PPAS Considerations

From: Bram Dorreman (bram.dorreman@gmail.com)
Date: Mon Jul 28 2008 - 16:15:09 UTC

  • Next message: Brad Young: "Re: PPAS Considerations"

    PPAS considerations
    
    Prioritysystem
    Up to and including June 2008 every satellite acceptable for our PPAS
    observation program was assigned a priority for observation. These ran
    from 1 (most important) to 6 (least important). In practice the
    satellites with priorities 4, 5 and 6 were barely observed. Therefore
    I considered a simpler system where only the priorities 1 to 3 are
    used:
    
    1. is the priority in case the satellite is found to accelerate or
    exhibits an unexpected brightness pattern
    2. is the priority when some extra attention is requested
    3. All other spacecraft with brightness variations of short duration
    
    Initially a satellite gets priority 3 if it appears that it has
    variable brightness.
    
    Variable brightness
    Each satellite has a brightness change while it moves from its rising
    point through its culmination point to its setting point. The
    brightness of the satellite depends on three things:
    
    a. the distance between the observer
    b. the phase, this is the angle between the sun, satellite and observer
    c. the size and nature of the surface that reflects the sunlight. Some
    objects have "diffuse" reflection and some have "specular" reflection.
     Typically the latter
    will have a larger range of brightness.
    
    The first two cases are to be calculated, the latter not. A satellite
    which does not or rotate or rotates very slowly (many minutes) will be
    relatively faint when it is in the same direction as the sun and gets
    brighter when it is more in the opposite direction. A satellite which
    rotates will continuously reflect sunlight by another part of its
    surface. If that surface is changing in size, the brightness is
    varying. This is the case with rocket stages that rotate around their
    minor axis, so tumbling. Often the magnitude variation is smaller at
    "full" phase and larger at "half" or worse phase. The PPAS is about
    these tumbling spacecraft.What we care about is the tumble period
    regardless of the nature of the variation.
    
    Sometimes I get observations within which a variation is reported
    while there is only one maximum stated. Then I do not know whether
    this is the normal brightness variation of steady objects or
    variations due to rotation.
    
    There are sometimes observations with two or three maxima and then no
    more. These are often satellites of which I know from experience that
    they do not, or sporadic tumble. One possible explanation is that some
    flat surfaces are situated that they just reflect the sunlight toward
    the observer. The satellite itself does not need to rotate.
    
    Processing of the observations
    Each submitted observation will be checked by me and corrected if
    necessary. This may be adjustments to the PPAS format, editing the
    comments and possibly add a reference to the notes file in which the
    too long notes are copied. Observations which do not comply the PPAS
    format, i.e. not mentioning a measured or guessed flash period are
    copied to an alternate file. Possibly subsequent observations can give
    rise to add those as yet to the PPAS file after some reconsideration.
    
    Format of PPAS-observations.
    This format was determined in the early nineties by several
    BWGS-members and the and is still in use. My contribution dealt with
    the mnemonics to keep comments as short as possible. The most
    extensive description of format and mnemonics can be found at
    
        http://www.satobs.org/tumble/flashpm.html # PPASformat
    
    On the encodings for the brightness variations described there I have
    the following additions:
    
    1. The pattern A'A' can also be expressed simply with A. If only one
    symbol, then it is obvious that it was counted on that maximum.
    2. The same applies to F'F' that may be replaced by a single F.
    3. Because the comma "," is used to indicate that the minima were
    counted, this should not be used as a separator between the different
    parts of a coded commentary. Since some years I use the semi-colon (;)
    as a separator.
    4. The word "mag" can be omitted for the magnitude value because the
    plus "+" with a number following it is in itself clear.
    
    An example of the four above-mentioned adjustments:
    Original:
    A'A' mag +3>7
    New format:
    A; +3>7
    
    Note references
    When I started sending the PPAS observations directly to Mike McCants
    in 2004, the original notes file was not available to me. In order not
    to use double allocations, I started again with notenummers but now
    preceded by a "b". Furthermore, the reference are like "note b1". Only
    later I realized that a reference "b1)" was better and applied this.
    From launch year 2007 I apply the original note number "n)". It is my
    intention once to join the two note files with the simplest note
    numbers.
    
    Reporting of the end time
    When drafting the PPAS format it was agreed to report the time of the
    last timed maximum (or minimum). In this way, with a TLE as close as
    possible to the observation, a satellite track can be reconstructed.
    For a serious analyst of PPAS observations there may be a lot to
    derive. There was even suggested to do a positional observation at the
    end of the flash observation. In practice, this never occurred perhaps
    because this is a great strain.
    
    Reporting of individual timings
    It is with some observers increasingly common practice that all
    individual timimgs were reported. I did it myself during some years
    and found it useful only when the satellite in question shows really
    different times between the maxima. For objects with regular flashing
    periods it has no additional information. I noticed by myself that
    there were sometimes differences of about one second between the
    minimum and maximum flashtime on a regular satellite. This has to do
    with how clearly defined the maxima are. With well defined flashes the
    differences show to be much smaller.
    
    My conclusion: You need not to include them if regular. With
    irregularities they can be very useful when determining a period.
    
    Detection of a flash period
    By following a satellite in a transit as long as possible the presence
    of a flash period can be detected. In case of short periods, many
    maxima can be counted. In case of long periods, only few, if any. It
    might be that a whole period (preferably between three successive
    maxima) lasts longer than that the satellite takes to transit. The
    latter also provides the upper limit of a well measurable flash
    period. A high orbiting satellite can be followed longer than a low
    one, giving more time to determine a flash period. For a geostationary
    object one can use the whole night. Note that the changing phase angle
    over a longer period if time can affect the accuracy of the
    measurement.
    
    Knowing by experience, I now follow a satellite with variable
    brightness as long as possible during a transit. In case of doubt,
    whether or not a variable brightness other than by change in phase
    angle will then become clear. I have experienced more than once that a
    satellite seemed steady and started flashing after its culmination
    point. It may even happen that in two successive transits one shows a
    steady object and the other a flashing one.
    
    There are also many small satellites with flat surfaces causing faint
    flashes. Such satellites can be seen steady, show some flashes times
    and then become steady again. These are not really PPAS objects.
    
    Observations per satellite
    A satellite with the properties of an PPAS object gets more
    inter­esting when more observations are available. In practice it
    appears that the Kosmosrocket, Zenitrocket and Centaurrocket are most
    observed. Faint satellites, because they are either small or have high
    orbits, are much less observed so that there can not be drafted a
    sensible evolution of the flash period.
    
    Flashes and flares
    A flash is a more or less short increase in brightness caused by the
    rotation of the satellite. A flare is mainly caused by the change in
    the phase angle or orientation of a satellite when the sunlight is
    reflected by a rather large flat surface.
    
    Operational Iridium cause "flares", tumbling Iridiums cause flashes.
    Iridium flares are predictable, flashes are not. I suppose that flares
    caused by Skymeds, Key Holes and Lacrosses can be predicted as long
    their attitudes can be predicted. Almost always such a satellite will
    flare under the same conditions in the same area of the sky (at nearly
    the same elevation and Azimuth).
    
    In summary:
    It is recommended to follow a potential PPAS-object for at least half
    of the available arc of visibility.
    It is recommended to determine as many maxima as possible when a flash
    period has still to be determined. The reporting of individual timings
    depends on the present regularity in those times.
    
    Last Note
    All observations are scanned, but not all will be written to the PPAS
    database. The less suitable observations are kept in a separate file.
    Possibly this file will be made available to other observers.
    
    
    Bram Dorreman,
    PPAS collector
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