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    Re: Cel nav in space
    From: Derrick Young
    Date: 2005 Jan 4, 10:24 -0500

    Years ago I worked on the SCOUT and later the Inertial Upper Stage of
    the space shuttle.
    The SCOUT was a highly successful solid propellant, two (later three and
    four) stage vehicle that was used to place a lot of satellites in
    near-earth orbits.  That missile used an analogue guidance system and
    was programmed to follow a "gravity turn trajectory".  There was some
    guidance available but it was more in having the vehicle not get too far
    away from the gravity turn it was supposed to be following.  We could
    put booster rockets on the first and second stages as well as adding a
    fourth stage on the vehicle - the fourth stage would allow for a boost
    into a higher orbit.  The nominal payload and orbit that we serviced was
    a 300 pound payload in a 110 nautical mile orbit.  With the known flaws
    in the guidance system, we had a nominal accuracy of +/- 150 nm.  BUT
    surprisingly enough, better then 98% of the flights got their payload
    where it was supposed to be.
    The Inertial Upper Stage (IUS) was an interesting project.  One of the
    proposals was to "fold" the SCOUT so that it would fit in the shuttle
    bay or to place stages into an orbital "storage bin" and assemble the
    missile in orbit.  Based on NASA's planned missions, an orbital launched
    SCOUT would be able to handle better than 90% of the interplanetary and
    high-orbit missions (this is from a thrust perspective) through about
    2015.  However, it would also have to be "modernized" since the analogue
    guidance system was no longer in production (1977-78).  NASA performed a
    market survey and found that none of the digital guidance systems
    available at that time would allow a solid fueled rocket to meet the
    mission requirements.
    Boeing proposed a liquid propellant IUS missile - which actually was
    safer to launch in the shuttle bay then the solid fuel motors of the
    SCOUT.  I designed the telemetry system, portions of the mission
    sequencer and guidance systems for the liquid fueled IUS.  We did NOT
    use any onboard radar systems.  What we did use was based on the IUS
    being attached to the shuttle by an umbilical cord.  Data would be
    passed from the shuttle's guidance to the IUS guidance systems (plural).
    Just before the shuttle moved to a safe distance, the cord would be
    disconnected and a timer would start running on the IUS.  The IUS would
    then "sit", keeping track of changes to the attitude and orientation by
    looking at an inertial guidance system (gyros).  The IUS could NOT make
    any changes until 45 minutes had passed to allow the shuttle to back
    away from the missile.
    Forty five minutes after the umbilical cord was disconnected, the IUS
    would "wake up".  It would then start searching the sky looking for a
    star of a specified color and intensity.  When it found a star with the
    right color and intensity, it would place that star in the center of the
    visible star field and then rotate around until it found a second star
    of specified color and intensity.  With two, it would then look for a
    third.  This would enable the onboard systems to determine its orbital
    position and orientation.  From there, the IUS would look at the end
    state of the mission profile (the desired high altitude orbit, inner
    planetary mission, etc) as well as other parameters and "decide" the
    length of each rocket engine burn as well as turning points, and places
    where it should seek stars to confirm it's position.
    The folks on the ground could over ride the on board systems by up
    loading a new end state, or the entire mission sequence events table.
    But everything was based on the IUS using the stars to determine the
    initial proper orientation.
    Everything was based upon determination of the initial orbit,
    orientation and position of the vehicle before anything else was done.
    The IUS could take up to an hour or more to find one of the initial sets
    of stars.
    Does that help?

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