From: mcgp1!flak@nwnexus.wa.com (Dan Flak)
Subject: MIJI
Organization: NCR Corporation - Law Department
Date: Tue, 25 Aug 1992 14:44:33 GMT


From mcgp1!flak@nwnexus.wa.com (Dan Flak)

wsun@undergrad.math.waterloo.edu said:

> Hi, could any one out there briefly explain to me how does radar jammer
> function? 

Let's talk about MIJI (Meaconing, Intrusion, Jamming and
Interference) which is a much broader concept.

Meaconing is the deliberate "spoofing" of a navigation aid such
as a TACAN or VOR. It's intention is to confuse the crew by
providing false bearings, distances, identifiers, etc. In
other words, the "meaconers" hope that the aircraft will get lost,
can't find its way to the target, inadvertently overflies where
it shouldn't, etc. (or to just plain old harrass).

Intrusion is putting misinformation into a voice circuit. (Sort
of like yelling "fire" in a theater when there isn't any).
Confustion is the aim again.

Jamming is an attempt to block either a navigation signal or a
voice signal. It can be unsophisticated such as broadcasting a tone
on a specific frquency or subtle such a rebroadcasting signals.

Interference is the unintentional broadcasting of an undesired 
signal which degrades the reception of the desired signal.
Friendliesas well as enemies can be part of the interference
problem.

> The book I read says that when the jammer is on, the enemy
> radar is unable to locate the friendly jets, therefore can't lock on
> any target. But why can't they just locate the source of the jamming
> signal and lauch a missile at the jammer, would that be possible?

There are missles designed to home in on radar antennas, but they
are mainly concerned with destroying fire control radars
associated with anti-aircraft batteries. There are ways to 
electronically counter jamming.

First of all, you can fight power with power. "He pulls a knife;
you pull a gun. He puts one of yours in the hospital; you put one
of his in the morgue". This works up to a point.

You could try frequency diversion. If the bad guys are jamming
frequency A, then you go to frequency B. A more sophisticated way
of doing this is to use multiple frequencies and switch among
them rapidly. (This is called frequency hopping). The bad guy's
going to have to spend a lot of money to cover all possible
frequencies. There are formulas which can predict how much of the
singal you need to get to have a good picture (or conversation), 
and how much noise you can have and still hear (or see).

Finally, you can use steerable antennas. Radar antennas are very
directional, and unless you a very close to the jamming source,
the only direction in which you'll see a problem is in the
direction of the jammer. (You'll see a line from the center of
the scope to where the jammer is). One of the airborne
communications radio systems I know of has an adaptive array
antenna with "ground" and "air" modes. In ground mode, it assumes
you are talking with a ground station, and that the strongest
signal must be the jammer. So it puts a "notch" in the antenna
pattern that direction. In the air mode, it assumes that you are
talking to your wingman who is only dozens of feet away, so it
puts a lobe in the pattern and blocks out the rest.

If I were jamming a radar, I would use other than brute force to
do so. I would retransmit pulses in such as way so as to produce
multiple targets at multiple distances (easy to do), or even at
different bearings. (See intrusion, above).

> Also, at certain range, the enemy radar will burn through the jamming
> signal and lock on to the incoming fighters. How does that work?

Inverse square law. If the jamming source and the target are not
co-located, then it's easy to see that if you can get close
enough to the target, you'll get a better "picture" of it.

If your question is how does a radar burn through when the jammer
and the target are co-located, then then answer is still the same,
but is obscured by a little physics.

Let me use the example of an interceptor coming up on a plane
carrying a jammer.

The jamming source appears to get stronger as the square of the
distance between it and the interceptor. So, at 1 mile it is 4
times stronger than at 2 miles.

So is the signal hitting the jamming aircraft from the
interceptor. At one mile, the signal from the interceptor hitting
the jamming aircraft is 4 times stronger than at 2 miles. Let's
say for the sake of argument that the signal arriving at the
jammer is 1 watt when the interceptor is at 2 miles and 4 watts
when its at 1 mile.

But the reflected signal only has to return 1 mile now instead of 2.

    So, at 2 miles we have a 1 watt signal returned over 2 miles. 

    At one mile we have a 4 watt signal returned 1 mile. 

The one watt signal at 2 miles is attenuated 4 times more than
the 4 watt signal at one mile (due to inverse square).

In other words, as the interceptor appraoches the target, the
signal of the jammer gets stronger by a power of 2 with respect
to distance, whereas the power of the returned tracking signal
gets stronger by a power of 4. Given a sufficently strong
transmitter on the interceptor, the signal to noise ratio can
become such that a recognizable signal occurs at some "burn
through" point.

-- 
 Get things done by the book when you can, and in spite of if when you have to.
       Dan Flak - McCaw Cellular Communications Inc., 5400 Carillon Point
  Kirkland, Wa 98033-7397, 206-828-8006, (usenet: flak%mcgp1.uucp@nwnexus.com)

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