KH-11 USA 224 recovered

Over the past week I twice tried to recover the KH-11 CRYSTAL ('Keyhole') USA 224 (2011-002A) but failed. Leo Barhorst and Cees Bassa however did recover it on the night of April 27-28, in an orbital plane which is 4 degrees more westward than its previous plane. This meant that on two previous nights when I was doing a 1.5-hours (one orbital revolution) photographic coverage of the old plane, it actually passed outside the FOV of my camera...

Last night, based on Cees' search orbit, I did observe it as it was passing through Lyra around 22:57 UT. The image above and below shows it, together with the old Japanese scientific satellite Tansei 3 (MS-T3, 1977-012A), which was captured as a stray in the same images. The Japanese satellite is moving in a much higher orbit, as can be seen from the much shorter trails. It slowly faded in and out, so it appears to be slowly tumbling.

The image below is a stack of 13 images (4 seconds exposure each, with 5-second gaps). The image above at the top of this post is a single image from this series. The images were made with the very fine Samyang 1.4/85 mm lens.

The plane change was probably done to keep the separation of this primary East plane KH with the primary West plane KH (USA 245) near 48-49 degrees (the angle between the primary East and West planes maintained over the past several years). This would also bring the separation with USA 161, the secondary East plane KH, to 25 degrees, similar to the current distance between the orbital planes of  USA 245 and USA 186 in the primary and secondary West plane.

I therefore expect that when we recover USA 161, the secondary East plane KH, it will be in an orbital plane about 25 degrees east of USA 224.

Chasing USA 186

After Greg did the last observations from the southern hemisphere mid-February, the northern hemisphere observers recovered the KH-11 CRYSTAL ('Keyhole') optical reconnaissance satellite  USA 186 (2015-042A) late March. Alberto Rango captured it on March 30 from Italy and several observers have followed since. I observed it on April 6 in early twilight. Below is one of the images, showing it passing west of Castor and Pollux in a still blue twilight sky:

The current elements are here. USA 186 is currently moving in a 269 x 467 km orbit.

Next is the hunt for USA 245, the other West (evening) plane KH-11 that should emerge out of its winter shadow blackout by now. The East plane KH's, USA 161 and USA 224 will not be recoverable until early summer.

USA 186, bright and fast

USA 186 (top) and an old Russian r/b (1988-039B, lower corner)

click image to enlarge

Yesterday evening at the end of twilight, I observed USA 186 (2005-045A) pass amidst some scattered clouds. It had cleared just in time.

Close to perigee, the satellite was moving fast. At 70 degrees elevation due East, it became bright (about mag. +1.5), and then briefly flared to mag -1 near 19:06:20 UTC (22 Sep).

A second bright object was moving lower in the sky, and slower. It was an old Russian rocket from the Kosmos 1943 launch in 1988, 1988-039B.

Unfortunately, it later became completely clouded, so I missed this morning's favourable pass of the ISS and Dragon CRS-4, just hours before berthing.

USA 186 manoeuvered on the 17th

USA 186 being half a minute late one hour after the manoeuvre, 17 Sept 2014, 19:32:02 UT.  Chinese satellite Yaogan 11 also visible  (click image to enlarge)

Ten days after the first post-summer-glareout observations of the KH-11 Keyhole/CRYSTAL optical reconnaissance satellite  USA 186 (2005-042A), it has made another orbital manoeuvre.

In the evening of Wednesday 17 September I was targetting the satellite in a somewhat hazy sky, using the 1.4/85mm lens and a FOV near the tip of the Big Dipper tail.

To my surprise, the satellite was over half a minute late with respect to a 3-day-old element set. This suggested a  manoeuvre. My observations were corroborated by video observations of Leo Barhorst in the Netherlands and visual observations by Pierre Neirinck in France, obtained during the same pass.

The image above shows one of my images. As it turns out, this image was taken perhaps only an hour after the manoeuvre! USA 186 is overtaking Yaogan 11 (2010-047A) in the image (the fainter shorter, upper trail). Yaogan 11 is a Chinese optical reconnaissance satellite.

Observations the following evening by Cees Bassa and me in the Netherlands showed the satellite running even more late by that time: it passed 6m 32s late, low in the west. My camera caught it very close to the image edge. A few hours later, Kevin Fetter in Canada captured it as well.

The Sept 17 and 18 observations suggest that the manoeuvre happened on Sept 17, just before I did my Sept 17 observations (perhaps only an hour before, i.e. less than one revolution!). The current orbital solutions vary a bit between analysts (the post-manoeuvre observational arc is still short), but they agree in that the manoeuvre slightly adjusted the inclination, raised perigee and lowered apogee.

