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

From: Jonathan McDowell (
Date: Sun Apr 08 2012 - 18:57:19 UTC

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

    Phil - this raises a more general question which I haven't seen addressed.
    What do you mean by sun-synchronous?
    Specifically, how much drift is too much? No orbit is perfectly sun-synch, just like
    with GEO the real question is what tolerance (in local time equator crossing for SSO;
    in longitude for GEO) is acceptable for your particular mission.
     It seems to me that if your orbit drifts by less than one hour in local time over the
    course of a year, it's reasonable to describe the orbit as sun-sync, and I believe all
    the retrograde Yaogans meet this test. Of course most US sun-sync satellites keep
    station very much more precisely - typically around 1 minute per year - but that doesn't
    mean someone is wrong if they describe a one-hour-per-year satellite as synchronous.
    In the absence of an accepted international definition it's a matter of taste what the
    tolerance should be. And for the North Koreans, if their satellite only works for
    a couple of weeks, say, then you only care how much the local time drifts during that
     - in the spirit of friendly pedantic argument,
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