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USA 86/116 more comments

Ted Molczan

In response to Allen Thomson's query about the state of the KH 11 constellation, Bjoern Gimle wrote about the puzzling orbit of what may be USA 116 or the old USA 86:

A more serious speculation, which I haven't checked yet, is that the object now known moved to the high orbit between my first and my second observation.

That is what I have been thinking. Looking at the observations between 10 Apr and 14 Apr, it seems possible that a manoeuvre took place sometime during 13/14 Apr. I have some ideas about what is going on, which I will summarize in the following two paragraphs, followed by some elaboration.

In short, over the past 8 or 9 months, both KH-11 planes have been moved westward, closer to where they were in early 1992. This appears to have been designed to correct an accumulated eastward drift of 3 to 4 degrees. This change was accomplished without greatly altering the standard sun-synch KH-11 orbital elements and constellation spacing. By mid-Apr'96, USA 33 appeared to have halted its drift, but USA 116 raised its perigee, thus accelerating its westward drift, apparently to decrease the RAAN spacing between it and USA 33. This may be in anticipation of a possible upcoming launch of a replacement for USA 33. There is a launch scheduled from Vandenberg AFB on 12 May 1996.

The new USA 116 orbit may be more than a temporary house-keeping measure. By raising the perigee 125 km to a mean altitude of 403 km, drag has been greatly reduced, conserving precious orbital maintenance propellants. Also, the groundtrack repetition rate has been increased from 59 revs in 4 days to 29 revs in 2 days, providing more opportunities to overfly areas of interest. This could become the new standard KH orbit once the replacement is launched.

In time, we have a good chance to determine the reason for the unusual new orbit of what is apparently USA 116. One possible interpretation is that the RAAN of the orbital plane is being moved to a new location. That seems reasonable given that the new orbit is not sun-synchronous, and its plane is drifting westward at about 0.06 deg/d, relative to a an exact sun-synch orbit. That is nearly 2 degrees per month!

Is that significant? I believe it is. I have just reviewed the drift of the planes of USA 33 and USA 27/86/116 since early 1992, and I have found that by late 1995, USA 33 was 3 deg east of its 1992 plane, and USA 86 was 4 deg east of the USA 27 plane of 1992. Over the next 8 months, both planes reversed direction and had moved west 2 deg by mid-April 1996.

Since mid-April, USA 33's plane appears to have halted its drift. This is about the time of USA 116's apparent manoeuvre, which has caused it to drift west even faster (much faster) than during the previous 8 months. Why? Here is a possible explanation.

In early 1992 (day 135), USA 33 and USA 27 were about 48.7 deg apart in RAAN. This was within about 0.2 deg of what I have long suspected to be the desired spacing, which is the Earth's rotation plus precession of the RAAN, during two KH-11 orbits about the Earth. But in mid-April 1996 (day 109), USA 33 and USA 116 were 49.6 deg apart in RAAN. By then, USA 33 had halted its drift, perhaps because it had reached its ideal RAAN. If the satellite's controllers need to re-establish the 48.9 deg spacing, then they would have to drift USA 116 a little further west. Its present orbit is doing just that, and it is closing the RAAN spacing relative USA 33 by about 0.04 deg/d. At this rate, the spacing would narrow to 48.9 deg on about 6 May'96.

Why the transfer? The Titan IV awaiting launch on 12 May'96 from Vandenberg AFB could offer a clue. USA 33 is very old, even by recent KH-11 standards, so it is reasonable to expect it to be replaced soon, certainly sometime this year. If the May launch turns out to be USA 33's replacement, then it would not surprise me to find it in a standard 14.77 rev/d orbit, with RAAN spaced about 48.9 deg west of USA 116. At about that time, USA 116 would drop its perigee to the standard altitude, and resume its sun-synch, 14.77 rev/d operation.

The main thing that troubles me about this scenario is that USA 116 would have raised its perigee by about 125 km and then lowered it a short time later, using a fair bit of propellant. That leads me to consider another possibility - that a new standard orbit will be established, close to that of USA 116's present orbit. The present orbit's higher perigee altitude greatly reduced drag, which saves propellant. Also, the new orbit has changed the groundtrack repetition rate from 59 revs in 4 days to 29 revs in 2 days or 58 revs in 4 days. Even if this is only a temporary orbit, it does preserves the standard repetition rate of 4 days and provides 2 days as a bonus.

Increasing USA 116's inclination by 0.48 deg would restore sun-synchronous precession, and barely change the present 29.07 rev/2 days repetition rate. If an exact 29 rev/2 days rate is required then, the perigee could be raised 23 km from the present 403 km, and the inclination increased by about 0.53 deg. In the past, the repetition rate was not exact, differing by about 0.05 deg, so the additional perigee raising does not seem necessary.

One problem with this scenario is that the ideal RAAN spacing would increase to 49.6 deg, which is where they were on day 109, before USA 116 changed its orbit! Of course, if we consider that the replacement for USA 33 can be launched directly into the desired plane, then this does not matter. What does matter is that since USA 116 will be operational for many years, it should be placed at or near the ideal RAAN in time for the launch of USA 33's replacement, without disrupting the present operations with USA 33. That appears to have been the theme of the past 8 months of orbital changes, both large and small.

I would be interested in other interpretations of the recent KH 11 orbital situation.

next up previous
Next: The rotation axis Up: Tracking classified satellites Previous: USA 86/116 speculations

Bart de Pontieu
Fri May 24 11:30:41 MET DST 1996