The new orbit is sun-synchronous and close to a 321 x 417 km orbit (it was 265 x 440 km before the manoeuvre), i.e. perigee was raised by about 55 km and apogee lowered by about 23 km. The new orbit is more circular, and starts to conform to the orbit I envisioned in October 2013. I suspect more manoeuvres gently raising perigee and lowering apogee until an approximate 390 x 400 km orbit is reached will occur over the coming half year.

An analysis using COLA suggests the manoeuvre(s) occured on 17 September, either near 17:46 UT or 18:25 UT. Or perhaps (and I favour that) it was a double manoeuvre, performed near both of these moments.

17:46 UT corresponds to passage through the ascending node on the equator, only minutes after passing through perigee. 18:25 UT corresponds to passing through apogee.

A manoeuvre to change inclination is normally done in one of the orbital nodes, or near the poles. A manoeuvre to raise or lower perigee is normally done while the satellite passes through it's apogee, and a manoeuvre to raise or lower apogee is normally done in the perigee. If either one of these (in the current case: the perigee) closely coincides with passage through one of the nodes, this is the ideal moment to change both peri- or apogee, and the inclination in one boost, which spares fuel.

It is very difficult to  adjust the inclination, change the apogee altitude and change the perigee altitude in one manoeuvre.

My favoured scenario is therefore that a first manoeuvre happened near 17:46 UTC in or near the ascending node (and near perigee). This lowered the apogee altitude from 440 to 417 km, and allowed a slight adjustment of the inclination at the same time. Half a revolution later, while passing through apogee near 18:25 UTC, a second manoeuvre was made to raise the perigee altitude from 265 to 321 km.

(click map to enlarge)

KH-11 USA 186 has stabilized its orbit

Note 15/09/2014 9:25 UT: corrected inadvertent apogee - perigee mix-up in 4th paragraph

USA 186 passing in early twilight of the evening of Sept 12, 2014
(click image to enlarge)

At the end of May, Northern hemisphere observers lost visibility of KH-11 Keyhole/CRYSTAL USA 186 (2005-042A) when the midsummer nights became too short. The orbital plane of the satellite was still drifting at that time, a process that started after a manoeuvre in mid-November 2013 (see earlier posts on this blog). The big question was, when that drifting would stop. I expected that when the satellite reached its new intended orbital plane it would manoeuvre into a stable sun-synchronous orbit again.

It now has done so, having manoeuvered probably on or near July 1. The orbital plane drift has stopped.

Kevin Fetter in Canada made a chance recovery of the satellite, the first post-summer glare-out sighting, on September 8: he was looking for another object and saw a "unid" in Low Earth Orbit pass through his field of view, that Cees Bassa was quick to identify as USA 186, in a new orbit. Over the next nights several other observers tracked it (including me on Thursday and Friday evening) yielding a first version of the new orbit it is in.

USA 186 passing close to Arcturus (top left) in the evening of Sept 11, 2014
(click image to enlarge)

The satellite has drastically lowered its perigee apogee by almost 500 km, and gently raised its apogee perigee by a few km. It is now in an approximately 265 x 440 km, 96.9 degree inclined orbit. This orbit is sun-synchronous again.

This means that the RAAN drift relative to the other satellites in the KH-11 constellation that had been going on since mid-November 2013, has stopped. It has finally settled at a RAAN distance of about 25 degrees from USA 245 (2013-043A), the primary West plane KH-11.

Comparing the new orbit to the old orbit suggests that the manoeuvre into the new orbit happened on or near July 1st.

In all, the satellite has kept itself pretty much to the expected scenario which I outlined on this blog in several posts in September and October 2013, e.g. here and here. Following the launch of USA 245 (2013-043A) into the primary West plane of the KH-11 constellation in August 2013, I had predicted that:

1) USA 186, at that time the primary West plane satellite, would migrate its orbital plane to the secondary West plane; 

2) USA 129, the extremely aged satellite in the secondary West plane, would be de-orbitted;

3) after a period of drifting, USA 186 would manoeuvre back into a sun-synchronous orbit again, stopping the RAAN drift, when reaching the intended plane location of the secondary West plane;

4) that in that manoeuvre it would drastically lower its apogee from near 1000 km to near 400 km and gently raise its perigee.

This all has basically happened. It differed on details with my predictions, but the bigger picture is pretty much as I anticipated.

What was somewhat unexpected, is that the satellite had its RAAN drift to a much larger distance with respect to the primary West plane (now occupied by USA 245) than I had anticipated. I expected 10, maybe 20 degrees. It turned out to be almost 25 degrees.

The perigee, although indeed raised, is slightly lower than I expected. The massive lowering of the apogee is exactly how I expected it to be however.

The current orbital plane makes it make passes near 8 am and 8 pm local time.

