Re: Why the GPS Orbits

From: Mgr. CSc. Antonín Vítek (
Date: Wed Apr 22 2009 - 07:08:36 UTC

  • Next message: David Brierley: "DMB Obs April 20-21"

    It's very interesting question. For example, the Chinese navigation system Compass with Beidou satellites is constructed in different way in comparison with GPS or Russian Glonass.
    The operational system Compass should contain at least 35 spacecraft in different orbits:
    27 in MEO (Medium Earth Orbit, period 12.8 hrs, inclination 55 deg, 3 different planes [i.e. 3 x 9 sats], rotated by 120 deg in RAAN) - this is very similar to the GPS and Glonass systems;
    5 in GSO (Geostationary orbit, period 24 hrs, inclination 0 deg);
    3 in IGSO (Inclined geosynchronous orbit, period 24 hrs, inclination 55 deg).
    But the philosophy ot Chinese system is also different. Computation of the user's position is not done byl tre mobile receiver, but in the system's control center and result is then transmitted to the user via S-band link.
    < ------------ Původní zpráva ------------
    < Od: Brooke Clarke <>
    < Předmět: Why the GPS Orbits
    < Datum: 21.4.2009 17:29:36
    < ----------------------------------------
    < 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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