Q: How do you "jam" radar? Same modulation or frequency? Higher power?
A: Depends on if you are talking Strawberry or Grape jam….ba-dump-bump
Seriously there are multiple types of jamming, and how it is done depends on what your goals are and what the radar is that you are trying to counter. Also, while this thread has centered on aircraft, because this is an aircraft oriented forum, lots of other platforms use radar, and thus jamming. Typically aircraft do NOT jam by using overwhelming power, aircraft always have a limited power budget compared to systems on the ground or on a ship. The exception here might be a specialized EA platform, such as the EA-6B, EF-111, or EA-18G, they might use high power, still less than the ground based radar they are fighting, but high power compared to other airborne platforms. Yeah, I know, the Raven is not around anymore, but I gotta get it in there anyway.
Airborne jammers, even high powered ones, are lucky if they have a few 10’s of thousands of Watts of transmitter power. Ground based radars typically start at this level and can easily go up to millions of Watts of transmit power. And because of the larger antennas generally found on ground based systems the ERPs are even more skewed.
Airborne jamming can be “stand-off” or “stand-in”, it can also be “self protection”. Typically a stand-off jammer (SOJ) is one who stands off from the target and protects other platforms with its jam energy. The SOJ might never enter the target area and its goals are to protect other platforms that do enter the target area. The stand-in jammer (SIJ) moves into the target area. It can be on one of the attacking platforms or it could be a separate platform that accompanies the strike package. Self protection would, obviously, mean you are using your jam energy to protect your own platform.
Jamming techniques can be loosely grouped in two categories, “noise” or “repeater” techniques.
Noise techniques typically attempt to just overwhelm the radar/s, or the radar operator display/s, with noise transmitted on the radar RF frequency. This noise (modulation) might be random or it might be structured to affect certain known performance parameters of the radar. Just think about trying to carry on a conversation in a very crowded and loud stadium or nightclub with a live band, often the noise is so loud it is hard to hear the person you are talking with next to you. If the noise happens to be at the same audio frequency and speech cadence as the person you are trying to talk with you are really screwed.
Repeater techniques typically target specific radars and are designed to display false, coherent, targets to the radar. This might range from one false target stronger than the real target return in an attempt to make the radar track something that is not there, to many false targets in an attempt to make the radar be unable to tell real from false.
A very old and simple, yet very cool, repeater technique is range gate pull-off (RGPO). To understand this you must understand that once a radar is tracking a target many of them put a “range gate” around the target. This range gate is a way for the radar to ignore all of the other returns except the one that has been designated the target of interest. The range gate is used to derive not only range information, but also to tell the radar this is the point in distance/space/time to sample for angle track data. In other words, it can be the cornerstone of the track when a system is in “auto” track and timing based on the range gate is sometimes called the “track gate”.
In this technique (RGPO) you allow the radar to establish a track on your platform. You then send out a pulse of energy in sync and on frequency to the radar, with zero time delay, so the returned jam pulse sets right on top of your platform from the radars point of view, right on top of the real echo. In essence you are making a beacon, or secondary surveillance radar, out of your jam power. You ramp up the energy in the returned pulse so that the AGC of the radar receiver rises, pushing the real target down to the bottom of the radar receiver floor and the fake target becomes very large in relation. You then start adjusting the timing of the fake pulse, adding (or subtracting, but it is easier to add time) a little bit more time for each pulse you return. Remember, time is distance to a radar. This makes the fake target walk off of the real target in the range domain, “out” range if you are adding time, “in” range if you are subtracting, and since the AGC is self adjusting to the fake target level the real target slides out of the range gate. Once you have walked the fake target off far enough you shut off the jam pulse, leaving the radar range gate looking at a point in time with no target present. Since the angle track data is being derived from the range gate timing the angles are also lost, and drift away.
Jamming is a subset of something called Electronic Counter Measures (ECM). ECM is just anything you do to deny the enemy the use of electronic systems. Naturally, the people who make radars and other electronic systems do not want you to defeat them, so they build in Electronic Counter Counter Measures (ECCM).
Remember what I said above about it being easier to add time to a jam pulse, and I also said adding time makes the return move out in range? An ECCM to this might be to bias the range gate to the “in” range side of the target (the closest side to the radar), instead of having the range gate try to maintain a neutral track around the target make the gate try to move “in” range a little bit between each target echo. This would keep the range gate on the in range part of the target, and when the RGPO walks “out” range it would increase the probability of the system staying on the correct, or real, target. Of course, the problem then is if a jammer subtracts time from the return pulse, instead of adding time, this bias makes the system even more likely to go with the fake pulse.
Why did I say it was easier to add time to a returned jam pulse rather than subtract time? Because your pulses must be in sync with the radar pulses to be affective. If my equipment listens for a radar pulse and then responds by sending out a pulse there is some delay in my equipment, it takes time to grab the pulse, recognize it is what I want, and to key up an identical pulse coming out of the jam transmitter. In so doing I am always behind the received pulse in time, making adding time easy, but subtracting it not so much.
So, instead, your equipment listens for a radar pulse, it then listens for a second radar pulse. The time between the two pulses is the Pulse Repetition Interval (PRI). In a previous post I talked about Pulse Repetition Frequency (PRF). PRI is simply PRF expressed as time, instead of frequency. A PRF of 100 times a second (100 Hz) would have a PRI of 1/100 second, or 10 milliseconds. So, you receive two pulses 10 milliseconds apart to establish the PRI and to start your sync. When the third pulse happens you have a jam pulse ready to go out on top of it. And you can then adjust amplitude and time form there to capture the range gate. And, since I am actually working on future pulses now I can subtract time from the next pulse, walking the jam pulse “in” instead of “out”.
So, the next ECCM a radar might have would be PRI stagger. Make each pulse come out at a slightly different time. Staggers have other advantages also, such as moving the MTI blind speed around.
Anyway, you can see that the game of EW is always in catch up mode. They come out with a technique to counter what you are doing, and you come out with a technique to counter their new technique, etc, etc.
T!
