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" Captain, the clouds are up and the Nav-system has gone down again " |
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the only craft that have the stars reliably in sight - satellites - already use that. |
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i find the best navigation to be other people. apart from in france |
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The link says " [i]ts "blue light" source star tracker, which could see stars during both day and night, would continuously track a variety of stars..." How is it possible to see stars during daylight? I mean yes, in theory, but how? |
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By not looking towards the sun, and by being outside the atmosphere,
so there's nothing to scatter the sunlight and make the background of
the sky blue. |
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Edit: but the link's talking about a plane, not a satellite. Maybe it only
works at high altitudes? |
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Maybe so. The phrase "blue light" also suggests that it's using tight spectral filtering - perhaps to see a wavelength that's not scattered much by the atmosphere, or is not abundant in sunlight. |
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Double-dichroic prismatic optical "notch" filter - we could tell you more, but then we'd have to kill you. |
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Imagine an upside-down S.A.B.S. Mk VII, and yet somehow strangely not. |
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//How is it possible to see stars during daylight? I mean
yes, in theory, but how?// |
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The systems use a few tricks, firstly, they're astro-
INERTIAL, so a standard inertial system is used to get a
general idea of location. These are set on the ground and
are very accurate at the beginning of the flight, over time
the error tends to accumulate. The point is, the inertial
system gives the information on where in the sky to look.
I can't find any detailed descriptions on how it worked, I
expect it may have been classified until the people who
really knew were a bit dead, but I expect you could
create a mask with holes for target stars and holes for
definatelynotstars, then you could sort of wiggle it
around looking for the biggest difference in light. |
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//The phrase "blue light" also suggests that it's using tight
spectral filtering// |
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UV would be ideal. UV tends not to bounce around or be
emitted by things that aren't stars. That's about the limit
of 60's optics anyway, they certainly weren't making 3nm
bandpass filters for notches in the solar spectrum. |
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I think the biggest factor is that it was fitted to the SR-
71, and therefore operated at 80-100,000ft where the sky
is less optically complex. Modern equivalents for low
flying aircraft exist, but even Northrop-Grumman thought
they were pretty natty in 2007, so it's clearly harder at
10,000ft than 100,000ft. |
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// they certainly weren't making 3nm bandpass filters for notches in the solar spectrum. // |
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So much you know, sonny boy ... |
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// the sky is less optically complex. // |
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No, it's not about complexity, it's the noise floor. |
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// Modern equivalents for low flying aircraft exist, but even Northrop-Grumman thought they were pretty natty in 2007, so it's clearly harder at 10,000ft than 100,000ft. // |
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The technical term is "A right bugger". |
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During the day, there is a fairly useful optical reference called "The Sun" (the celestial object, not the brand of laminar anus-cleanser) which is quite useful if you have an automatic digital sextant. |
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At night, celestial navigation is Baked and WKTE. |
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If you're high enough (in altitude, not pharmacologically) then it is possible to see stars during daylight hours. |
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It's when you drop down into the soup of the lower atmosphere that things start to get difficult... |
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//useful optical reference called "The Sun"// |
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Turns out it's a pretty noisy neighbor. I read a few bits of
RADAR stuff and apparently it can be used as a reliable
source of broadband microwave emissions. I even found a
couple of papers from the 40's where passive reading
from the moon were possible. Hell, with a modern AESA
RADAR the sun and moon should be easy as pie to see.
You could probably get an active return off the moon, if
you know your speed, altitude and the moon position
then the time of flight would probably be enough to
triangulate from a single reference. Or you could use a
closer reference, give the folks in the ISS a bit of a scare. |
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