sonar

Hi everyone,

I am quite of a newbie in electronic but I learn quickly, here is my project:

I've been making plan for an obstacle-avoiding robot, and was originally planning to use a Sharp IR range finder for it (10 - 80cm), but after seeing other sonar range finders, and other people's projects (the majority of the ones I see seem to use sonar), I was wondering:

Which is better: IR or sonar?

'Cos IR recievers can be affected by ambient IR light, and the best i;ve seen goes up to 150cm, whereas I;ve seen sonar ones up to 7.4m, roundabout, and they can't be affected by ambient sound (I think).

3 Friends and I are building an autonomous tennis ball collecting robot for a 4th year design project for our undergrad.

We need a way for the robot to keep track of its position on the tennis court (in doors) and we are thinking of using 2 beacons to emit signals with a receiver on the robot. Has anybody done something like this before? Has anyone heard of pre-packaged solutions that do just this? A USB module would be ideal (we are using a netbook to control the robot), but we are not at all opposed to building the positioning system from scratch.