# RE: North Korea satellite: retrograde ground track displayed in launch control centre

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Wed Apr 11 2012 - 20:14:15 UTC

• Next message: Bob Christy: "Re: North Korea satellite: retrograde ground track displayed in launch control centre"

```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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