Meanwhile, there are indications that USA 245 (2013-043A) in the primary West plane has manoeuvered. Russell Eberst still observed it in it's last known orbit from Scotland on Sep 7. Then Bjorn Gimmle from Sweden observed an unknown object on Sep 10, that I suspect is USA 245 after a perigee raising orbital manoeuvre conducted between Sep 7 and Sep 10.

[note 14/09/2014: Mike McC identified Bjorn's object as a Russian r/b near decay]
[note 15/09/2014 9:25 UT: corrected inadvertent apogee - perigee mix-up in 4th paragraph]

Tracking USA 161

click image to enlarge

I am a bit behind with posting the image above taken a week ago, on 3 June 2014 . It shows the KH-11 Keyhole/CRYSTAL optical reconnaissance satellite USA 161 (2001-044A), which was recently recovered.

With new observations including mine, the orbit is now getting better defined. During the winter blackout, the orbit of the satellite appears to have been further circularized to a 389 x 391 km orbit, by a small perigee rise.

In the image above, another object is also visible: a Falcon 9 r/b, 2010-066K, at over 5000 km altitude at that time.

Sorting out the confusion: USA 161 and IGS 8R

Okay, so yesterday considerable confusion arose about the current orbit of USA 161 (see previous post). This was due to the (luckily shortlived) confusion between two objects: the real USA 161, and a Japanese spysat that was briefly mistaken by me for USA 161.

The object which I photographed during the night of May 30-31 and which Björn Gimmle photographed from Sweden on April 22, turned out to be not USA 161 but, as Cees Bassa pointed out, another classified object we had "lost" in the winter blackout: IGS 8R (2013-002C), a Japanese radar imaging satellite launched early 2013.

So the image below actually shows IGS 8R flaring brightly, not USA 161 as I initially thought:

click image to enlarge

Luckily, Russell Eberst in Scotland observed the real USA 161 on June 1. Together with the observation by Leo on May 23, this means we do have an idea of the current orbit of USA 161 now, although further refinement through more observations is necessary. What is clear, is that USA 161 still is in the same orbital plane it was in when we lost it in August 2013. It's RAAN difference with the primary East plane KH-11 USA 224 is still 20 degrees, and it's orbit is sun-synchronous and about 385 x 393 km (these are approximate values which are subject to change, as the current orbit is preliminary and needs some refinement with more observations). The current KH11 Keyhole/CRYSTAL constellation now looks like this:

The short-lived confusion of yesterday could arise because both objects (IGS 8R and USA 161) currently move in a similar orbital plane. This can be seen in the image below, where the IGS 8R orbit is yellow and the USA 161 orbit is light grey:

This is the kind of confusion that can arise when multiple objects who's orbit have not been recently updated, move in a similar orbital plane. It does not only happen to us amateur trackers: even the professionals at JSpOC sometimes confuse objects.

Actually, this situation ended positive with a double recovery: that of USA 161, and that of IGS 8R.

[AGAIN UPDATED] USA 161 recovered (?) - and new ideas on changes in the KH system

CORRECTION 1 June 2014: The object I observed on May 31 turns out to be NOT the KH USA 161, but one of the IGS objects, IGS 8R (2013-002C) which we had 'lost' in the winter blackout, just like USA 161. Their orbital planes happen to be very close (with a few tenths of a degree in inclination and a few degrees in RAAN) at the moment.

This renders the whole story below incorrect and hence moot.

Luckily, Russell Eberst did observe the real USA 161 last night (June 1) and his observations fit with Leo's observations from May 23. More in a new post later. (this new post is up now here)

ML, 01/06/2014

- STOP PRESS - (31 May 2014, 19:00 UT). Okay. Cees Bassa thinks last night's object is not USA 161. See here. So read the story below with caution: the jury is still out on all this.

Although it all was/is a bit confusing, it appears USA 166 (2001-044A), the secondary East plane KH-11 Keyhole/CRYSTAL has finally been recovered. [UPDATE: OR PERHAPS NOT???]. It had not been positively observed since August 2013 and over the past half year had been hiding in the northern hemisphere winter blackout.

USA 161 ?. IGS 8R last night

(click image to enlarge)

Initially, Leo Barhorst in Almere (NL) appeared to have recovered it on May 23. However, his observed object now turns out to not have been USA 161 but an unidentified other object.

Last night it was clear, after a very rainy week. Suffering from a bout of insomnia, I took advantage of the clear skies by conducting a 30 minute plane search, from 01:00 UT to 01:31 UT (May 31), using the EF 2.0/35 mm wide angle lens.

Just as I was about to abandon the effort at 01:30 UT, a bright and fast satellite appeared with a direction that was correct for USA 161 in a generally correct area of the sky. I managed to capture it on two images, the second one of which is depicted above. The object was about mag +2 when I first visually picked it up, and then becoming brighter as it produced a slow flare to mag -2 near 01:30:27 UTC. The image above shows the slow flare moment. It moved fast, producing long trails on the 10 second images with the 35 mm wide angle, so it evidently was near perigee.

