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    Star Tracker - Space Shuttle and the Inertial Upper Stage.
    From: Derrick Young
    Date: 2020 Apr 4, 14:53 -0400

    Sorry if this is a bit long.  I don’t post very often but some of these discussions bring back a lot of good memories.

    In 1978, I went to work for Boeing in Seattle.  Thought I was going to work on airborne radar systems.  During my intake sessions, I met my new boss, who asked me to follow him to another location.  No, I was not going to work on the radar systems, but I was going to work another program.  The Space Shuttle program.  Specifically, the Inertial Upper Stage (IUS), that would be launched from the bay and used to place payloads in high earth orbit and interplanetary missions.

    The initial problem faced by the IUS was that as long as it was connected to the shuttle, it received continuous updates on it position and altitude.  But as soon as it disconnected, it lost the updates.  So, it would know where it was, but not where it is currently located (should sound familiar).

    The IUS had to wait 45 minutes from the time/point of disconnect from the shuttle before any of its systems would activate (to allow the shuttle to back away a “safe” distance).  It could and did use DR to determine what it thought was the location/altitude.  But that “best guess” was not good enough to determine the actual location/altitude needed to get the payload into the correct orbit.

    So, after the 45 minutes of waiting, it would activate the star finder mechanism and the thrusters.  It would start “looking” for a known star, bring that to the center of it’s field of view.  Then start a slow rotation about it’s axis to locate another (the second) known star.

    That would stop the rotation, and it would search for a third known star within the field of view.

    Once the three stars were located, it could calculate the current location, based on the IP of the location when the disconnect with the shuttle occurred.

    That computed position would serve as the base for the calculations needed to determine the trajectory/boost needed to get the payload into the needed orbit.  There was a lot of calculations that could be done by the onboard systems, but these could also be accomplished by NASA and then uploaded.

    Anyway the reason that I write this email is because the concept behind the star tracker is not new.  Just basic CELNAV, The IUS did this using an 8-bit computer using an operating system written in JOVIAL (“Jules' Own Version of the International Algebraic Language") and the actual software also written in JOVIAL.

    Now, we have apps that tell us that same information and we have computers that are a lot more powerful and smaller than those that were used in the space program.  Just look at your phone or (for some of you) watch. 

    Thanks for bringing back good memories.


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