How about for check points on an ultra-marathon desert run, where you are crossing the Sahara. Each check point could have an RFID reader that gives feedback on

How about for check points on an ultra-marathon desert run, where you are crossing the Sahara. Each check point could have an RFID reader that gives feedback on the racers. If Bob doesn't get to the next checkpoint in say 2 hours, you know he's probably lost.