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The image above shows the KH-11 ADVANCED CRYSTAL/"KeyHole" optical reconnaissance satellite USA 186 (2005-042A) rising over the roof of my house in late twilight of March 28, 2016.
It had passed perigee at 260 km altitude just 10 minutes earlier and was at about 285 km altitude when I photographed it. At that low an altitude, it races over the sky (these are 5 second images with a 50mm lens). Clearly visible by the naked eye, I watched it rise over the roof, and then slowly flare to mag. -1 near 19:17:55 UT:
click image to enlarge
USA 186 is the oldest of the remaining KH-11 ADVANCED CRYSTAL satellites. After the de-orbit of USA 161 late 2014, there are now three KH-11's left on-orbit:
(a) USA 186 (2005-042A) pictured here;
(b) USA 224 (2011-002A), and;
(c) USA 245 (2013-043A).
USA 224, the primary East plane satellite, and USA 245, the primary West plane satellite, are separated by 48.7 degrees in RAAN. USA 186, the secondary West plane satellite, is 24.3 degrees (half the separation of the two primary satellites) West of USA 245 in RAAN:
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All the orbits are sun-synchronous, meaning that they maintain an orbital precession coupled to the daily precession of the sun.This ensures that they make passes at roughly the same time each day, imaging terrain with roughly similar solar elevations. This helps interpret imagery, as the shadow patterns will be similar on images from nearby days and changes in shadow pattern will stand out.
USA 224, the primary East plane satellite in the constellation, makes daily passes around 13h local solar time. It's ground track repeats each 4 days. USA 245, the primary West plane satellite, makes daily passes around 9:45h local solar time, also with a ground track repeating each 4 days. USA 186, the secondary west plane satellite, makes daily passes around 8h local time, with a repeating ground track each 10 days.
Basic data ():
sat orbit mlt DN repeat plane
USA 124 265 x 1009 km* 13:00 4 days primary E
USA 245 268 x 1020 km 09:45 4 days primary W
USA 186 260 x 456 km 08:05 10 days secondary W
* September 2015 (sat not yet recovered in 2016)
("mlt DN" means mean "local time of passage through Descending Node")
Note that due to the winter blackout in the Northern hemisphere we haven't observed USA 224 for seven months: the orbit shown here assumes it has kept up its September 2015 orbit. We will know for sure when we recover the object in early summer.
The previous secondary West plane satellite in the constellation, USA 129 (de-orbited in late April of 2014) used to make passes near 9h local time. By changing the RAAN difference between the primary and secondary satellite from 10 degrees to 24.3 degrees after USA 186 moved from the primary to the secondary plane and took over from USA 129 following the launch of USA 245 (see earlier post here and links theirin), the pass time was shifted to one that occurs one hour earlier.
As can be seen in the table above, all current KH-11's have perigee at a roughly similar altitude of ~260-270 km. The primary plane satellites have apogee at ~1010-1020 km. The secondary plane satellite has a much lower apogee, at ~455 km altitude.
The exact reason why the secondary plane KH-11's lower their apogee is unclear. USA 129 was the first to lower apogee connected to its move from the primary West plane to the secondary West plane in 2006. USA 161 repeated this pattern in 2011 when it was moved from the primary East plane to the secondary East plane (since the de-orbit of USA 161 late 2014, this latter secondary East plane no longer exists). Various options can be considered:
(a) some operational constraint, e.g. aim for a particular ground-track repeat interval;
(b) some operational constraint, e.g. related to image resolution (this is unlikely);
(c) some operational constraint, e.g.different imaging role compared to the primary plane satellites;
(d) making it easier to de-orbit the satellite near end-of-life: if fuel reserves are low, a lower apogee would allow de-orbit from apogee if necessary, so one does not necessarily have to wait for perigee to be located over the southern Pacific.