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CHAPTER 40

Planetary Radar

Steven J. Ostro

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

1. Introduction 4. Prospects for Planetary Radar


2. Techniques and Instrumentation Bibliography


3. Radar Measurements and Target Properties

P

lanetary radar astronomy is the study of solar system
entities (the Moon, asteroids, and comets, as well as
the major planets and their satellites and ring systems)
by transmitting a radio signal toward the target and then
receiving and analyzing the echo. This field of research
has primarily involved observations with Earth-based radar
telescopes, but it also includes certain experiments with
the transmitter and/or the receiver onboard a spacecraft or-
biting or passing near a planetary object. However, radar
studies of Earth’s surface, atmosphere, or ionosphere from
spacecraft, aircraft, or the ground are not considered part
of planetary radar astronomy. Radar studies of the Sun in-
volve such distinctly individual methodologies and physical
considerations that solar radar astronomy is considered a
field separate from planetary radar astronomy.

1. Introduction

1.1 Scientific Context

Planetary radar astronomy is a field of science at the inter-
section of planetology, radio astronomy, and radar engineer-
ing. A radar telescope is a radio telescope equipped with a
high-power radio transmitter and specialized electronic in-
strumentation designed to link transmitter, receiver, data
acquisition, and telescope-pointing components together
in an integrated radar system. The principles underlying

operation of this system are not fundamentally very dif-

ferent from those involved in radars used, for example, in

marine and aircraft navigation, measurement of automo-

bile speeds, and satellite surveillance. However, planetary

radars must detect echoes from targets at interplanetary

distances (∼ 105 –10^9 km) and therefore are the largest and

most powerful radar systems in existence.

The advantages of radar observations in astronomy stem

from the high degree of control exercised by the observer

on the transmitted signal used to illuminate the target.

Whereas virtually every other astronomical technique re-

lies on passive measurement of reflected sunlight or natu-

rally emitted radiation, the radar astronomer controls all the

properties of the illumination, including its intensity, direc-

tion, polarization, and time/frequency structure. The prop-

erties of the transmitted waveform are selected to achieve

particular scientific objectives. By comparing the properties

of the echo to the very well known properties of the trans-

mission, some of the target’s properties can be deduced.

Hence, the observer is intimately involved in an active as-

tronomical observation and, in a very real sense, performs

a controlled laboratory experiment on the planetary target.

Radar delay–Doppler and interferometric techniques

can spatially resolve a target whose angular extent is dwarfed

by the antenna’s beamwidth (that is, its diffraction-limited

angular resolution), thereby bestowing a considerable ad-

vantage on radar over optical techniques in the study

of asteroids, which appear like “point sources” through

Encyclopedia of the Solar System 2e©C2007 by Academic Press. All rights of reproduction in any form reserved. 735