RFID radar

Retroactively posted April 7, 2005

I learned another new term today, "harmonic radar".

This is a technique that has been used by people doing animal tracking for a little while. Basically, you plant a very lightweight, simple tag on the animal (consisting of a Schottky diode and a particular length of wire, pretty much), and then blast the vicinity with a standard directional radar pulse. The tag picks up the signal passively and reradiates it at a harmonic frequency (doubled, typically), which you can pick up without worrying about confusion from radar backscatter by other objects in view.

The impressive bit is the spec. They have been putting tags on bees and butterflies, for goodness' sake! We're taking milligrams here, with wire lengths of a centimeter or few, in the GHz bands they typically use. Effective range is up to a kilometer, giving you bearing to less than a degree. I haven't found a hard number for typical distance resolution, but I'd wager a few meters is a fairly sane expectation, since they tend to use these to calculate flight speeds and so forth at scanning intervals of a few seconds. Obstructions aren't a problem, and this can even work underground.

The central transceiver is evidently able to be converted from commericial search-and-rescue equipment, on the typical budget of a grant to study butterfly flight patterns, to set the economic scale involved. ;-)

I somehow suspect that running a radar dish in the middle of the Lab would probably provoke some interesting FCC and interference issues, but on the other hand something like this would be way cool for people-tracking on scales ordinarily dominated by per-person costs. With a little effort you could cover the whole campus.

The extreme simplistic tags don't provide any individual identification like RFID tags do, so either you would want to (a) run experiments where only statistical information is needed, (b) engineer a fancier tag on the same principles as normal RFIDs, or (c) track identity by continuous motion monitoring until the person "gives themself away" by logging into a terminal, swiping their ID card somewhere, or staying in their dorm room / office for long enough to say it's probably them.