Robot tracking RF transmitter

We have been given the project idea to build a rescue bot that will find a victim trapped in some sort of debris . It is assumed that the victim will have some sort of transmitter ,either RF or ultrasonic.The bot must sense the transmitted signal and find the victim,avoiding obstacles in its path. We are planning to do the obstacle avoidance using ultrasonic sensor. But we are at a dead end on detecting the victim. Which would be better - ultrasonic or RF ? pls help us.thanks in advance for any ideas,suggestions and circuits.

Ultrasonic is better because it has higher near-field strength than RF and it is able to "Wrap" the signal around corners and overcome most direct path obstacles. RF is a bit cheaper, but its broadcast limitations aren't worth the savings for mobile robotics applications.

How are you going to determine direction to the victim? Are you going to implement radio bearing? Or victims transmitter will have GPS on it and can send you it's coordinates?

Now we are doing it this way. An ir led array is modulated by a 4khz signal, which will be used to identify the victim,on a line tracer track for the bot. The bot will look for this signal.
I think it will be difficult to implement the "search for victim" without some sort of track,like searching an entire room, is it not?

The problem is that although IR is directional it is very difiaclt to separate direct IR signal from reflected. You have to measure received IR signal power for taht and compare but it is very complex especially in closed room. I can think fast about any good solution.