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    Re: NASA Artemis missions - navigation
    From: Frank Reed
    Date: 2022 Nov 26, 11:29 -0800

    Thanks for the link. Quoting from there:
    "One set of the system’s sensors are known as star trackers – sensitive cameras that take pictures of the star field around Orion."

    The star field around Orion? Wow, that could get confusing! This is what happens when you name your spacecraft after a constellation. Ha ha. :) 

    The article says:
    "By comparing the pictures to its built-in map of stars, the star tracker can determine which way Orion is oriented."

    An interesting feature of these off-the-shelf star trackers is that most of them work from an internal star database (and have done so for decades). That is, when they are pointed at a star field, they don't output what they see in terms of apparent x,y coordinates of star images. And they don't output "photos" of what they see. Instead they do the processing on-board the tracker from a built-in database and output the direction they're facing as a pure RA and Dec direction. All they send to the spacecraft's main gudiance computer is that "finished product".

    I have to wonder whether there's been any trend away from processing directly on the star tracker. This sort of distributed computing has become less attractive as the cost of general computing has plummeted. The disadvantage of processing on the device itself is that it's not usually possible to tweak the code of the tracker. It "just works", or it doesn't. Apparently this has been an issue on the current mission. The star trackers are misbehaving, and it's believed that this is due to glare. It's always night in space so you can see all the stars... great! But, not so great, it's always brilliant-bright high noon in space at the same time so if any sunlit component is too close to a star tracker it can be blinded. A small modification in code could address this, but can the star trackers be updated? Anyway, they are "working the problem", and it doesn't sound too serious.

    Frank Reed

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