"Heat seekers for dummies" advice and clarification wanted.

[pathology_doc]

Okay, so I'm reading "From Fireflash to Skyflash, a history of air to air missile firings in the RAF" (See the books section for details) and am reading about the "chopper disc" used in the guidance head of heat-seeking missiles.

I understand that the basic idea is that by alternating transparent and opaque 'bars' of varying width (as you transit to the outer rim of the spinning disk) you get a degree of frequency modulation of the incoming signal (i.e. the hot exhaust or flare or whatever the missile is tracking) proportional to the distance from the centre of the field of view that the guidance package/autopilot interprets as being proportional to the angular error. I'm a long way from understanding how the thing is done electronically, but fine with the concept.

However, I'm not so sure about understanding (again in broad) how the direction of that error is derived. The phase of the signal is mentioned, but what's not stated - and what would seem to be implied - is that there surely has to be some sort of reference signal for that phase to be compared against, which the missile uses to determine what is, for want of a better term, "up" - and therefore which way it should turn (as opposed to how hard it should turn and when it should stop turning, both of which I imagine could be handled by differentiating the degree of modulation electronically and suppressing the control deflections & d(theta)/dt of its own long axis as d(freq)/d(theta) approached zero.

 

[RP1]

Well, you could just have the rotation of the chopper produce a signal when it was at some reference point (say 12 o'clock) and compare the phase of that pulsed signal with the target pulsed signal. The phase difference is then proportional to the bearing. This doesn't even need to know "up" as it's just a bearing to translate into control deflections. IIRC early missiles flew with a fixed orientation, though.

RP1

 

[pathology_doc]

*Slaps forehead* Silly me, of course. Thank you

Naturally by "up" I meant 'something the missile regards as its internal reference direction', which may or may not at any particular time correspond to the absolute (terrestrial) vertical (which is why I said "for want of a better term"). So we're on the same wavelength there.