  Planetary radars have obtained unique science measurements about solar system
bodies and they have provided orbit determinations allowing spacecraft to be
navigated throughout the solar system. Notable results have been on Venus,
Earth's "twin," and small bodies, which are the constituents of the Sun's
debris disk. Together, these results have served as "ground truth" from the
solar system for studies of extrasolar planets. The Nation's planetary radar
infrastructure, indeed the world's planetary radar infrastructure, is based on
astronomical and deep space telecommunications infrastructure, namely the radar
transmitters at the Arecibo Observatory and the Goldstone Solar System Radar,
part of NASA's Deep Space Network, along with the Green Bank Telescope as a
receiving element. This white paper summarizes the state of this infrastructure
and potential technical developments that should be sustained in order to
enable continued studies of solar system bodies for comparison and contrast
with extrasolar planetary systems. Because the planetary radar observations
leverage existing infrastructure largely developed for other purposes, only
operations and maintenance funding is required, though modest investments could
yield more reliable systems; in the case of the Green Bank Telescope,
additional funding for operations is required.
