I have been asked by a airsoft game operator if it is possible to create an "Echo Detector" for a Stalker, Shadow of Chernobyl scenario "cheaply". An echo detector is a device that alerts the player when an "anomaly" is nearby, and beeps more quickly as the player gets closer. So I will need to create both an "anomaly" device and a detector device. The longest range needed is approximately 50 feet (I think, still waiting on this information).
Since I havn't received any specific information about the prop, i'm going to break this into two steps. The first step is just to get the Arduino to detect when it is within about 50 feet of a device. 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. The accuracy can be fairly low. If it could just tell the difference between max distance, 2/3 max distance, and 1/3 max distance, that would be good. 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? I could use the RF pair to sync the time on the two devices, or a realtime clock if that would be accurate enough. Are there any bluetooth modules that can be queried for signal strength?
I'm probably going to end up building these below cost, so any help is appreciated.