Position Sensors (GPS)
Sun Angle a Java Script to compute the angle of the sun based on location, date, and time.
About using start light to improve INS or GPS operation, Don Hyde [DonH at AXONN.COM] says:
There are a bunch of simplifying facts...
1) The favorite star is Canopus, as I understand (I'm no astronomer) it's the brightest, it's close to the ecliptic, and there are no other bright stars nearby to be confusing.
2) You know pretty nearly where you are by your inertial system, you're merely trying to improve on it, so you can point your telescope pretty close.
3) The image is a single bright point. Your "retina" only needs 4 cells. You move "up" or "down" until the "up" pair of cells matches the "down" pair of cells, and similarly for the "left" and "right" ones. You confirm that you're pointing to the right star by checking the absolute brightness of what you see. If it's not bright enough, it's the wrong star, you were looking for the brightest one.
To get aligned in 3 dimensions, you need one other star, preferably about 90 degrees away. I think Polaris is the favorite for that, the familiar north star.
4) An inertial navigation system intrinsically gives your location "inertially", that is to say, it doesn't turn with the earth, so it doesn't move around the sky with time of day or seasons, as long as the initial position that was set just before launch was chosen that way.
For a weapon system that could be launched at any time, I guess it must be able to use an alternate star in case the one it wants happens to be on the other side of the earth at the time.
Star tracking should be able to null out the gyro drift, at least during the coast phase, though I can't see how it could help for accelerometer errors.
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