R: Re: North Korea satellite: retrograde ground track displayed in launch control centre

From: satrack@libero.it
Date: Wed Apr 11 2012 - 21:13:44 UTC

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    I agree with Bob. The image represents the orbit in ECI frame (it does not
    take into account the Earth rotation). If it was plotted accounting for the 
    Earth rotation 
    (ECEF) it would result visibly bent like a S (in the 3D perspective).
    >----Messaggio originale----
    >Da: bob@zarya.info
    >Data: 11-apr-2012 22.36
    >A: "Seesat-L"<SeeSat-L@satobs.org>
    >Ogg: Re: North Korea satellite: retrograde ground track displayed in launch	
    control centre
    >The image displayed on the LCC screen is not a ground track - the line 
    >is an orbit in a representative 3D view. The website that displayed the 
    >video cut off the top of the image with a message box when I paused the 
    >it for the screen capture. The full view of the north pole showed the 
    >orbit rising and disappearing over the Earth's limb in a similar way to 
    >the Orbit Architect view.
    >If the LCC image seen in animated mode, I suspect the satellite will 
    >move around the ellipse, and the Earth will rotate underneath it. The 
    >Korean software and Orbit Architect seem to display in the same mode.
    >Bob Christy
    >On 11/04/2012 21:14, Ted Molczan wrote:
    >> Bob Christy wrote:
    >>> I have used a piece of iPad software to reproduce the screen image
    >>> showing the orbit. It tells me the inclination is 97°.4.
    >>> There are copies of the two images here:
    >>> http://www.zarya.info/Gallimaufry/Unha3SS.php#3D
    >> I was stunned to read this, since there is a very noticeable difference 
    between 94.45 deg and 97.45 deg ground tracks.
    >> My first thought was that perhaps I had been too hasty in minimizing the 
    parallax problem arising from the 3D depiction,
    >> but I believe that I have found the actual explanation, which if confirmed, 
    is far more interesting, with possibly
    >> profound implications.
    >> I had a hunch that the software that Bob used to determine the 97.4 deg 
    inclination did not allow for Earth's rotation
    >> as the satellite moved through its orbit. As an experiment, I temporarily 
    set the Earth rotation rate to zero in my
    >> ground track program, and calculated the sub-satellite points of the 97.45 
    deg orbit I posted earlier today. Next, I ran
    >> the program with the correct Earth rotation rate, and plotted the 94.45 deg 
    orbit. I found that for at least the range
    >> of latitude of interest, the change of longitude with latitude agreed to 
    within a fraction of one degree, which seems to
    >> prove the point.
    >> This may well be the explanation for the odd 94.45 deg orbit implied by the 
    track displayed in North Korea's LCC. My
    >> first hypothesis was that a typo had been made in entering the inclination 
    into the display software, such that 97.45
    >> deg was entered as 94.45 deg, but the difference between inertial and 
    rotating Earth is close to 3 deg for a quasi-polar
    >> orbit.
    >> If the ground track displayed in North Korea's LCC did not account for 
    Earth's rotation, then this could have
    >> implications for the reliability of the NOTAM coordinates, depending on how 
    they were computed. But before, I go there,
    >> I want to do a bit more checking of the new hypothesis, and invite others 
    to do likewise.
    >> Plot the 97.45 deg orbit with the Earth's rotation rate set to zero:
    >> 1 79802U          12103.11415512  .00000000  00000-0  00000-0 0    06
    >> 2 79802  97.4500 182.0500 0002000 359.9726 179.8827 15.21000000    03
    >> Plot the 94.45 deg orbit with the correct Earth rotation rate:
    >> 1 79802U          12103.11415511  .00000000  00000-0  00000-0 0    05
    >> 2 79802  94.4500 182.0500 0002000 359.9726 179.8827 15.21000000    00
    >> The longitudes will differ of course, but for a given change of latitude (e.
    g. between 20 N and 15 N) the change of
    >> longitude should be about equal in direction and magnitude for both.
    >> Ted Molczan
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