Abstract
The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of two in a very narrow cone around the direct return direction when multiple scattering occurs in a weakly absorbing, disordered medium. So far, this effect has not been investigated in terrestrial snow in the microwave spectrum. It has also received little attention in scattering models. We present the first characterization of the CBOE in dry snow using ground-based and space-borne bistatic radar systems. For a seasonal snow pack in Ku-band (17.2 GHz), we found backscatter enhancement of 50–60 % (+1.8–2.0 dB) at zero bistatic angle and a peak half-width-at-half-maximum (HWHM) of 0.25°. In X-band (9.65 GHz), we found backscatter enhancement of at least 35 % (+1.3 dB) and an estimated HWHM of 0.12° in the accumulation areas of glaciers in the Jungfrau-Aletsch region, Switzerland. Sampling of the peak shape at different bistatic angles allows estimating the scattering and absorption mean free paths, ΛT and ΛA. In the VV polarization, we obtained ΛT = 0.4 ± 0.1 m and ΛA = 19 ± 12 m at Ku-band, and ΛT = 2.1 ± 0.4 m, ΛA = 21.8 ± 2.7 m at X-band. The HH polarization yielded similar results. The observed backscatter enhancement is thus significant enough to require consideration in backscatter models describing monostatic and bistatic radar experiments. Enhanced backscattering beyond the Earth, on the surface of solar system bodies, has been interpreted as being caused by the presence of water ice. In agreement with this interpretation, our results confirm the presence of the CBOE at X- and Ku-band frequencies in terrestrial snow.
Highlights
The scattering of electromagnetic waves in any type of medium can be used to characterize some of its structural properties
We present the first characterization of the coherent backscatter opposition effect (CBOE) in dry snow using 5 ground-based and space-borne bistatic radar systems
An important effect, the coherent backscatter opposition effect (CBOE), can enhance the radar backscatter return by up to a factor of two. This effect has rarely been considered in descriptions of the backscatter return from snow in monostatic 20 radar experiments, because even though the CBOE is present, its magnitude cannot be quantified without a bistatic reference measurement
Summary
The scattering of electromagnetic waves in any type of medium can be used to characterize some of its structural properties. An important effect, the coherent backscatter opposition effect (CBOE), can enhance the radar backscatter return by up to a factor of two. This effect has rarely been considered in descriptions of the backscatter return from snow in monostatic 20 radar experiments (where the transmitter and the receiver are co-located), because even though the CBOE is present, its magnitude cannot be quantified without a bistatic reference measurement (where the transmitter and the receiver are at separate locations). An opposition effect ( referred to as "opposition peak", "opposition surge", "enhanced backscattering", "hot spot", and 25 similar) is any phenomenon in which electromagnetic (EM) radiation scattered from a particular medium exhibits an increase in intensity in the direct return direction and its vicinity. Opposition effects occur at a variety of wavelengths and scattering media, and are caused by a variety of underlying physical phenomena (Hapke, 2012)
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