The primary science goal of the NASA-sponsored ANITA project is measurementof ultra-high energy neutrinos and cosmic rays, observed via radio-frequencysignals resulting from a neutrino- or cosmic ray- interaction with terrestrialmatter (atmospheric or ice molecules, e.g.). Accurate inference of the energiesof these cosmic rays requires understanding the transmission/reflection ofradio wave signals across the ice-air boundary. Satellite-based measurements ofAntarctic surface reflectivity, using a co-located transmitter and receiver,have been performed more-or-less continuously for the last few decades.Satellite-based reflectivity surveys, at frequencies ranging from 2--45 GHz andat near-normal incidence, yield generally consistent reflectivity maps acrossAntarctica. Using the Sun as an RF source, and the ANITA-3 balloon borneradio-frequency antenna array as the RF receiver, we have also measured thesurface reflectivity over the interval 200-1000 MHz, at elevation angles of12-30 degrees, finding agreement with the Fresnel equations within systematicerrors. To probe low incidence angles, inaccessible to the Antarctic Solartechnique and not probed by previous satellite surveys, a novel experimentalapproach ("HiCal-1") was devised. Unlike previous measurements, HiCal-ANITAconstitute a bi-static transmitter-receiver pair separated by hundreds ofkilometers. Data taken with HiCal, between 200--600 MHz shows a significantdeparture from the Fresnel equations, constant with frequency over that band,with the deficit increasing with obliquity of incidence, which we attribute tothe combined effects of possible surface roughness, surface grain effects,radar clutter and/or shadowing of the reflection zone due to Earth curvatureeffects.
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