That's some kind of flight/operational manual for the astroinertial navigation system (ANS), pretty detailed. Quite amazing that it's been declassified - you can see the link is from the Smithsonian Air and Space Museum. It also discusses details of how missions are loaded and planned with respect to the ANS. It also says that it has planet avoidance information, which gives you some idea of how sensitive it is (obviously the Sun and Moon are avoided).
I've not found any indication of what wavelength was used, but I would imagine the detector is a single infrared photodiode. By blocking visible atmospheric light (scattered), you can see the stars behind. Stars are point sources for all practical purposes, so you can have as large a magnification as you like without resolving them. What that means is if you have an large-aperture telescope, you'll still collect all the light from a star even if your field of view is tiny. Hence why this uses a scanning telescope to sweep around the sky.
Given the technology of the day, it's unlikely they had a multi-pixel detector (maybe a very low res array). So one approach you could use for this is two photodiodes looking slightly apart - by subtracting the signal of one from the other you might be able to correct for local atmospheric light (if the field of view is narrow enough that you don't expect two stars in the same field). Essentially a real-time dark frame. I have no idea how it actually works in practice though. I'm sure nowadays this can be done with a single wide angle camera.
It also seems like it has some strong initial conditions based on e.g. where the aircraft took off as well as references from the onboard IMU, aircraft attitude, etc. And if I read it right, the pilots can also indicate when they've reached waypoints to calibrate the system.
Very cool! Can you expand on the idea that it's just a single IR photodiode? This would get rid of visible atmospheric light, but wouldn't there still be residual IR light from the sun (whatever isn't absorbed by the atmosphere) interfering with collecting the light from stars?
From the sibling comment seems like I had the right idea. The main restriction I was thinking of is what technology was used at the time. The SR 71 used film for imaging, and the CCD array was developed later on and probably wouldn't have enough sensitivity even if there was an early prototype. You'd also need an InGaS(?) detector for SWIR and I'm not sure when they first came out.
For this application you want as sensitive as you can get, since you want to detect minute differences in background intensity. That points to a big ole photodiode.
Hence why I suggested that they use an off axis sensor to do online dark subtraction. Locally if there was no star, the diodes should give you the same intensity. If a star enters the field of view of one, but not the other, then you have a detection. It's more complicated than that apparently, but the idea is sound.
From the patent it actually looks like they use some kind of optical chopper and a single diode. A chopper is a disc with slits cut out, in the fold of view. If there's a star that doesn't fill the field of view, the chopper will give you a periodic signal as it rotates. If the sky is empty then you'll get roughly the same signal all the way round. (from a quick skim)
Typically the FOV of the telescope is too narrow to see the sun directly, so you only have to worry about scattered light from the atmosphere (hence the IR-pass filter).
Referencing the link from my other comment, the nutator and optical modulator referenced in the patent combine to remove the residual sky background and apply FM modulation to the star aligned with the telescope OA.
By measuring the phase and depth of the FM modulation, you get the angle and distance of offset between the star and OA.
That's some kind of flight/operational manual for the astroinertial navigation system (ANS), pretty detailed. Quite amazing that it's been declassified - you can see the link is from the Smithsonian Air and Space Museum. It also discusses details of how missions are loaded and planned with respect to the ANS. It also says that it has planet avoidance information, which gives you some idea of how sensitive it is (obviously the Sun and Moon are avoided).
I've not found any indication of what wavelength was used, but I would imagine the detector is a single infrared photodiode. By blocking visible atmospheric light (scattered), you can see the stars behind. Stars are point sources for all practical purposes, so you can have as large a magnification as you like without resolving them. What that means is if you have an large-aperture telescope, you'll still collect all the light from a star even if your field of view is tiny. Hence why this uses a scanning telescope to sweep around the sky.
Given the technology of the day, it's unlikely they had a multi-pixel detector (maybe a very low res array). So one approach you could use for this is two photodiodes looking slightly apart - by subtracting the signal of one from the other you might be able to correct for local atmospheric light (if the field of view is narrow enough that you don't expect two stars in the same field). Essentially a real-time dark frame. I have no idea how it actually works in practice though. I'm sure nowadays this can be done with a single wide angle camera.
It also seems like it has some strong initial conditions based on e.g. where the aircraft took off as well as references from the onboard IMU, aircraft attitude, etc. And if I read it right, the pilots can also indicate when they've reached waypoints to calibrate the system.