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Comment by jessriedel | Hacker News Reader

Of course, celestial navigation doesn't work in the daytime. It also seem weird to write an article aimed at readers who have never heard of it, but not mention the existence of terrain contour matching, which is widely used on cruise missiles and works any time of day.

https://en.m.wikipedia.org/wiki/TERCOM

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> Of course, celestial navigation doesn't work in the daytime.

Even with a simple sextant you can use the sun. And the SR71's system did operate during daytime using starlight.

I was wondering about that. I guess at the altitudes the SR71 operated at, at least the brighter stars would be visible.

They don't have to be visible to the human eyes, just visible to instruments. With band filters and a narrow field of view you can get a stronger signal that sticks out from the the approximately uniform sky background. I mean the altitude helps, but it's not necessary.

Celestial navigation works in the daytime. Star tracking was part of the SR-71 G&C system. The stellar inertial platform was aligned preflight with the aircraft at rest. Day or night made no matter.

It had many initialization modes, at least one of which required tracking two stars within 5 minutes. It's alluded to at [1].

[1] https://www.sr-71.org/blackbird/manual/4/4-40.php

Thanks for the correction. Does the point still stand with regard to cloud cover?

Sure it does - consider that the sun is a celestial object. Sailors for hundreds of years have been shooting a noon sight.

A noon sight works at noon. The vast majority of daytime is not noon.

A noonsite is just the base reference. Once you’ve done that and compared it in the table you have your position. Any site after that is the delta until the next noonsite.

Do they fly high enough that atmospheric scattering and brightness become small enough to still use it ?

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