XBradTC

You’re probably aware by now that the US Air Force and Navy both use the AGM-88 High Speed Anti-Radiation Missile (HARM) to suppress enemy air defense radars during combat.

The missile detects the emissions of the enemy radar, and steers itself toward it. A proximity fuse detonates the warhead just above the transmitting antenna, and sends out of shower of tungsten shrapnel to shred it and put it out of action.

It shouldn’t surprise you to learn that our potential enemies field similar missiles. These, as a class, are generally known as ARMs, Anti-Radiation Missiles.

We tend to think of US air superiority as being based on bad ass fighters like the F-22 and F-15 being controlled by the mighty E-3 Sentry AWACS. Which, sure. But we only have 33 AWACS. And so the Air Force, Marine Corps, and the Army, all operate a fairly large number of ground based radars, some for the fighter control mission, and many simply for the military equivalent of routine air traffic control. For instance, the USAF has about 60 AN/TPS-75 radars to support operations worldwide.

Not surprisingly, the USAF would rather not have enemy ARMs hitting their TPS-75s, both because they need the radars, and because there are people in the shelters operating them that they would prefer not get blown up.

We’re all used to seeing the pencil thin beam of a radar sweeping around a circular scope. And for the most part, the antenna array is designed to do just that, direct as much of the R/F energy into the smallest possible beam. Unfortunately, no system is perfect. Some R/F energy is actually emitted all around the compass. This “spillage” of R/F energy is generally referred to as the sidelobe (the main beam being the main lobe).

ARMs don’t actually track the radar via the main lobe. Instead, they detect the side lobes and home in on that.

Obviously, sidelobe suppression techniques become important to the radar designer. Antenna design, active cancellation (similar to noise cancelling headphone technology) and deception techniques all can come into play.

One deception technique to use a decoy.

For the TPS-75, there is a dedicated and associate decoy system, the TLQ-32. The Tickle 32 is actually a fairly sophisticated piece of equipment.  It isn’t enough that the decoy simply transmit on the same frequencies as the radar. It has to actually mimic the genuine sidelobes of the radar, and the transmission characteristics of the main beam (such as pulse width and pulse repetition frequency) in real time. And so, up to three Tickle 32s can be connected to the TPS-75 via fiber optic cable and placed in such a manner as to draw any ARMs away from the genuine radar. The fiber optic cable allows the main radar signal processor to send a  message to each decoy what the next radar pulse will look like (modern radars tend to shift these things around a bit to avoid jamming).

As to the post title? Apparently, the location where the operators place the TLQ-32s, where they hope the enemy missiles will harmlessly impact, is known in the trade as the ARM pit.