A Precision Planetary Range-Tracking Radar

A closed-loop range-locked radar system developed by the Jet Propulsion Laboratory has recently had great success range tracking the planet Venus. It has provided measurements to the planetary mean-tracking point with peak minute-to-minute variations less than 2.25 to 3 Km in range. Over a one-hour tracking period, a mean tracking point can be determined to 0.5 km. A scattering-law calibration of the planet is made each day, measuring the mean-tracking-point-to-planetary-surface distance to within 3 km (nominal). The subearth point-to-radar distance is thus measured to a nominal accuracy of 3.5 km. Tracking behaves as a first-order linear ``range-locked´´ loop with ephemeris aid, and is practically calibration free. Data obtained during the 1964 conjunction showed that the ephemeris not only contained a range error, but also a range-rate error of 18 km per day. Deviations from this rate correspond to surface features whose height can be estimated. Such data will be invaluable in determining, to a greater degree of accuracy than ever before attainable, the orbital constants of the earth and Venus.