What is a planetary radar?
Planetary radar is a radio telescope with a built-in radar transmitter capable of observing planetary targets such as asteroids, comets, planets and their moons. The transmitter emits microwave signals that reflect from the radio telescope's dish towards the target and back the same path from the surface of the target. Measuring the changes to the well-known transmitted signal allows the observer to derive information on the target. The animation below shows the path of the signal, simultaneously demonstrating that only small part of all the initial signal power is received. The signal weakens as a function of the distance squared, which is why the signal has to be extremely powerful, the collecting area of the dish as large as possible, and yet, the quality of the radar measurement depends strongly on the distance of the target.
The word radar is originally an acronym for RAdio Detection And Ranging. Planetary radar is not used as a survey telescope to discover new asteroids, but to post-characterize them after the discovery. The most common measurement is to detect the distance (range) to the target as well as its radial velocity. The delay of the signal to the target and back reveals the distance, and the change in the wavelength (or frequency) through doppler effect gives the radial velocity or indications of the rotation of the asteroid. The doppler effect is illustrated in the pictures below in these two different cases. Note that the velocity of the signal is substantially slowed down for the animation (in comparison to the rate of movement of the asteroid) in order to clarify the change in the wavelength. The wavelength of the transmitted signal is centimeter-scale depending on the band the transmitter uses. In Arecibo, 12.6 cm (2380 MHz) is the default wavelength, whereas Goldstone uses 3.6-cm waves (8650 MHz).
Another characteristic of the radar signal is its polarization, that is, the orientation of the vibrations. The changes in the signal's polarization give indications on the surface properties of the target. In modern radar observations the radar signal is usually circularly polarized although a linearly polarized signal can be also used. The circular polarization is mainly used to prevent the rotating effects of the ionosphere's charged particles to the polarization. When the signal hits the target's surface, the handedness of the circular polarization may change as shown in the animation below. If the surface is smooth, the handedness fully changes. If there are wavelength-scale boulders or craters on the surface, part of the signal remains in the same polarization. The ratio of the echo power in the two different states, the circular polarization ratio, is thus used as an indication of the roughness of the surface. Also the material of the surface may have an effect on how well the polarization remains through the scattering process.
In addition to the polarization, also the relative brightness of the echo reveals information on the target. This characteristic is called the radar albedo. Metallic asteroids and icy moons tend to look brighter in the radar images than rocky asteroids. Also, more solid surfaces reflect the signal brighter than very porous surfaces. Therefore, the radar albedo can be used to give indications on the abundance of metal/ice or the surface density of the target.
All animations above have been programmed by myself using python.