2013 May 19
Sunday, Day 139
Unha 3/Kwangmyongsong 3 to Sun-Synchronous Orbit
There has been a lot of discussion over North Korea's use of the phrase "sun-synchronous orbit" in respect of Kwangmyongsong 3. Information on the launch vehicle impact zones indicates that the launch trajectory will result in a near-90° inclination. It has led to discussions ranging from mis-translation to North Korea being downright untruthful.
Jonathan McDowell has pointed out that yawing the third stage through something in the region of 50° before ignition will allow the trajectory to be changed sufficiently to make the orbit sun-synchronous. Unha 3 will need to achieve an inclination of 97°.4 for its stated target of 500 km altitude.
It is an ambitious manoeuvre for a first launch to orbit. The previous attempt in 2009 resulted in the third stage not igniting so the manoeuvre is unknown territory.
The map shows an approximation to the curved trajectory that Unha 3 will follow during the third stage firing if a sun-synchronous orbit is the real target. If not, it will follow a continuation of the line taken by the second stage up to its impact zone.
Update - 2012 Apr 11
Early April 11, Ted Molczan posted an item on the Seesat-L discussion board.
He pointed to a video available on the web that appeared to show a 3D model of Kwangmyongsong 3's intial circuit of the globe.
The video was showing a computer screen that was set up for reporters on site in North Korea to see. This image is an extract from the video. It has been corrected from an original that was slightly distorted and made the Earth appear elliptical. It depicts an orbit around the Earth resembling what might be expected for Kwangmyongsong 3.
There is a white dot to the south-east of Taiwan that indicates the postion of the satellite about the time of separation from the rocket - see the large map further up the page.
Whether it is representative of the intended orbit or was a simulation for the benefit of the assembled press corps is not known.
The second picture was created using a piece of software called Orbit Architect, available for the Apple iPad. Orbit Architect allows touch screen gestures to vary all of the fundamental values of an orbit at will and displays it as a 3D representation that can be moved around to show views from different angles.
A low circular orbit and the Earth's orientation have been set up to resemble, as closely as possible, the image from the video. It was then posssible to read off the orbital parameters needed to produce it.
The orbital inclination, the crucial value, is 97°.5.
It is the expected inclination for a 500 km circular orbit if it is to be sun synchronous.
NOTE - the images DO NOT show the ground track. Both show the orbit as a closed ellipse with the Earth depicted as it would be at the moment the satellite is at the point indicated by the white dot.
Page Date: 2012 Apr 8
Updated: 2012 Apr 11
Copyright © Robert Christy, all rights reserved
Reproduction in whole or in part without permission is prohibited