Re: 88-18A geoflasher results

From: Bjorn Gimle (b_gimle@algonet.se)
Date: Sun Apr 28 2002 - 04:39:05 EDT

  • Next message: Alan Pickup: "Decay watch: 2002 April 28"

    > At 23:55 27/04/02, Kevin Fetter wrote:
    > You must have a stop watch that measures time in 1/1000 sec.
    > My watch only measures at 1/100 sec. As for my timing accuracy,
    > its 40/100 second or 0.4 seconds. That's 0.2 seconds to star
    > the watch and 0.2 seconds to stop the watch. How good can you
    > measure time to.
    >
    Tony answered: ...
    > on a known minute. Thus the consistency of the reaction time is important,
    > for the period measurement, not its absolute value. I entered the cycle
    > number and flash time into an Excel spreadsheet, and fitted a staight
    > line for the cycle number vs Time. The slope of this line is the
    > best estimate of the period. Whats more EXCEL gives me a formal standard
    > deviation of the slope ( and the period), and residual plots.
    > The SD for most days was .003 or .004 seconds. The difference of the
    observed
    > times and the fitted times was always equal to or better than 0.2 seconds,
    > with quite a few .05 secs or better.
    > This technique is of course quite time consuming and only applicable
    > to slow moving satellites like 88-18A.
    
    I use the program SYNODIC that I wrote, which plots Tony's "difference of
    the observed times and the fitted times". Then it's easy to spot the bad
    timings, and even more important, if there is a phase shift, ie a different
    surface starts showing its flashes during a long observed arc, or during an
    intentional gap to avoid filling your stopwatch memories. This can result in
    two parallell lines, with a shift of a half period, or some other more or
    less explainable fraction, and I can use "all" observations to find the best
    common period, which is NOT necessarily close to (last-first) flash time
    divided by an integer!
    
    There is "always" a synodic effect, ie the observed period (P) is
    longer or shorter than the physical rotation period (Pp), because
    the Sun-Sat-Obs angle is changing during the observation.
    
    In the case of geosynchronous satellites, observations on consecutive
    days will span an integral number of physical rotations.
    If P/2 > SD * 86400 / P you can get a VERY accurate flash period, BUT :
    
    In most (all?) geometries I can think of, there is always a daily
    phase shift, and P should be close to (the lap covering one day)
    divided by (an integer +-0.5), but Pp is lap/integer.
    For an even number of days P = lap / integer.
    
    (To determine if Pp is <P or >P you need very accurate absolute timings of
    the same pass from two different observers, or Bart de Pontieu's DRA method
    from very accurate lap times over a large range of Sun-Sat-Obs angles - and
    some luck)
    
    -- bjorn.gimle@tietotech.se (office)                         --
    -- b_gimle@algonet.se (home)  http://www.algonet.se/~b_gimle --
    -- COSPAR 5919, MALMA,    59.2576 N, 18.6172 E, 23 m         --
    -- COSPAR 5918, HAMMARBY, 59.2985 N, 18.1045 E, 44 m         --
    
    
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