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!