The object could not be matched to any known object, but did appear to move in the general secondary East orbital plane of the KH-11. Visually, the slow bright flare it showed was quite typical for the KH-11 too. So I was (and am) fairly certain it is USA 161. Or not? Problem was, my observation was difficult to match with Leo's observation a week earlier...

After I reported my observations, Ted Molczan next managed to positively match my object to a UNID observation by Björn Gimmle in Sweden on April 22 (so over a month earlier), proceeding to fit a very reasonable orbit. The suggestion of this all is, that Leo's object from May 23 was something else, as it does not fit well with the other observations.

The preliminary orbit calculated by Ted, which needs to be refined by further observations, suggests that USA 161 made a manoeuvre into a (compared to the orbit it was last seen in in 2013) slightly higher orbit of 411 x 425 km, with the orbital inclination changed by half a degree to 97.52 degrees. This is the same orbital inclination as USA 129 (1996-072A), the former secondary West plane KH which we have recently "lost" and which is suspected to have been de-orbitted, was orbiting in.

Like the drifting secondary West plane KH USA 186 (2005-042A, see several previous posts on this blog for a discussion), USA 161 is no longer sun-synchronous as a result of this manoeuvre, and hence in what appears to be a plane transfer orbit. Its distance in RAAN relative to the primary East plane KH-11, USA 224 (2011-002A), has increased and will keep increasing steadily until it makes a corrective manoeuvre (which I suspect will happen near June 12, see below).

The orbital constellation for May 31 looks like this:

(image removed)

click image to enlarge

The current situation is that the two new primary plane objects, USA 224 and USA 245, are keeping a more or less steady plane distance (in terms of RAAN) of 48.5 degrees. The two (older) secondary plane satellites, USA 186 and USA 161 however have now both lost sun-synchronicity, and are both drifting outwards with respect to the orbital planes of their corresponding primary plane satellite.

Frequent readers of this blog will remember that I initially expected USA 186 to manoeuvre back into a non-drifting sun-synchronous orbit when the RAAN difference with USA 245 was 10 degrees, early February. That didn't happen. As a secondary option, I then thought it would manoeuvre when the difference was 20 degrees, early May. That didn't happen either. I then was at a loss as to what "they" were planning to do with USA 186.

Now USA 161 has been recovered and turns out to have been manoeuvered into a drifting plane-changing orbit as well, just like USA 186, I am getting a possible idea again about what they might intend. Please note: I have been wrong twice before, so my track-record in these kind of predictions is not quite good :-p

Nevertheless: assuming that symmetry is what is being aimed for, I think both objects (USA 186 and USA 161) will manoeuvre back into a non-drifting, sun-synchronous orbit on or near June 10 to June 12.

On May 31 the RAAN difference between USA 245 (primary West) and USA 186 (drifting secondary West) was 23.0 degrees. The RAAN difference between USA 224 (primary East) and USA 161 (drifting secondary East) was 23.7 degrees. Their rate of drift is different: it is -0.11 degrees/day for USA 186 and +0.056 degrees/day for USA 161 (i.e., USA 186 is drifting twice as fast as USA 161). These differential drift rates mean that at some point in time, both satellites will reach a matching value in RAAN difference with their primary partner, i.e.be at similar RAAN distances from their primary partner on the same day. This is depicted in the diagram below:

(image removed)
click diagram to enlarge

What can be seen from the diagram, is that this moment will occur in about 10-12 days from now, near June 10-12. On June 12, both satellites (USA 186 and USA 161) will reach a matching difference in RAAN of 23.3 degrees with their primary partners USA 245 and USA 224. Interestingly, this value is very close to 0.5 times the RAAN difference between the primary plane satellites, USA 224 and USA 245 which are 48.5 degrees apart in RAAN.

But please be advised: until now, I was wrong each time I thought I could make sense of it....

USA 224 recovered, USA 186 still drifting, and looking for GPS IIF-6 20 minutes after launch

For various reasons, I am a bit late in keeping the reader up to what is happening to the KH-11 Keyhole/CRYSTAL system of optical reconnaissance satellites.

USA 186 (2005-042A), the secondary West plane KH-11, is still in a non sun-synchronous orbit and hence still drifting westwards. It is drifting for over half a year now. The difference in RAAN with USA 245, the primary West plane KH-11, is now over 20 degrees (21.8 degrees on May 19th). I am very curious as to when the drifting will stop, if ever. If it continues to drift for many weeks to come, we should contemplate whether perhaps the satellite is "dead", i.e. has lost manoeuverability. Problem is that NW European observers temporarily have lost visibility of the satellite, due to the current short nights. Tracking all comes down now to observers in the US and southern Europe.

