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.
In my misspent youth USAF in its supreme wisdom assigned me to a remote US radar station. We were second line of observation against attacking aircraft, late1970’s.
We contended with simulation of enemy counter measures we called jammers more modern would be termed counter measures. The idea was to fuzz our radar, we had the single horn and antenna rotating kind.
Transponders on civil aircraft are the central asset for en route air traffic control. Later in my work life I was exposed to FAA processes. Radar control in USA done by airport surveillance radars, connected with one or more local airports.
Enroute control is done with aircraft squalk gps/inertia nav position and altitude. The ATC computer has the flight plan, as long as the aircraft report equals plan everything OK. If squalk don’t equal plan a flash appears on a scope, and a human calls on radio.
Large, heavy military aircraft, especially transports, have the navigation gear to fly under beacon control.
There is no space in fighters, nor do they want anyone to know, fighters do not have full beacon squalk. They do have minimal gear to tell friendlies not to shoot, but that is not open to public.
The South Carolina F-35 was not squalking.
Jammers have grown in variety and complexity since I-was near a radar.
paddy:
Were you on what they called the DEW line or what is now (93) the North Warning System (NWS)?
Military transponders, called IFF (identify friend or foe) have nothing to do with radar spoofing. Active spoofing (besides URN, unsynchronized random noise) is tailored to the previously investigated specific (usually pulse/doppler) radar that is being evaded. Typically, pulse samples at the radar frequency and repetition rate will spoof the radar’s direction determination because they are much stronger than reflections. A cat and mouse game has over the years has radars putting range gates and frequency gates on reception in order to ignore spoofing. But, with Range Gate Pull-off and Frequency Gate pull-off, the spoofer will start transmitting inside the gates, transponding in coincidence with the reflection. Then, after gradually pulling the gates away, it will apply the pulse timing delays (or anticipations) that will spoof direction.
I got the impression that aircraft started having transponders in the late 1960s, but there was recently a big upgrade called ADS-B which gives a lot more information to the controllers. Then again, it’s still all radio. I was flying in a small plane in Canadian air space when the controller contacted the pilot and asked him to make a turn and change elevation because they were getting a bad signal from his ADS-B transponder. We complied and got a nice polite Canadian thank you when the turn and elevation change gave them a better signal.
The electronic warfare thing was already active in the 1960s, but by the 1980s military aircraft had lots of electronics to screw up targeting. These were met with ground based countermeasures and so on. There has been something like this going on with drones. They were a real menace before ground based units started jamming and spoofing their control communications. I’m sure everyone is a few generations into this, though Elon Musk took it to a new level when his boss in Moscow gave him an order.
Kaleberg:
Were you in the US Air Force. You appear to have some knowledge like Paddy (above).
Is this about that Marines F35 that went down, with the pilot ejecting using his ’emergency parachute’ & found slightly injured. (Why didn’t he use the regular one?) The wreckage has been found.
These planes are designed to be stealthy, Apparently stealth-mode was turned on when he bailed out.