Electronic – Detecting absolute position

PIR sensors usually have 2-4 pixels. That, coupled with the intended optics that appear to be designed with "blind spots" (so something moving in/out of them will cause a change) wouldn't really be that helpful.

As tyblu suggests, an IR camera—the cheap ones are NIR, not LWIR (thermal)—may be more helpful, and may even be able to couple it with some FPV (first-person view) goggles for R/C aircraft to make your own hack NVGs.

That said, some FPV goggles and IR cameras can be kinda spendy, so paying just a little more for the convenience (batteries, mechanical package, etc) of some commercial NVGs (or just a scope) might be worth it.

Rather than transmit the marker ID code from the marker to a central receiver, perhaps it would be simpler to transmit the marker ID code from a central transmitter to the marker.

The central transmitter (perhaps a 38 kHz IR transmitter or some wireless transmitter) would send, in effect, "Marker number 22, please turn on for the next 20 ms on my mark: NOW". (Ideally, while that marker is glowing for those 20 ms, the central transmitter is sending out the ID of the next marker to turn on).

Since that one central transmitter is controlling the timing, you won't have to deal with markers getting out-of-sync and accidentally transmitting at the same time.

Hopefully you can place that central transmitter close enough to the position tracker, so that if any marker can't see the commands sent by the central transmitter, that marker wouldn't be in the visual field of the position tracker anyway.

That also gives you the flexibility of using the data you get back from your position tracker to dynamically adapt which markers you select:

• If some markers seem to be motionless or extremely slow-moving, perhaps you only need to check on them once every 3 seconds or so.
• Perhaps you can check up on the latest positions of more than one marker at a time; something like "OK, marker number 22 and 23, please turn on for the next 20 ms on my mark: NOW".
• If some markers are not visible from this vantage point, perhaps you only need to check if any of them have re-entered the visual field once every 3 seconds or so.
• The time slots you free up with the above techniques could be used to track the remaining markers at a somewhat faster update rate than you could if you simply cycle through every marker in a fixed pattern.
• If, say, marker #22 is so close to marker #23 that receiver of #23 is blinded when marker #22 is active, you could shuffle the order you turn on the markers so that the "#23 please turn on" message happens a few slots before the "#22 please turn on" message.