Meanwhile, Russell Eberst in Scotland recovered USA 224 (2011-002A), the primary East plane KH-11, on May 9th. It is in a 260 x 1006 km orbit, which means it has slightly lowered its apogee. Before the winter blackout it was in a 258 x 1023 km orbit. The difference in RAAN with USA 245, the primary West plane KH-11, is now 48.5 degrees.

My own first observation of USA 224 was in the night of May 16-17. The image below shows it crossing through Corona borealis:

click image to enlarge

USA 161 (2001-044A), the secondary East plane KH-11, has still not emerged out of the winter blackout. Meanwhile, USA 129 (1996-072A) has gone missing since April 24 (see a previous post). There is a good chance it has been de-orbitted.

The current KH-11 constellation now looks like this (where the current orbital configuration of USA 161, in red, is uncertain, and USA 129 left out as it is no longer in its old orbit, and presumed de-orbitted):

click images to enlarge

In the early morning of May 17 (evening of May 16 in the US) and after a one day delay due to bad weather, a new GPS satellite, GPS II-F6 was launched from Cape Canaveral on a Delta IV rocket. It would pass over the Netherlands some 20 minutes after launch, still ascending and still attached to the 2nd stage. A number of search orbit had been published, but it looks like none of these was very accurate. I visually observed a bright UNID near 00:24:00 UT (May 17) moving just a few degrees to the 'right' of Altair on a trajectory parallel to the predicted ones but some 20 degrees cross-track in a southern direction. It was already descending over the roof when I picked it up, so I had no time to snap a picture alas. It did not match any known object so I am quite confident it was GPS II F-6 on its way to orbit. It was bright, about mag +1 to 0.

KH-11 USA 129 is missing, USA 186 has still not manoeuvered

USA 129 (96-072A), the oldest of the KH-11 Keyhole/CRYSTAL/KENNAN optical reconnaisance satellites, has gone missing. The last observers to see it were me on April 22 and Russel Eberst on April 24. The photo below shows one of my images from April 22, with USA 129 passing near Castor and Pollux:

click image to enlarge

Somewhere between that date and May 1, when various observers noted it missing, it disappeared.

There is a possibility that it has been de-orbitted, as it is over 17.5 years old now and appears to be 'redundant' after the launch of USA 245 and plane move of USA 186 (see various earlier posts on this blog). On the other hand, we should be cautious and not too hasty: in the recent past (Feb 2014) we erroneously wrote USA 129's eulogy before, and it turned out it had just manoeuvered. Maybe it did this time as well. A dedicated plane watch I did in the evening of 3 May between 20:41 -21:05 UT yielded nothing.

Meanwhile, we had expected USA 186 (2005-042A) to manoeuvre early May. But up to yesterday May 4th it hasn't. Maybe it will do in the coming days. On May 1st the difference in RAAN with the main West plane KH, USA 245, was 19.8 degrees. At a drift rate of 0.11 degrees/day, it reached 20 degrees the past weekend. If it hasn't manoeuvered by the end of the coming week, it will become interesting. Unfortunately, it is disappearing in evening twilight for my location these days.

The image below shows USA 186 crossing Canis minor in deep evening twilight of May 2nd:

click image to enlarge

I imaged USA 245 (2013-043A), the current main West plane KH-11, last Saturday evening. In evening twilight, it was visible in the same camera field with FIA Radar 1 (2010-046A):

click image to enlarge

An update on the evolving KH-11 Keyhole/KENNAN/CRYSTAL system

In several previous posts from September 2013 onwards, I analysed and prognosed changes to the KH-11 Keyhole/KENNAN/CRYSTAL optical reconnaissance satellite system. Main focus was on the changes, notably in the orbit of USA 186 (2005-042A), following the addition of USA 245 (2013-043A) as a replacement for USA 186 in the Keyhole primary West plane.

Indeed, as predicted, USA 186 was moved out of the primary West plane, and started to drift towards the secondary West plane in mid-November 2013 following a manoeuvre that changed its inclination and made it loose sun-synchronisity.

USA 186 is still drifting westwards. It currently is moving westward at a rate of 0.11 degree/day relative to USA 245 (2013-043A), the primary satellite in the West plane since the autumn of 2013. The difference in RAAN between the two satellites currently is slightly over 16 degrees:

 click images to enlarge

Late 2013 and early 2014, I expected that the drift would end early February 2014, when the RAAN difference between USA 245 and USA 186 reached 10 degrees, the old separation between USA 186 and USA 129, the former primary and secondary West plane satellites. I was proven wrong with my prediction, when USA 186 continued to drift past that date.

I then turned to a second scenario: that the intended RAAN difference between USA 245 and USA 186 will be 20 degrees, similar to the difference between USA 224 and USA 161 in the East plane.

