Charlie Springer wrote:
On Thu, 2 Jun 2005 07:40:35 -0700, Eunometic wrote
(in article .com):

The limitation of the AM seeker lies in the performance of the AM
detection (here x-axis) circuits, as the average signal from the
detector becomes quite weak in one direction thus producing poor
tracking performance in this axis. A scheme to resolve this is what is
termed frequency modulation (FM), whereby the number of spokes varies
with the radial distance from the centre of the reticle.

What do you call the no spokes version where you need to know the angular
position of the reticle? Say, from a synchronous motor's phase. I tried to
make one for tracking stars once long ago with a PMT. I still want to make
one work.

-- Charlie Springer

I'm not sure I understand what you mean. There were 'rossete scans'
that ended up in latter versions of sidewinder thse use a sort of
rotating and oscialting mirror. These also ended up in early German
infrared seekers intended for terminal homing on the Wasserfall missile
but actually derived from infrared imaging systems such as "Spanner".

The Basic AM seekers consists of a rotating transparent disk the half
segment of which is 'greyed' out with a fine speckled pattern of dots
to let in half the infrared light, the other half might consist of a
few dozen spokes. As it rotatres the infrared image produces either a
steady flat signal from the greyed portion or a series of high
frequency pulses from the spokes. The average instensity is the same.
A low pass and high pass filter distinquises the two and the phase
relative to the position of the disk determin the angle though not the
distance from the center. In one of the German versions intended for
the X-4 missile the disk did not rotate but the whole missile did
instead. A single gyroscope spun up at launch by a gramm of gunpowder
acting through a commutator kept track of "up"

Sidewinder I believe never had gyroscopes but used little wind driven
turbines in the tail acting as gyroscopes that mechanically acted on
little tail elevators to roll stabalise the missile. It rotated so
slowly it didn't matter to the seeker. Brilliant.

FM seekers are like AM ones only have another ring (or two or three)
with a different spoke pattern around the disk to widen the acquisition
angle but make the tracking more precise.

I think rossete scans took over a long time ago due to their higher
resistence to jamming and now imaging array systems.

The British Redtop missile (used on the lighting inteceptor) used a
different more sophisticated pattern. Its rotating disk 12 scimitar
spokes each of which had a different curve and width to the scimitar as
it widened toward the periphery. Thus the phase determined the angle
and the 'length' of the pulse the distance from the central axis. I
guess you'd call it 'phase modulation'.

I guess you could used 4 infrard photodetector arranged in a pie shape,
you even buy these from electronics suppliers such as RS components,
but the usual problem with these is that each must have exactly the
same gain and chracteristics.