses that can be sensed, the microwave indices of refraction of snowpacks and their underlying media, and the integration time required to depress the autocorrelation noise floor below the autocorrelation signal. Findings of this paper are that microwave travel times within dry snowpacks over frozen or thawed soils, or over ice, could be deterministically measured for snowpack thicknesses between 10 cm and 2 m using wideband AR sensors having 10-GHz center frequencies, 1-GHz bandwidths, and 1-ms integration times.