With the current rate of precession of 0.1111 degrees/day relative to USA 245, the RAAN difference between USA 186 and USA 245 will reach 20 degrees near May 1. So I expect USA 186 to manoeuvre into a lower, sun-synchronous orbit on or near that date. The KH-11 constellation will then look like this:

click images to enlarge

An open question is what will happen to USA 129 (1996-072A), the over 17 year old former secondary West plane satellite. I still think it will be de-orbited somewhere this year. But I have been wrong before.

Note: the orbits for USA 129, USA 186 and USA 245 in the West plane in the images above are based on recent observations. The East plane satellites USA 161 and USA 224 have not been observed since autumn 2013, and will not be visible from the Northern hemisphere before mid-April. There is therefore a possibility that one or both have moved, but since we have no information on their current whereabouts, I have depicted them in their 'old' orbital planes of the autumn of 2013 relative to the West plane.

A flare show by USA 245

Wednesday evening saw a dynamic atmosphere, where clear skies intermittently were broken by fields of fast moving scattered clouds.

I targeted all three West plane KH-11 Keyhole/CRYSTAL satellites that evening: USA 129 (1996-072A), USA 186 (2005-042A) and USA 245 (2013-043A).

USA 245 had a pass during a period where patches of clouds occupied the southern sky up to the zenith. The image below shows it rising through Leo, over the roof of my house, amidst scattered clouds:

click image to enlarge

USA 245 twice flared brightly during this pass. Such flares are caused when a reflective surface on the satellite (solar panels, an antenna panel, a dish or some other part of the structure) acting like a mirror reflects sunlight directly towards the observer. I love it when they do that, it is always spectacular.

The first flare was a slow bright one up to magnitude -2 peaking near 20:27:28 UTC (26 March) just above the back of Leo:

click image to enlarge

One-and-a-half minutes later at 20:29:03.64 UTC, while passing through Uma, it flared again, a more brief glint/flare to magnitude -1:

 click images to enlarge

USA 129 was also observed, low in the west while emerging from earth shadow. It only faintly registered on one image, allowing one position determination. USA 186 was not seen, but this was likely due to unfavourable circumstances (it was too faint for the lens used) as Cees Bassa did observe it, on-time, the next day.

First post winter-blackout observations of KH-11 Keyhole/CRYSTAL USA 129

One by one the KH-11 Keyhole/CRYSTAL optical reconnaissance satellites are emerging from the Northern hemisphere winter black-out. Following USA 186 (2005-042A) on Feb 21, and USA 245 (2013-043A) on March 3, USA 129 (1996-072A) has now been recovered as well on March 21.

USA 129 was last seen by Greg Roberts in South Africa on Feb 12.  It had hence not been seen for over a month until it was recovered by me and Leo Barhorst last Friday evening. Below is my recovery image:

recovery image of USA 129, 21 March 2014

click image to enlarge

This means all three objects in the West (evening) plane have now been recovered. Recovery of the two East (midnight) plane objects (USA 161 and USA 224) will have to wait until mid-April.

I went to my secondary site in the Cronesteyn polder in the evening of March 21, with the explicit aim of recovering USA 129. I had aimed the camera, with the 1.4/85mm lens, at 33 degrees elevation, at a point covering the orbital plane of the satellite where it should just have emerged out of earth shadow.

And it did, 104 seconds late and 0.8 degree off-track from the orbital elements based on Greg's last observation of Feb 12. Running a series of images for several minutes, I snapped it on two images.

In Almere (also in the Netherlands), some 55 km Northeast of me, Leo Barhorst also captured it using his WATEC camera, and reported it as an "unknown".

Over the coming days, we hope to capture more positions as the satellite gradually becomes more visible.

click image to enlarge

That same evening, I photographed two other classified objects: USA 186 (2005-042A, above, with two strays Russian rocket boosters in the image as well) and SAR-Lupe 5 (2008-036A), a German military Synthetic Aperture Radar satellite.

click image to enlarge

SAR-Lupe 5 was captured by accident, when I was making a test image to check lens focus. Two satellites showed up in the test image: the very old Russian weather satellite Meteor 1-5 (1970-047A) and SAR-Lupe 5. As I had not carefully timed this image (I did not expect a classified object to be in it), Meteor 1-5 provided a welcome time calibration for the image. SAR-Lupe 5 was 57 seconds early and 1 degree off-track relative to 10-day old orbital elements.

USA 245 now also recovered

One by one the KH-11 KENNAN/CRYSTAL ("Keyhole')  optical reconnaissance satellites are emerging from their winter blackout on the Northern hemisphere (see a previous post). After USA 186 (2005-042A) in the secondary West plane late February, now USA 245 (2013-043A), the new primary West plane satellite, has been recovered.

