RE: KH 11-8, USA 86, 116

Ted Molczan (molczan@fox.nstn.ca)
Wed, 1 May 1996 13:17:59 -0400

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.

Elaboration
-----------
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.

bye for now