# Tricky Transponders

General Disclaimer: I don’t know what I am writing about. I am an economist not an electical engineer. This should go without saying, but just in case, remember that I don’t know what I am typing about.

Radar is used to determine where airplanes are. One use is air traffic control for civil aviation — the airplanes cooperate. Another is military — hostile aircraft do not cooperate.

First in civil aviation, the system is far removed from the old system of measuring radio waves reflected by the airplane. Instead airplanes have transponders which detect the radar signal and respond with their identifiyier (tail number) location (by GPS ?) altitude and velocity.

I think military aircraft can also use transponders — to deceive adversary radar.

For example, if the transponder sends a signal similar to the passive reflection but with a delay of a microsecond, it will report a position 300 meters further from the radar than the actual position. If the radar sends out pulses at regular intervals (one microsecond pulse every milisecond) then the transponder can mimic a position closer to the radar by anticipating the arrival of the new pulse.

another example, radar uses the doppler effect — the signal reflected by an approaching object is blue shifted and from a distancing object is red shifted. The reflection is fairly weak. A transponder can deceptively report that it is rapidly approaching by sending a blue shifted signal. I think it is possible to send a signal equal to constant minus reflected intensity so that there is no way to determine the speed of approach.

Or a tricky transponder can tell the adversary radar that it is close and rapidly approaching (that tends to get attention).

For old fashioned rotating dish radar, the tricky transponder can deceive also about the direction from the radar to the transponder. Generally radar sends out a beam and determines the direction to the target from when the most intense part of the radio beam hits the target and is reflected. I think a tricky transponder can mesure the amplitude of the incoming radio waves and send a psuedo reflection equal to constant minus actual reflection. I think this makes it impossible to determine the actual direction from radar to target.

all of this is about information processing and sending out a not super strong radio signal. I assume all information processing problems are now trivial — a chip costing a few cents can solve the problem in less than a microsecond.

I think tricky transponders are very cheap compared to (ineffective) stealth technology and might be worth exploring.

Again recall I don’t know what I am typing about.