Re: Satellite Elements from Astrometric Observation.

From: Greg Roberts (grr@telkomsa.net)
Date: Tue Mar 17 2009 - 12:19:07 UTC

  • Next message: Tony Beresford: "satobs 8597 march 17"

    Afternoon John
    
    I could supply you with some of the images I have taken - I would rate them as 
    fair quality for several reasons. I can provide the approximate centre of the 
    field of view as well as field of view. Are you okay with FITS images- this has 
    the time of the end of exposure in the header file.
    
    Now normally my images are taken with the camera stationary which means the 
    geostationary satellite will appear stellar and the stars trail with the left 
    edge of the star trail corresponding to the time the exposure was recorded. The 
    magnitude limit will typically be around 12-14th magnitude.
    
    I could also supply some FITS images where the camera is tracking at sidereal 
    rate so stars appear stellar-like but there will be some edge distortion and 
    field rotation as Im using an altazimuth mounting . I do NOT normally use this 
    mode for satellites since my geostats will then trail and I loose sensitivity - 
    however this is probably the type of picture that video observers will take of 
    leo satellites ...
    
    Anyway if of any interest please let me know and Ill be happy to send. I could 
    of course send the images as JPEG files and identify field and time of exposure 
    in the naming of the file.
    
    As far as satellite elements from astrometric observation - (not what you are 
    offering to do) but since I "have the floor" Id like to say how I would possibly 
    like it to work:
    
    One takes a picture - FITS or JPEG or extracts it from an avi file so you have 
    the time and the satellite name. One also has an idea of how close the object 
    was to prediction in time and position so one starts with a list of satellites 
    and corresponding time of observation and approximate error in time and position 
    ( say to 2-3 secs and position say 0.5 degrees) as well as the elements used for 
    the observation as well as the matching images.
    
    Now it would be NICE if this is all one has to do- the program reads the 
    satellite name and time and program computes the expected position. Now comes 
    the tricky part- the program needs to identify this position against the stellar 
    background in the image so the program needs access to a database to at least 
    mag 12 stars. The program will then try and pattern match the expected star 
    background against the observed background, starting with the predicted position 
    and, if not matched, then correct the predicted position by the estimated time 
    difference noted and/or positional uncertainty, so means sliding the obtained 
    image along the new predicted satellite track and finding a match. This should 
    not be too hard a job for todays pc's if the images were stellar but they will 
    not be - at least in my case, so it means "plotting" the end of each star trail 
    to get the stars x,y coordinates. Then the program also needs to be able to 
    identify which is the "intruder" - ie the satellite and, if the satellite is 
    trailed which is the end you want to measure.
    
    It will also probably to necessary to be able to rotate the expected star field 
    to match the orientation of the image which may have been taken with an altaz 
    /equatorial or other type of mounting. The observer should usually be able to 
    supply a reasonably good estimate of the field of view which will narrow the 
    pattern matching somewhat.
    
    Once one has the RA/Dec of the satellite the orbit reduction is pretty 
    straightforward as such programs already exist and in some cases the code is 
    available - eg Scotts software for example. It is also possible to get a 
    satellite magnitude from the intensity but I would not put too much faith in 
    this ...
    
    I think the entire procedure is pretty straight forward but I think its beyond 
    my programming capabilities .
    
    Anyway just some ideas...
    Cheers
    Greg
    
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