Sunday, February 07, 2016

Inconsistent DPRK versus JSPoC orbit claims for Kwangmyŏngsŏng 4

This post is a brief update to my more elaborate post of earlier today here.

As I wrote in that post, western military tracking of the N-Korean satellite places it in an orbit with perigee at 465 km, apogee at 502 km and an orbital inclination of 97.5 degree.

It is interesting to compare this with the (English) radio announcement of the DPRK itself, which you can hear here.

In that bulletin, the orbit is given as having perigee at "494.6 km", apogee at "500 km", and an orbital inclination of "97.4" degrees.

Compare this to JSpOc data: 465 km, 502 km, 97.5 degrees.

The DPRK apogee perigee altitude does not match the JSpoC data, which gives a clearly lower apogee perigee at 465 km. This could in theory be due to initial errors in the JSpOC tracking data (the first few orbit determinations are always less accurate). But the magnitude of the difference is such, that I doubt that.

Assuming that the numbers in the DPRK radio bulletin are not based on actual North-Korean tracking data, but instead based on a pre-launch desired orbit, then maybe this could indicate that the satellite did end up in a slightly lower orbit than intended.

We'll see if the difference keeps standing with more western tracking data added...

[update: Jonathan McDowell suggested that perhaps the Koreans used another spheroid (earth shape) to refer to. I doubt that: not only would I expect similar discrepancies in the apogee altitude as well in this case (the apogee altitudes given by JSpOC and N-Korea are close), but moreover, 30 km is a large difference. I know of no ellipsoid that differs by as much as 30 km from the WGS84 ellipsoid]

[update 2:  Bob Christy makes some very interesting remarks on his webpage, which also support the idea that orbit insertion did not go as intended].

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