  From an elevation of ~38 km, the balloon-borne ANtarctic Impulsive Transient
Antenna (ANITA) is designed to detect the up-coming radio frequency (RF) signal
resulting from a sub-surface neutrino-nucleon collision. Although no neutrinos
have been discovered thus far, ANITA is nevertheless the only experiment to
self-trigger on radio frequency emissions from cosmic-ray induced atmospheric
air showers. In the majority of those cases, down-coming RF signals are
observed via their reflection from the Antarctic ice sheet and back up to the
ANITA interferometer. Estimating the energy scale of the incident cosmic rays
therefore requires an estimate of the fractional power reflected at the air-ice
interface. Similarly, inferring the energy of neutrinos interacting in-ice from
observations of the upwards-directed signal refracting out to ANITA also
requires consideration of signal coherence across the interface. By comparing
the direct Solar RF signal intensity measured with ANITA to the
surface-reflected Solar signal intensity, as a function of incident elevation
angle relative to the surface {\Theta}, we estimate the power reflection
coefficients R({\Theta}). We find general consistency between our average
measurements and the values of R({\Theta}) expected from the Fresnel equations,
separately for horizontal- vs. vertical-polarizations.
