Orbital Focus - International Spaceflight Facts and Figures
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USA 179

Tyneside, UK
2022 May 19
Thursday, Day 139

Maintained by:

USA-179 Analysis:


Through long-term measurement of the Mean Motion, it is possible to monitor how its natural rate of change varies with time. The technique is periodically to measure the slope of the straight lines in the plot of Mean Motion against time.

This plot shows measured values of ndot/2 from June 2006 through to August 2007.

The value changes cyclically between bounds of .0000100 and .0000132. The line of best fit varies between .0000102 and .0000127. However, peaks and troughs do not repeat exactly from cycle to cycle. The trough in Autumn 2007 was not as low as the previous one.

The period is about ten months which coincides with the length of time taken for the orbit plane to complete one rotation round the Earth's polar axis when measured against the line joining the Earth to the Sun. In all probability, the variation is solar driven in some way (gravity or solar wind?).

USA 179 delta n

Page Date: 2012 Aug 12

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