The ideas that immediately come to mind are those cheap RF transmitter/receiver pairs that you can get on ebay for like $2, but i'm not sure of the range.
There's going to be a lot of these devices on the field, so I don't want them interfering with eachother.
Other options are IR, Bluetooth, and ultrasonic sensors.
I don't think I could get GPS modules cheaply enough, or accurate enough for this.
The second step is getting some idea of how far apart the two devices are.
To get an idea of the distance, the usual method is to send out a pulse at a known speed and see how long it takes to come back.
I don't think the arduino microprocessor is fast enough to do this with RF or IR. Would this be achievable with an ultrasonic transmitter and receiver?
Are there any bluetooth modules that can be queried for signal strength?
I'm probably going to end up building these below cost
How about a combination of RF and ultrasonic? RF will go on and off around 50ft unreliably. Sound will show when you're within 20ft. Less than that you can use the ping (sound) idea?