Re: North Korea satellite: NOTAMs inconsistent withclaimedsun-synchronous orbit

From: Jonathan McDowell (
Date: Sun Apr 08 2012 - 20:30:02 UTC

  • Next message: Bob Christy: "Re: North Korea satellite: NOTAMs inconsistent withclaimedsun-synchronous orbit"

    > There may not be a accepted international definition inscribed in a treaty
    > somewhere, but there is precise mathematical definition and that's good
    > enough for me:
    Oh Brian, I couldn't disagree more! I'm familiar with that paper and similar
    calculations, of course, but that's a definition of the sun-synchronous
    property. There are NO satellites which PERFECTLY have this property.
    For a practical definition of an orbital regime you MUST define a range.
    (It's not physics if it doesn't have error bars :-))
    I have plotted currently orbiting debris objects and active payloads versus their departure
    from perfect sun-synchronicity, using the most recent TLE data available
    to me. I think the data clearly show that a sensible pragmatic definition
    of SSO, in agreement with the wild guess I made before doing these plots,
    is about plus or minus one hour per year in drift rate. I propose that this
    range, corresponding to a first order precession rate of 0.9446 to 1.027 deg/day,
    be adopted as a working definition. (This would exclude the postulated 88 deg orbit of
    the NK satellite).
    > Sure, being off by a few fractions of a degree in inclination is not a huge
    > deal, but in the case of North Korea (which is what started this
    > discussion) the inclination based on their announced azimuth isn't even
    > retrograde.
    And so what? As Bob Christy pointed out this corresponds to a few minutes
    plane drift over the probable life of the mission, so as a *practical* implementation
    of sun-sync ("are we monitoring the weather at the same local time each day?")
    they may consider it fair enough even if it's not what the rest of the world 
    would call sun-sync. I agree it's a stretch, if that's their intention, but
    I don't think it rises to the level of a falsehood.
     - Jonathan
    Seesat-l mailing list

    This archive was generated by hypermail 2b29 : Sun Apr 08 2012 - 20:31:07 UTC