What are the trade-offs to use a radio relay orbiter also as a radar science instrument?

Question

Any landed mission could benefit from having an orbiting radio relay satellite to help enhance its data rate to Earth. For polar landers on for example the Moon or Mercury it would be a necessity. For a lander on a distant object such as Europa, Uranus or a comet it would be especially helpful, easing the mass, electric power consumption and complexity of the lander somewhat.

What is required to complement such a radio relay orbiter so that it can second also as a powerful radar to study the object it is orbiting? Would the communication frequency be useful also as a science radar, for surface reflection or ground penetration? What are the synergies and problems with combining high data rate interplanetary radio communication with a capable multi frequency science radar? Could every orbiter with a radio dish be used as a science radar instrument too?

EDIT: Oops, I happened to log myself in with an old little used user name. I'm actually LocalFluff.

Answer 1

Cassini did this. Its high-gain antenna was used for both communications and radar.

Some tradeoffs:

1. time: if you want to use the same dish antenna for communications and radar, you need to divide the dish's use between both. Most interplanetary missions store their data and send it at intervals anyway (with 8 hours/day scheduled for communications), so this is not a major problem.

2. time: radar science requires the dish to be pointed at the planet, this could interfere with pointing other instruments. I'm thinking of New Horizons, which had its dish at a 90º angle to most of the instruments.

3. weight: a radar needs a powerful transmitter (more so than communications to Earth), which uses some of your power, weight, and space budgets.

What is required to add a radar option: you need a radar transmitter in addition to the radio transmitter (and ditto for the receivers), and a way to connect them to the dish in turn. With modern electronics you may be able to run one transmitter for both tasks.