Retrotechtacular: Radar Jamming

It’s been said that the best defense is a good offense. When aloft and en route to deliver a harmful payload to the enemy, the best defense is to plan your approach and your exit carefully, and to interfere with their methods of detection. If they can’t find you, they can’t shoot you.

As of May 1962, the United States military was using three major classifications of radar jamming technology as described in this week’s film: the AN/ALQ-35 multiple target repeater, the AN/ALQ-55 communications link disrupter, and the AN/ALQ-41 and -51 track breakers. The most important role of these pieces of equipment is to buy time, a precious resource in all kinds of warfare.

The AN/ALQ-35 target repeater consists of a tuner, pulse generator, transmitter, and control panel working in concert to display multiple false positives on the enemy’s PPI scopes. The unit receives the incoming enemy pulse, amplifies it greatly, repeats it, and sends them back with random delays.

The AN/ALQ-55 comm disrupter operates in the 100-210MHz band. It distinguishes the threatening enemy communication bands from those of beacons and civilians, evaluates them, and jams them with a signal that’s non-continuous, which helps avoid detection.

Finally, the AN/ALQ-41 and -51 track breakers are designed to break enemy lock-on and to give false information. It provides simultaneous protection against pulse ranging, FM-CW, conical, and monopulse radar in different ways, based on each method’s angle and range.

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

23 thoughts on “Retrotechtacular: Radar Jamming

    1. I’m pretty sure police radar uses the Doppler effect. For the most primitive implementation, this is essentially an oscillator hooked up to a transmitter for the “gun”. The receiver filters the signal reflected off your car through a Schmitt trigger and the resulting square wave is fed to a frequency counter.

      The radar jamming in the video worked against echo location, not changes in frequency.

      TL;DR I don’t think so.

      1. Yes it can. just transmit frequency of the radar and send a pulse rate equal to 20mph less than your current speedometer reading.

        This is exactly how old school police radar jammers work. Problem is more and more are using the laser, and short driving a faceted mirrored car with no flat or retroreflective surfaces there is no way around that.

    2. Bad idea. I don’t know how common it is for police to actually do so but at least some of those radar guns have a headphone jack. The officer can actually listen to the return signal. What does it sound like when you jamming signal is mixed in? I don’t know but I’m sure it doesn’t sound the same as a slower driver without a jammer. He’s coming after you!

      1. He may find the signal strange, but unless the radar gun has clear lock on your speed, he would have no justification for going after you. Even if he can clearly hear the higher pitch of a speeding vehicle behind your false signal, if the radar gun is only reading the slower signal he would thus have no evidence beyond having heard something odd. The whole point of radar guns was for police to have a reliable way to collect evidence of a vehicular crime, and that’s why these things are calibrated periodically by certified technicians. If that radar gun was “giving off strange noises” while he read your speed, that alone would probably be enough to invalidate the reading as evidence of your actions. Now that being said, all he would need to do to over come that problem is point the radar gun at a nearby car that is going the speed limit and he could use that as evidence of you speeding. Not an exact piece if evidence, but better than admitting that his gun may have been malfunctioning.

        Of course all of this is glossing over the fact that it is quite illegal to “jam” any frequency, and then its also even more illegal to do so from a moving vehicle. It’d be cheaper to just pay the speeding ticket really.

        1. Plus the fact that all they have to do when employing your pace car method is write the ticket for how fast they think you were going and instead of writing ‘Radar’ on the ticket, write ‘Pace’. I got a radar detector a few years ago (best $30 I’ve spent on my car) and I’ve gotten two tickets since then: one being paced by an unmarked car and one from a motorcycle cop with Lidar.

    3. Jamming police radar is ILLEGAL, as in the FCC says you can’t harmfully interfere with radio transmissions, along with a slew of state laws. Jamming IR based LIDAR guns is another story, not federally illegal, but some states do have laws against in on the books. But LIDAR jamming is a “thing” for sure, and even some LIDAR makers have gone to great lengths to defeat jammers.

      ALSO – Police radar is CW Doppler radar, CW meaning “constant wave” – there are NO PULSES! NO TIME OF FLIGHT! It simply transmits a steady frequency, say 10.5Ghz X-band 24.05 – 24.25GHz K band and around 35Ghz for Ka band, depending on the maker of the radar unit.

      In CW Doppler, you transmit a constant frequency, and in the case of police radar it is most commonly from a gunn effect diode, which is the oscillator and transmitter all in one. The only “L.O.” would be for receiving and extracting the Doppler frequency. You focus that energy with a horn antenna that has dielectric lens (you’ll see these on the dashboard, with a white-ish clear or dark colored lens pointing down the road) and that signal hits a target, your speeding car, and your car reflects (actually, re-radiates) a portion of that energy back to the police radar’s antenna. Since your car is moving towards or away from the police radar, the original frequency is shifted up or down respectively depending on if you are closing or receding on the radar, that is the “Doppler shift”, which is directly related to your speed in relation to the radar, the counting unit does a little simple math and produces your velocity, or speed on the display.

      And yes ALL police radar units MUST produce an audio tone, which is in fact the Doppler shift in audio. A lot of cops turn it off because it’s an annoying whine. But some like it because they have learned to tell your speed just from the pitch, so they don’t have to keep looking at the display. They have squelch settings so the thing is silent until it starts getting a return signal off of a target. Or you can turn the squelch off and listen to the noise floor static, if you’re into self punishment.