USA 186 in the secondary West plane imaged on March 11

(click image to enlarge)

During wintertime the KH-11's are not visible from the Northern hemisphere: they are eternally eclipsed by the earth shadow during these months. Only late February/Early March they reappear. Observations during the Northern hemisphere winter season solely rest on the shoulders of our single Southern hemisphere observer, Greg in South Africa. He starts to lose sight of the West plane KH's late January to early February.

I did a failed attempt to recover USA 245 on March 3, continuously imaging its last known orbital plane for 19 minutes (10 minutes before and 10 minutes after the nominal pass time). Nothing was seen. I should have kept on the attempt longer: Russell Eberst in Scotland was more lucky that evening and got a single position. It turned out it was even more late (36 minutes) than the time period covered by my attempt.

The next person to observe it was Cees Bassa in the Netherlands on March 8. The next day, March 9, it was finally my turn and I could do my first post-winter-blackout observations! Several other observers (e.g. Jon Mikkel in Spain, Leo Barhorst in the Netherlands) also picked it up around this date.

I was nearly fooled on March 9 by a bright object that appeared about two minutes before USA 245, moving along a similar track. It turned out to be the French optical reconnaissance satellite SPOT 4 and not USA 245 being too early. Luckily, I kept photographing and soon captured the real USA 245.

The visibility of USA 245 has since been rapidly increasing. The satellite is currently already making zenith passes for my location. The image below I shot on March 11:

USA 245 in the primary West plane imaged on March 11

(click image to enlarge)

The hunt is now on for the third of the West plane objects, USA 129 (1996-072A) which should be slowly emerging from invisibility, at first only very low in the North, but soon higher in the sky during the last week of March. It was manoeuvered into a much lower orbit near January 27 (see a previous post). It will be interesting to follow it, as I still suspect it to be de-orbitted, perhaps later this year.

The East plane midnight KH-11's USA 161 (2001-044A) and USA 224 (2011-002A) will start to emerge from darkness about mid-April and have currently not been seen for several months.

Imaging USA 186 after the winter blackout

In yesterday's post I mentioned that while the southern hemisphere window on the evening Keyhole/Crystal satellites ended early February, for us in the northern hemisphere it is just starting. After Greg in South Africa did the last southern hemisphere observations of USA 186 (2005-042A) on Feb 12, Cees in the Netherlands did the first northern hemisphere observations on Feb 21.

Yesterday evening (Feb 26) I did my own first post winter-blackout observations of USA 186. I captured it on several images, including the one below which shows the satellite shortly after merging from eclipse on 30 degrees altitude in the N-NW:

click image to enlarge

As current passes at 52 N are still restricted to visibility very low in the northern sky, I could not target the satellite from my regular town center location (which has obstruction by buildings in the north). I therefore did a short bicycle trip to a spot 2 km southeast from my home, in the Cronesteyn polder on the eastern outskirts of Leiden. Visibility is horizon to horizon there.

As I expected the satellite to be faint this low in the N-NW sky, I used the 1.4/85 mm lens instead of the 2.5/50 mm lens I normally use on the KH-11 satellites. The satellite registered well on the images, and was some 10.3 seconds early on a 5-day-old elset. It is evidently still drifting in RAAN (see previous post). As visibility improves over the coming weeks, it will be interesting to follow it.

I also targetted some parts of the geostationary belt, but have not come yet to measuring those images (probably this weekend).

If weather cooperates the coming week, I will return to this observing spot to try to recover the new primary West plane KH USA 245 (2013-043A), which hasn't been seen since Greg's observations of January 11, i.e. for almost two months. USA 129 is not visible from 52 N yet.

USA 186 is defying the schedule

Over the past months I have posted a number of analysis and prognosis with regard to the likely changes to the KH-11 Keyhole/CRYSTAL constellation of optical reconnaisance satellites, following the launch of USA 245 (NROL-65, 2013-043A) into the KH primary West plane on August 28, 2013. The most important of these posts can be found here (Sep 16, 2013), here (Oct 12, 2013) here (Dec 22, 2013), and here (Feb 1, 2014).

One of my predictions was that USA 186 (2005-042A) would be moved from the primary West plane to the secondary West plane, 10 degrees west in RAAN from the primary plane.

Indeed, it initially seemed to keep to my prediction as mid-November 2013 USA 186 made a manoeuvre that involved a 1-degree inclination change. As a result it lost its sun-synchronous precession rate and started to drift westward relative to the other KH-11 satellites, moving orbital plane out of the primary West plane towards the secondary West plane. Its precession rate was such that it would reach a 10-degree difference in RAAN with the new primary plane satellite, USA 245 (2013-043A) near Feb 6. I therefore expected USA 186 to manoeuvre near that date, a manoeuvre that should entail an orbit circularization including a significant lowering of the apogee (after which the orbit would be sun-synchronous again and the westward drift would stop). So as Feb 6th neared, we held our breath.

