Past and future of the KH-11 Keyhole/Evolved Enhanced CRYSTAL constellation (part 3)
That discussion involved a look at past configurations, especially the relative positions of the orbital planes of the primary and secondary satellites in the constellation. Based on these past configurations, I concluded:
"After these orbital re-arrangements, USA 224 and USA 245 will be the primary satellites in the East resp. West plane. If history is to go by, their orbital planes will likely be separated by about 48 to 50 degrees. The orbital plane of the secondary East plane satellite, USA 161, will probably be 15 to 25 degrees east from that of USA 224. The orbital plane of USA 186, now the secondary satellite in the West plane, will most likely eventually be located 5 to 15 degrees west from that of USA 245."
This weekend I mapped the history of the KH-11 constellation in somewhat more detail than I did for my previous post (where I only looked at 2007, 2011 and 2013). Based on this more detailed analysis, I think I can constrain the distribution of orbital planes even further. The orbital plane of the secondary West plane satellite, a role USA 186 will take early 2014 if I am correct, will be located 10 degrees west of the primary West plane satellite. The orbital plane of the secondary East plane satellite, USA 161, will maintain to be located 20 degrees
I can say this with some confidence because this seems to have been the intended nominal KH-11 constellation over the full past 8 years.
Since 2005 I keep an archive of the frequently updated classfd.tle orbit files calculated by Mike McCants: they are based on amateur observations that include mine. From my archives I extracted orbital elements for the KH-11 Keyhole/CRYSTAL satellites with an epoch in early July, for each year between 2005 and 2013. Next, since the orbit epochs in question sometimes differ by a few days, I used Scott Campbell's SatFit software to normalize them all to the same epoch, day yy182 (where yy is the year and 182 is the day number), i.e. July 1st of each year.
By normalizing to the same epoch, the RAAN values of the orbits become directly comparable. RAAN stands for Right Ascencion of the Ascending Node, and this value maps the orientation of the orbital plane in space for the epoch in question. More precisely, the RAAN value gives the angle of the orbital plane in earth-centered space, with respect to the direction of the Vernal equinox at the epoch in question.
The diagram above maps the RAAN values (in degrees) at July 1 for each KH-11 satellite in each year in the period 2005-2013. The diagram below plots the same data, but then expressed as the difference delta (in degrees) between the planes of the primary East and West plane satellites, as well as the delta between the planes of the primary plane satellites and the secondary plane satellites:
A clear systematics can be seen to it. The primary East and West plane satellites (always the newest satellite in each plane) are 48 to 50 degrees apart. The secondary West plane satellite is in an orbital plane 10 degrees more westward than the primary West plane satellite. The secondary East plane satellite is in an orbital plane 20 degrees more eastward than the primary East plane satellite.
These are (of course) similar values to what I reported before, but now much more clearly constrained, documented over a larger time span, the diagrams visualizing the intended spatial arrangement very well. An arrangement that has basically been stable over the past 8 years. Changes in the arrangement amount to satellites switching roles (and orbital planes), but the basic orbital planes that make up the constellation remain the same.
The top diagram visualizes how satellites have switched from primary to secondary roles (and from orbital plane) as new satellites are added to the constellation.
For example, we see USA 129 (1996-072A) switch from the primary West plane to the secondary West plane in 2006-2007, following the launch of USA 186 (2005-042A) into the primary West plane. It does so by changing its orbital plane by 10 degrees. And we see USA 161 switch from the primary East plane to the secondary East plane in 2011-2012 (actually late August 2011), following the launch of USA 224 into the primary East plane. It does so by changing its orbital plane by 20 degrees.
A similar switch will no doubt occur early 2014 (see my previous post), when (if I am not wrong) USA 186 switches its orbital plane by 10 degrees from the primary West plane to the secondary West plane, once USA 245 (freshly launched into the primary West plane last August 28) becomes fully operational.
What can also be seen, is how USA 116 (1995-066A) started to drift away from its orbital plane after 2006, and was next de-orbited in 2008. The suggestion is that this satellite had almost ran out of fuel by 2006, as a result of which it was no longer an option to counter the drift by periodic manoeuvres. By 2008 it was drifting too far from the intended constellation, and the last fuel reserves were then used to de-orbit it. The secondary East plane was then left empty until three years later, in 2011, USA 161 (2001-044A) took on the role previously filled by USA 116. This happened after USA 224 (2011-002A) was launched as a replacement into the primary East plane. As of late 2011, the KH-11 constellation can hence be considered complete again (I have pointed out earlier, in my previous post, that the 2008-2011 gap in the secondary East plane was caused by the delay and then cancelling of the FIA optical program).
Note 12 Oct 2013: a follow-up on this post, discussing other orbital parameters than orbital plane, can now be read here.