This may seem like a stupid question...but what about when it was cloudy? Can I assume the BFF was flying above the clouds most (or all) of the time?

Yes, haze and clouds were a problem at low altitudes, but most of the time the aircraft was above the clouds. The Aurora Borealis (northern lights) was potentially a problem; the system included an aurora filter.

Was the star tracked manually by the navigator (as in, did they have to manually “look for” and keep track of it)? Fascinating article, but I’m not grokking how it was used in practice.

The device has a spiral search mechanism to find the star. Then it locked onto the star and automatically tracked it. So this was unlike the Apollo star tracker where the astronaut has to manually aim at the star.

Thanks, I was looking through the article for exactly that. Does it lock on to a configuration of stars?

Really curious how they did this mechanically.

I'll probably write another article on the star tracker itself. But I can give you a quick summary of the spiral search mechanism. It was electromechanical: a motor turned a resolver, a device with coils to generate sine and cosine from the shaft angle. This gives the X and Y deflections for a circle. These signals went through potentiometers that were also turned by the motor to produce constantly growing magnitudes, so you get a spiral. But you need to slow down the motor as you spiral outwards since you're covering a much larger linear region. So the motor also turns a stepping switch that progressively reduces its speed.

Once the system finds a star, a complicated feedback mechanism keeps it locked onto the star. There is a spinning slotted disk in front of the photomultiplier tube. If the star is off center, the output will peak when the slot lines up with the star. Thus there is an error signal with phase that indicates the direction to the star. This signal is demodulated to produce X and Y signals that change the aim to move towards the star.

I would absolutely love to read something about that - thanks for putting in the work and sharing it.

I have a buddy working on restoring a set of binoculars that were attached to the Target Bearing Transmitter system for a US sub from the 50s. Last I heard he was able to find someone that actually had parts of the original schematics for it so that he’s able to machine some new pieces.

These things are definitely a labor of love.

Am I right in thinking it didn't matter which star it locked onto, and it didn't need to know which star it was? Would it be a problem if it locked onto another celestial body (e.g. Venus)?

No, it needed to lock onto the right star, the one that matched the coordinates. Otherwise, it would be pointing in a random direction. The navigator would check against three different stars to detect an error.

The system could also use planets or even the sun for navigation. A special filter was used with the sun to avoid burning out the photomultiplier tube.

Ah, so it could be used in the daytime. I read the whole article assuming it was only useful at night. (When else would you be flying a bomber and need high accuracy?)

The SR-71’s star tracker was so sensitive it could track stars during the day.

https://theaviationgeekclub.com/the-sr-71-blackbird-astro-na...

being halfway to space probably helped

I would love to read a more detailed article on this device. I came back from reading the article with this exact question.

Reads like a labour of love. Thanks for sharing.

We couldn't find a wiring diagram so I had to trace out every wire.

Since the article doesn't mention: I've read that ICBMs used celestial navigation. Is this similar to what contemporary missiles used? Do we even know at this point?

As I understand it, the star altitude is measured relative to an artificial horizon.

How did it determine "down" in a moving airplane? Was it essentially doing the high-tech equivalent of dangling a rock on a string with some dampening (in a gyroscopic cage to avoid being affected by the airplane's rotation), or something smarter?

When I looked into whether astronavigation would be solvable cheaply or somehow trivially using modern hardware, I found this a surprisingly difficult problem even on a static platform - inclinometers that would get you down to 0.01° accuracy (which would still translate to a ~1 km positional error if I'm not mistaken, roughly what a skilled sailor is supposed to be able to do with a sextant) are expensive even today.

With a moving, shaking platform that's changing position (i.e. a perfect gyro will point perfectly in the wrong direction after a few minutes of flight) and might be flying turns (which makes "down" point in the wrong direction) that seems hard to solve.

The B-52 star tracker used a gyroscope to determine vertical. The Astro Tracker was stabilized by a bunch of motors and synchros so it matched the gyroscope. Thus, the Astro Tracker was a stable platform even as the aircraft pitched and rolled. (Footnote 4 in my article shows the vertical gyro attached to the Astro Tracker.)

> Was it essentially doing the high-tech equivalent of dangling a rock on a string with some dampening (in a gyroscopic cage to avoid being affected by the airplane's rotation), or something smarter?

Yes, that is essentially how a gyroscopic artificial horizon works.

Consider that the local horizon changes relative to an inertial frame (the stars) as you travel across the surface of a sphere, so even if you could build a perfect gyro that remained stationary in the inertial frame you would need to update the local down as you move. The solution is to slightly weight the gyro cage to bias it to the local down.

Now, consider that, in a properly-coordinated turn, the passengers (and gyro) will feel that gravity points straight to the floor :) So the time-constant of the damping is important.

I assume the constant is usually chosen short enough that the system will "forget" turns quickly, in exchange for becoming useless while turning?

Still, getting this whole thing accurate to probably one minute of arc is insane, especially with the gyro and star tracker linked only via motors and synchros. So the total error is the sum of any deviation of the gyroscope from the actual down direction, the error in measuring the gyro angle, the error in setting the star tracker to that exact angle, and then all other errors the system introduces. Then you need to take multiple separate measurements at different times and compensate for the movement, and a one-degree difference means you're over the wrong city (or in Europe, country) so the end-to-end accuracy must be much better than that.

And sailors supposedly did that with a sextant to something like 0.01° on a moving ship.