RE: Magnitude observations

From: Ted Molczan (molczan@rogers.com)
Date: Sun Jul 11 2004 - 14:06:08 EDT

  • Next message: Bram Dorreman: "Re: Magnitude observations"

    Ed, although I am open to the idea of a magnitude-only format, I am far from
    convinced that we really need one.
    
    There are a number of significant advantages to combining positional and
    magnitude observations:
    
    - time and position provide a check against accuracy, which I make use of in my
    analysis
    
    - when there are more than one observation per pass, phase angle and range can
    be computed based upon a circular orbit approximation derived from time and
    position. This method was used successfully by Rainer Kracht in late 1995-96, to
    compute std mag from a huge archive of Russell Eberst's observations. See his
    messages in the SeeSat-L archive.
    
    A magnitude-only observer could produce useful data, as long as he or she takes
    care to measure time accurately, as well as magnitude.  An observer careless
    about time accuracy could make a mess of the database, yet it would be difficult
    to detect, due to the normal scatter in magnitude data.
    
    I am not arguing for or against. If there are likely to be many accurate
    observations forthcoming from magnitude-only observers, then we should
    accommodate them, but let us take time to consider the pros and cons, and how
    best to address the latter.
    
    Having said that, if there is going to be a magnitude-only reporting format,
    then I like your "first cut" effort, and I have appended my proposed second cut.
    
    It fits within the same number of columns, but does away with hyphens,
    semicolons, and decimal points, to make room for:
    
    - satellite catalogue number. There are some people who strongly prefer to use
    the international designation; others who prefer the catalogue number, so as a
    compromise, let us accommodate both.
    
    - full three alpha characters in the international designation. Unlikely, to
    arise, but a piece of debris large enough to track visually may some day receive
    a 3 character ID, so might as well be prepared. Since col 14 will be seldom, if
    ever, used, I suggest that we start the next field in col 15.
    
    - four digit site number. I do not like using observers' initials, because it is
    too easy to encounter people with the same initials. One important issue to be
    resolved is whether or not magnitude-only observers should be issued site
    numbers from the same pool of numbers used for positional observations. I can
    see arguments pro and con. I lean toward separate numbers, because they are a
    scarce resource. Sure, there are 10,000 possible values between 0000 and 9999,
    and there are few positional and magnitude observers, however, once issued to a
    particular site, a number cannot be used again, so over time, as observers enter
    and leave the hobby, and/or change observing sites, the code numbers will be
    depleted. I do not have an answer, but this must be given serious consideration.
    
    - full four digit date as a matter of good practice
    
    Strictly speaking the phase angle and range are not part of an observation, but
    are computed from the satellite's known orbit and the observer's coordinates.
    However, including them would be a convenience - as long they are computed
    accurately. That depends in large part on the accuracy of the elements and the
    ephemeris generator. 
    
    If those values are to be included, then I recommend that they be computed by a
    standard computer program designed to produce formatted reports from data
    entered by an observer. Once the observer enters the satellite's IDs, obs date
    and time, the program would find the object's elements within a text file of
    2-line elements provided by the user, compute the phase angle and range, and
    enter them into the report.
    
    One concern is that users may use elements that are sufficiently outdated as to
    produce erroneous phase angle and range. therefore, I suggest adding a field
    after Range, containing an estimate of the prediction accuracy. Most likely,
    this would be estimate of the along-track position error that would occur if the
    rate of decay of the elements used was in error by, say, 10 percent. A too large
    value need not necessarily invalidate the observation, but analysts might use it
    as a flag to perform a check of the phase angle and range, using elements they
    know to be up-to-date and reliable. I am able/willing to write the necessary
    data entry software for use under MS Windows.
    
    Ted Molczan
    
    
    00000 001111111111 22222222 2333333 3334 444 444 555555 555666666666677777777778
    12345 789012345678 01234567 9012345 7890 234 678 012345 789012345678901234567890
    nnnnn yynnncccSite YYYYMMDD HHMMSSs MMmm MMm PPP RRRRRR Remarks
    26362 00 25C  1234 20040711 0854060 031  03  049    279
    
    Col 01-05 satellite's catalogue number
    
    Col 07-14 satellite's international designation
        07-08 year
        09-11 sequence, right-justified
        12-14 piece, left-justified
    
    Col 15-18 Site number, right justified
    
    Col 20-27 observation date, UTC, yyyymmdd format
    
    Col 29-35 observation time, UTC, HHMMSSs format, with no variations allowed
    
    Col 37-40 observed magnitude, MMmm format, show leading zero, do not show
    trailing non-significant zeros. Format accommodates negative magnitudes, by
    placing (-) sign in col 37.
    
    Col 42-44 estimated accuracy, MMm format
    
    Col 46-48 phase angle, deg (optional?)
    
    Col 50-55 range, km (optional?)
    
    Col 56-80 Remarks
    
    
    
    
    
    
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