RE: Phobos-Soil Project seeks observations of engine burns from South America

From: Ted Molczan (
Date: Thu Nov 03 2011 - 22:36:28 UTC

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    I am nearing completion of an Excel spreadsheet to generate ephemerides using state vectors extracted from the SPICE
    trajectory files available on the Phobos-Grunt project site. This is intended to provide prospective observers with more
    precise predictions.
    If I understand correctly, JPL-Horizons will provide ephemerides shortly after the spacecraft reaches orbit. They should
    be considered definitive, but in the meantime, the spreadsheet can assist in planning.
    In addition to the SPICE files, the project site has issued a pair of TLEs, for epochs immediately preceding the two
    burns. Together, they provide reasonably accurate coverage of burn #1, but neither is sufficient for burn #2, especially
    in its later stages.
    Here is a sample ephemeris, for Carlos Bella's pass that includes part of burn #1:
    It covers the portion of the pass >= 5 deg above the horizon. It provides the key information: date and time, indication
    whether the engine is planned to be firing, solar illumination, topocentric coordinates as AZ/EL and RA/DEC, and angular
    velocity. Also included is some non-essential but interesting information: range from observer to spacecraft,
    sub-satellite coordinates and altitude above geoid.
    The code in this spreadsheet is from stand-alone applications I have written and used extensively over 25 years.
    Basically, I have substituted the state vectors from SPICE for those that would have been generated from TLEs with SGP4.
    Provided the port into VBA was bug-free, it should be reliable, and limited tests support that belief.
    A small impairment of accuracy arises from the difference between the reference frame of the SPICE data (J2000.0) and
    that of TLEs (TEME). Ideally, I would have converted the J2000.0 state vectors to TEME, but did not have code handy to
    do so. The errors that arise probably are quite small, certainly in comparison to those that would arise from use of the
    TLEs, and much smaller than the large FOV optics likely to be used to observe.
    Ideally, I would have used the SPICE data with an appropriate ephemeris generator, but given the short time that
    remains, I am more confident using familiar code and systems, than in my ability to quickly master something new.
    For now, the spreadsheet is available only by request (please e-mail me off-list). This is to enable me to reach users
    in the event that serious problems arise, or updated state vectors are issued.
    I know that not everyone uses Excel, so please feel free to send me your site coordinates, and ASAP I will send you a
    custom ephemeris, like the one shown above.
    Later tonight, I hope to provide the promised tips on observing strategy, and respond to the comments and questions
    regarding plume brightness that resulted from my message yesterday.
    Ted Molczan 
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