Sunday, 22 December 2013

USA 186 (Keyhole KH-11/Advanced CRYSTAL) is moving orbit, as expected

In a series of previous posts culminating in the October 12 summarizing post here, I scetched a scenario of what I think will happen to the Keyhole/KH-11/Advanced CRYSTAL constellation of high-resolution Optical reconnaissance satellites following the launch of USA 245 on August 28.

The first part of that scenario now seems to be happening: USA 186 has moved orbit.

This happened slightly earlier than I anticipated, but it does seem to be the first change in a series of changes right along the lines I expected.

The KH-11's are currently almost inobservable from the northern hemisphere (and hence my location) due to the "winter blackout". In the southern hemisphere, where it is summer, South African observer Greg Roberts has however been tracking them.

On December 10, Greg failed to recover USA 186 (2005-042A) in its old orbit. Earlier I predicted that this would happen at some point, as the satellite would likely be moved several degrees in RAAN from the primary West plane to the secondary West plane, which are 10 degrees apart in RAAN. See my earlier post here for a discussion of primary and  secondary orbital planes.

This made Greg next search for USA 186 in orbital planes more west of the original one. Indeed, on Dec 17 he recovered USA 186 in a more westward plane.

The new orbit as calculated by Ted Molczan from Greg's orbservations shows that the satellite lowered its orbital inclination by almost a degree, to 96.9 degrees. This manoeuvre probably happened on or near November 12th.

As a result of the inclination change the orbit is no longer sun-synchronous and hence its rate of precession changed. As a result its RAAN is currently shifting westwards relative to the other KH-11's. On December 17 the RAAN of USA 186 had already shifted westwards by 4 degrees. I suspect it will keep precessing until it reaches a value 10 degrees west of what it initially was (see my earlier predictions here, where I predicted this shift in RAAN), close to the aged West plane secondary satellite USA 129 (1996-072A). This shift will have been accomplished by early February at the current rate of precession (0.868 degrees/day or -0.12 degrees/day relative to the sun. Taking into account the RAAN precession of USA 245, they will have a separation of 10 degrees in RAAN by February 5).

USA 186: old orbit (red) and new orbit (white, December 17 plane)
The new orbit is still precessing westward over time. I expect
this will stop once it reaches the RAAN of USA 129 (grey)

I also suspect that next the satellite will reduce apogee altitude to attain a near-circular 390 x 400 km orbit, after which it will be sun-synchronous again. Indeed, the change in inclination to 96.9 degrees indicates as much as this inclination value fits a 390 x 400 km sun-synchronous orbit. As a result, USA 186 would start to move in an orbit very similar to USA 161 (2001-044A) in the secondary East plane in terms of apogee, perigee, inclination and eccentricity as well as in ground-track repeat patterns.

The initiation of these moves comes two months earlier than I expected, suggesting that USA 245 (2013-043A) which was launched into the primary West plane last August 28, needed less check-out time after launch than was the case with USA 224 (2011-002A).

As USA 186 is now moving to take the place of the aged USA 129 satellite, I expect the latter to be de-orbitted any moment.

Below diagram depicts the current constellation (December 17th), with USA 186 on the move westwards between the primary West plane (now occupied by USA 245) and secondary West plane (occupied by USA 129). See my earlier post here for a discussion of primary and  secondary orbital planes.

It will be interesting to see whether the drift in RAAN of USA 186 relative to USA 245 indeed stops at a 10 degree difference (the former separation of the orbital planes of USA 186 and USA 129), or whether it perhaps continues up to 20 degrees (the separation of the orbital planes of USA 161 and USA 224 in the East plane).

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