True Story No16


Where exactly is that aircraft?

I heard there's never been a new radar that worked properly. That was said to me by a professional that worked at a factory making radars so I guess it's probably true.

The usual sort of radar uses an aerial that goes round and round. It supplies a picture, to a cathode ray tube and its called a Plan Position Indicator or PPI. The radio beam coming out of the aerial is quite narrow and the narrowness is a function of the size and shape of the dish. The bigger the dish the narrower the beam.

The conical section of the beam compared with a sphere of radiation coming from an imaginary point source equates to the gain of the aerial. The beam comes out of the aerial pretty fast, and having bounced off a target, heads back to the dish at much the same speed. The bounced back bit is really weak because the reflection is spread around and only a tiny bit ever gets back. Luckily the gain of the aerial is much the same in receive as it is in transmit so the tiny reflection gets magnified because of that. Also, the higher the transmitted power, the higher the received echo so a long range radar has a very powerful transmitting device such as a Klystron or a magnetron and a very sensitive receiver much like the type used to receive domestic satellite.

If the aerial just goes round, and one knows where its pointing when an echo is received, one can work out the direction and range of a target quite readily.

However it's not that easy to work out it's height.

One way is to use a second type of radar, similar to the first, but which, when pointed at an echo found by the PPI equipment, is then nodded up and down.

The first gives you information that you can use to work out a range and a direction and the second radar gives you the height of a target.

With these three bits of information you can direct an aircraft to investigate whats going on.

There's actually another type of radar used before that event though, just in case the target was friendly. That type activates a transponder in a friendly aircraft which sends a message back to say who it is. This is an IFF (it distinguishes between Friend and Foe).

A single radar which combines direction, range and height is often called a 3D radar.

One type, developed by another firm, has an enormous array of small aerials, the signals from which can no doubt be processed by computers to give the required information.

Another type, the subject of this story was much more compact.

It made use of an odd effect; when a microwave signal is passed down a tube with a specially shaped hole at the end, the angle at which the signal emerges from the tube is proportional in some mathematical way to its frequency, some call it a squint angle. Therefore if one pointed the special tube or waveguide towards a target and varied the frequency of the signal up and down one would get the same effect as a mechanically nodding aerial.

So that's the way it was designed.

The aerial went round and round and the frequency of the transmitted signal whizzed up and down at the same time but much faster. The result was a received target echo, which could be processed into direction, range and height.

The designers went away and drew up a system with a transmitter, a receiver and a few computers and tried to sell it to potential customers. As it was such a clever idea it sold well.

Many years later the first system was tried out at a real radar site and with a real aeroplane.

Everything seemed OK.

Direction, range and height readings were calculated and plotted by the computers.

The plane was tracked and it moved across the PPI display. Not a raw display, straight from the aerial, but one carefully worked out by the computers.

The height was indicated in little characters next to the picture of the plane and its IFF showed it was the right one.

It was noticed however, by the engineers, that the plane was taking evasive action!

It was climbing then diving, then climbing again, then diving.

The height reading showed it to be moving up and down over a thousand feet at a time and pretty regularly too.

"Now fly level", was the request.

"I am flying level", was the response.

Something was amiss.

In fact the effect of quite a serious problem had been seen.

The radar aerial pointed slightly downward, only by a few degrees.

This was to get a near horizontal signal at the highest squint angle.

Have you ever noticed when you're watching TV and a plane flies past you get a periodic fade as it goes between your aerial and the TV transmitter? It starts off hardly noticeable then gets worse then fades away as the plane moves out of the direct line.

This is caused by the addition of two signals at the TV aerial. One is the direct signal from the transmitter and the second is the one bounced off the aeroplane. At critical distances the two signals can alternately nearly cancel out and nearly double in strength.

The 3D radar aerial was getting the same effect.

Signals from the target were bouncing off the ground in front of the aerial and combining with the direct signals.

The height processor was confused and mistook the effect to be variations in aircraft height.

The customer hadn't really noticed what was going on and thought that everything was fine.

Going back to the (radar) drawing board wasn't the answer because it was a fundamental problem caused by the laws of physics.

The answer turned out to be easy.

There were computers involved in the conversion of raw data to displayed data wasn't there?

So make them work a bit harder.

Write an algorithm (that's a sort of equation) to average out the height variations. A sort of filtering process (or "fiddle factor" depending on your point of view).

Nobody outside the Company ever found out…until now!

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