      There are stationary mode radars which are pretty simple, but the cop must be sitting still for it to work. Most today include a means to measure the cruisers speed, via an odometer input signal as well as reading the Doppler shift of the cruisers motion in the RF return as well. It just simply subtracts the cruiser speed out of the equation to capture only your moving speed. With DSP, they can measure more than one speed, or pick the faster speed if more than one target is being tracked. it’s still up to Johnny Law to decide which car is “visually speeding”.

      CW Doppler speed enforcement radar is pretty dumb actually, it doesn’t need to be phase coherent or do much beyond transmit a steady frequency and receive a shifted frequency. Most of them were as simple as a transistor radio for years, with no DSP or anything fancy beyond some counter IC’s and LED displays. They started out by a guy that made guitar amplifiers (Kustom) and he likened them as being more audio processors than “radar”.

      Pulsed mode radar is not used for speed enforcement, because range is not important. You know you have a target because they only work via line of sight and most cars tend to travel in pretty much a straight line and on the ground, they aren’t fighter aircraft. The expense and complication of pulsed mode radar wouldn’t be cost effective or very useful for that kind of purpose.

      Another fact about police radar and lidar is that you have to be moving towards or away from the transmitter for it to work, or cosign error will start subtracting from your measured speed. So if the radar antenna is pointing across a road at a right angle to traffic, it won’t measure much of anything. it has to be pointing down the road, either at oncoming or receding traffic to work correctly. If a cop is sitting at a right angle to the road in the median or on the side, he either has got his antenna turned to point down the road, or he is aiming with a hand held unit.

      Lidar is more or less like pulsed radar, except it uses an IR laser and it actually uses “time of flight” of the laser pulse to measure range (distance). It fires pulses of IR laser energy at a target, and starts a timer at the exact same time as the leading edge of each pulse – the laser pulse strikes the target and is reflected (actual optical reflection) back to an avalanche photodiode detector, at which time the timer is stopped. That “time of flight” is used to determine the distance to the target, and if you repeat that process hundreds of times a second, if the target is moving towards or away from the transmitter, the distance will change at a rate directly related to the velocity. Your Speed…

      It’s much easier to do this with a laser and photodetector, because you don’t have to worry about noise as much, there is some, but optical filters are trivial and the coherency of laser light is much more stable than pulsed RF, and dealing with spurious emissions and all of the things that make pulsed radar more complex and expensive.

      Lidar has been pretty easy to jam in the past, starting with simple brute force IR LED based jammers which just blinded the detector and prevented it from picking out return pulses. Then came adding pulses to the jamming LEDs to barrage the detector with false returns, again blinding it to the real returns. Now there are systems that use laser diodes and lookup tables to send precise returns to fool the lidar units into thinking there are no moving targets. If the lidar unit keeps getting pulses back that are not shifting in timing, then whatever is returning a signal must not be moving…. right?

      Well, now some companies, one really, are invested in thwarting the jammers by using ECCM adopted from the defense world and clutter reduction techniques. So it’s becoming a cat and mouse game of sorts. highway electronic warfare!

      And yeah, I work in radar for a living. However; not police radar, but I do know just about everything about it as well as owning a couple of different speed radar units and Lidars as novelty related to my job, which is also my hobby, which is also my job ;)

  1. Film from 53 years ago. Today, electronic warfare at least ten times more complex, ten times smaller, and yes – ten times more expensive. But with all that advancement, do we only need one tenth as many of them? Watch the film two or three times then think long and hard about it – our enemies are. At-ease.

    1. The potential enemy also has smaller and better electronic military technology, we sold it to them after all.
      And no matter how modern the weapon, we still can’t contain the hordes, in fact the weapons get more precise and people multiply, so it becomes harder to contain a large number of enemies even when armed relatively primitively.
      And so the sheer population growth alone means you need more weapons if you decide to engage in war.

      As for electronics being ten times smaller, I think you mean 100 times at least?

      And last but not least, why bring up modern times?

    2. Did the title imply anything to you? In particular that ‘retro’ prefix could indicate to you that this is indeed spectacular tech from the past. If you had seen any other editions, you would have noticed that every episode features the same kind of instruction movie made by the US armed forces, usually during or just after WW2.
      And in perspective, almost every other tech from that era has been superseded or made obsolete by more modern tech, there is nothing special about this particular field.
      Your hostility towards ‘your enemies’ is pathetic, even if you meant all this ‘ironically’.

    3. Heh!

      “Today, electronic warfare at least ten times more complex, ten times smaller, and yes – ten times more expensive.”

      You got the last part right, maybe….

  2. Many moons ago I worked in ECM and quite familiar with passive as well as active ECM. The ALQ-55 used three Y-180 Vacume tubes in the final. I had never seen these valves used any where else. For some reason I find them on eBay quite often! Oh, and whilst they say that it can distinguish between civilian communication and military…Not all the time! Ask Cherry Point MCAS about the East Coast black out 1967.

    1. I was one of the techs in the ecm shop that year at j2 trained on alq 55,51, 41 and alq 76 pods and ale 32 chaff dispenser. Then went to j1 DaNang. The EA6A was great and USMC paved the way for modern electronic warfare .

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