Bot Chasing Bot

Anyone try to make a bot that can chase another bot?

I was thinking one bot would transmit infrared (a beacon) and the other would receive it. IR remotes work well for 10-20'. While I've only got that too work from 6' using a receiver like a tsop2438, that is for another thread.

The question here is how to find the direction of the transmitter. IR receivers either respond or don't respond. I was hoping a photodiode or IR detector like http://www.sparkfun.com/commerce/product_info.php?products_id=241 could provide an analog value that would indicate signal strength. The sender would broadcast regularly. But when I tried it with a photodiode, it's maximum range was about 6 inches. Another thread on Arduino Forum indicates the Sparkfun IR detector also has a short range.

Any ideas for a beacon-detector pair that would work from 6-20'? Again, the detector needs an analog signal that indicates 'hotter' or 'colder' as the receiver changes direction or gets closer.

I was thinking of modulated IR as they are the easiest to work with. The IR receivers cost about $1. IR LEDs are like $.20.

Pololu has a beacon that uses 4 receivers and 4 LEDs. This makes sense as the $5 in parts is cheaper then a motor to rotate them.

I guess as the bot changes direction slightly the receivers indicate if the chaser is still in pursuit. I don't see how the IR alone can give distance, but maybe distance is not necessary to chase. Could also make the bot kind of pan back and forth when it is hunting for a signal.

I was also reading about the Rhomba IR invisible wall. It acts as an invisible wall to prevent the vacuum from falling down stairs. This could be just an LED in an opaque tube (or snout) to keep the light beam narrow.

This solution does not seem very satisfying, but I guess it could work.