Sidewinder engineering stoy/divide by zero

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.

On Thu, 2 Jun 2005 22:39:21 -0700, Eunometic wrote
(in article . com):

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".

There is a tracker form with a reticule that is half transparent and half
opaque and spun by a synchronous motor, so there is a reference for the
position of the reticle over time. The amount of time and the angle over
which the target is obscured generates the error signal. When perfectly
centered the signal is constant (half is always blocked).

If I could center it well enough, I could half mask the secondary of a
Cassigrain and spin it. I just find the analog solution more satisfying than
a digital image tracker.

I thought the turbine wheels in the fins of the Sidewinder were stabilizers
and generators, so it didn't need any batteries. I may be thinking of
something else.

-- Charlie Springer

Charlie Springer wrote:

On Thu, 2 Jun 2005 22:39:21 -0700, Eunometic wrote
(in article . com):

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".

There is a tracker form with a reticule that is half transparent and half
opaque and spun by a synchronous motor, so there is a reference for the
position of the reticle over time. The amount of time and the angle over
which the target is obscured generates the error signal. When perfectly
centered the signal is constant (half is always blocked).

If I could center it well enough, I could half mask the secondary of a
Cassigrain and spin it. I just find the analog solution more satisfying than
a digital image tracker.

I thought the turbine wheels in the fins of the Sidewinder were stabilizers
and generators, so it didn't need any batteries. I may be thinking of
something else.

Not generators. The AIM-9B-J used a gas-grain generator for power.

Guy

Charlie Springer wrote:
On Thu, 2 Jun 2005 22:39:21 -0700, Eunometic wrote
(in article . com):

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".

There is a tracker form with a reticule that is half transparent and half
opaque and spun by a synchronous motor, so there is a reference for the
position of the reticle over time. The amount of time and the angle over
which the target is obscured generates the error signal. When perfectly
centered the signal is constant (half is always blocked).

If I could center it well enough, I could half mask the secondary of a
Cassigrain and spin it. I just find the analog solution more satisfying than
a digital image tracker.


I think I know how to do it.

You need to understand "phase sensitive demodulators". There are like
rectifiers whose polarity of rectification is dependent on a reference
phase. In their most basic form they consist of vibrating relays,
biased diodes or amplifier valves or transisters whose
gates/grids/bases are switched by a phase reference such that the
signals polarity is demodulated according to its phase.

So, consider, as the disk rotates you want to resolve into rectangular
x-y co-ordinates to opperate your star tracking device or missile fins.

To generate the x co-ordinate for instance you have a square wave
reference of period equal to the rotation of the disk and in phase with
it (generated by a light source and photocell on the priphery for
example or a commutating switch, or your stepper reference etc). This
is the reference phase for the demodulator. The demodulator could be
as simple as a fast relay or it could be electronic. The demodulator
will generate negative or postive value depending on whether the
'target' is in the positive (right) or negative (left).

For the y (vertical axis) co-ordinate you need to have a second square
wave 90 degree delayed created by a second light/photocell or some such
internal reference.

It's crude as you only get +/-x and +/-y signals. You don't really
need a square wave, sinusoid is uselff. Sin-Cos potentiometers also
resolve polar to rectangular co-ordinates.




I thought the turbine wheels in the fins of the Sidewinder were stabilizers
and generators, so it didn't need any batteries. I may be thinking of
something else.

I'm pretty sure they acted as gyros mechanically on the fins to roll
stablise it.



-- Charlie Springer

On Thu, 2 Jun 2005 22:39:21 -0700, Eunometic wrote
(in article . com):

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 slots or pattern may have formed the front end of a chopper amplifier,
which was the common way to work with infrared sources in the instrument
business at the time. Was there an internal reference IR source and detector
and a difference circuit? This might elliminate ECM and other common mode
noise.

-- Charlie Springer