Why the GPS Orbits

From: Brooke Clarke (brooke@pacific.net)
Date: Tue Apr 21 2009 - 15:28:19 UTC

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    Hi Ted:
    I just read your "Updated KeyHole elements" and how "it's
    manoeuvres relate to its mission".  That brings home the idea that the orbit 
    parameters of all satellites have been given a considerable amount of 
    consideration by whoever put them up.
    The obvious question is why does the western plane KeyHole operate in a lower 
    perigee orbit?  There must be a big benefit in doing that to justify all the 
    expense it entails.  Better photo resolution?  Less predictable pass time?
    That got me thinking about why the GPS satellites are where they are.  For 
    example the WAAS GPS service is by geosynchronous orbit sats that can only be 
    parked over the equator.  This is an advantage for WAAS corrections i.e. it 
    allows corrections for the area covered by that sats antenna pattern.
    But why not also use 24 hour period sats in different orbital planes, for 
    example near 90 degrees?  I say that because it seems to me that most of the 
    regular 12 hour period GPS sats spend most of their time near the horizon for 
    most locations away from the equator.
    Have Fun,
    Brooke Clarke
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