[AR] Re: Star trackers at high altitude
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 1 Apr 2023 14:30:11 -0400 (EDT)
On Sat, 1 Apr 2023, Jim Davis wrote:
"For an aircraft operating about 80,000ft, stars would be visible at all
hours of the day. At that altitude the sky is black, not blue; the
aircraft appears to be in space."
My first thought was that this couldn't be right; I was reminded of
claims by moon hoaxers of why no stars were visible in photos taken on
the moon. The lunar surface is so bright during the lunar day that it
drowns out any starlight.
*If* some of the surface is in the camera field of view! Which it
naturally is in the lunar-surface photos, since photographing the surface
(or someone/something standing on the surface) is typically the point of
the photo. That means setting the exposure short enough that the sunlit
surface etc. isn't overexposed. With an exposure that short, yeah, faint
things like stars simply don't show up, especially since photographic film
has a rather limited "dynamic range" (the range of brightnesses it can
reproduce).
(As do imaging systems with a small number of bits per pixel -- ordinary
digital cameras usually have 8-bit pixels, but scientific instruments
often have 10 bits or more because of this. Try photographing a black
cat! Unless you can get just the right shot, with a fairly dark
background, the cat tends to look like a black blob, with no visible
detail. I'm told that the same thing can happen in pictures of people
with very dark skin.)
Thinking on this further I began to have doubts. At 80,000 ft there is
no dust to scatter sunlight and perhaps the aircraft would be far enough
above the earth that the reflected light from the surface wouldn't be
that bright.
The key issue is bright stuff *in the same field of view*. On Earth's
surface in daylight, we've got that bright blue sky, which is scattering
from the atmosphere (mostly from the air itself, not from dust -- even
pure air is not actually 100.0000000% transparent). At 80,000ft, not so
much, although I've heard the SR-71 sky described as being very dark
purple rather than outright black.
We've tested spacecraft star trackers outside on clear nights. It works,
even though the sky isn't as dark as it ought to be because of all the
light pollution from Toronto.
And yes, spacecraft star trackers routinely work on daylit spacecraft --
it's just a question of not having any light-scattering surfaces in the
field of view, and not having the field of view *too* close to the Sun
(baffles help but are not perfect).
Can stars be tracked at that altitude during the daylight hours? If so,
at what altitude does this become practical?
I wouldn't think it would be any problem. The Snark strategic cruise
missile of the 1950s similarly used stellar-inertial guidance -- inertial
updated by a star tracker, because inertial systems weren't then good
enough to do the whole job -- and it had a ceiling of only about 50,000ft.
Henry
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