And nothing happened. USA 186 did not manoeuvre.

It is still drifting westwards, at a rate of  0.12 degrees/day relative to the other KH-11 satellites. My prediction failed.

click images to enlarge

Greg Roberts in South Africa did a good job in tracking USA 186 right up to February 12. As his southern hemisphere summer observing window was coming to an end, he could no longer follow it after that date. Luckily, it is coming in reach of northern hemisphere observers, and Cees Bassa in the Netherlands picked it up on February 21 with the first Northern hemisphere observations of 2014.

Now USA 186 has not manoeuvered, it is time to entertain my alternative scenario which I presented near the end of this post on Dec 22 and this post on Feb 1.

That alternative scenario is that the drift will continue until the difference in RAAN between USA 186 and USA 245 amounts to 20 degrees (instead of 10 degrees). This is a RAAN difference similar to that between the primary and secondary East plane satellites, USA 224 and USA 161. It would create a 90-degree angle in RAAN between the outermost, secondary East and West plane satellites (USA 161 and USA 186).

At the current drift rate, these values will be reached early May.

It is clear that the current, drifting orbit of USA 186 is not an intended end state. The orbit is not sun-synchronous, a must for an optical reconnaissance satellite. The inclination change it made mid-November 2013 is such that a manoeuvre into a ~380 x 400 km orbit similar to USA 161 in the secondary East plane will restore a sun-synchronous precession rate. So that appears to be the intended goal in the future. The current non sun-synchronous orbit is meant to let the RAAN drift up to a desired value. The question now is, what final RAAN value relative to the primary plane is intended.

My guess, now it has turned out to be not 10 degrees, is 20 degrees.

Meanwhile, another question is what they intend to do with the "old" secondary West plane satellite, USA 129 (see the post here).

USA 129 does a Mark Twain!

As I wrote in a previous post last week, the KH-11 Keyhole/CRYSTAL optical reconnaisance satellite USA 129 (1996-072A) had not been seen since January 27. We therefore presumed it might have been de-orbitted, as it is very old (17+ years, the longest operational lifettime of any of the KH-11's) and USA 186 is to take over its former orbital plane in two days from now.

We were wrong: USA 129  is still alive!

Greg Roberts observed it from S-Africa last night, after a dedicated 2.5 hour plane scan, and observed it over two passes. A quick fit to his observations by Ted Molczan suggests that the perigee of the satellite might have been brought significantly down, to 240 km (was 310 km). More observations are needed to say anything more about this.

This is something new. I really did not expect USA 129 to manoeuvre into a new orbit.

It should be noted that Ted Molczan already had a hunch about this: as USA 129 was running a bit late when Greg observed it on Jan 27th, Ted felt this could indicate it had made a manoeuver in the hours just prior to Greg's observation. It is now clear he was right: kudos to Ted!

The question now is: what does it mean? Do they have some new purpose for the satellite? At its age of 17+ years, that would be amazing! This is option #1 and perhaps the preferred option. The new orbit appears to be sun-synchronous, which is preferable for an operational optical reconnaissance satellite.

Or is this all in preparation for a de-orbit later (option #2)?

If they are running low on juice for example, they might have opted to bring the perigee of USA 129 down as far as possible and next use natural decay to bring down USA 129 even more, to say 150 km, and then do a final de-orbit burn (option #3). This is a scenario a bit similar to what NASA did with UARS in 2011 (except that they could not do a final de-orbit burn and had it re-enter uncontrolled, something which I don't expect for USA 129). But that is (extremely) wild speculation.

It will be interesting to see what happens with USA 129 the coming days, weeks and months.

Another interesting moment will be reached in two days from now: will USA 129's younger sister ship USA 186 (2005-042A), indeed be boosted into a more circular orbit, as I expect?

[UPDATED] USA 129 de-orbitted [NO!]? And USA 186 about to manoeuvre?

UPDATE 04 Feb 2014: USA 129 was NOT de-orbitted! Greg recovered it on Feb 3. It appears to have manoeuvered into an orbit with a much lower perigee. More here.

USA 129, the oldest orbiting member of the KH-11/CRYSTAL optical reconnaissance satellites, appears to have been de-orbitted during the past week.

(click image to enlarge)

USA 129 on 28 September 2013
RIP ?

On January 27, Greg Roberts in South Africa observed USA 129 (1996-072A) and USA 186 (2005-042A), two of the west plane KH-11 satellites. When he observed again on January 30, USA 129 was gone. He could not spot it on two good passes that evening.

This non-observation raises the serious possibility that USA 129 has been de-orbited somewhere between Jan 28 and Jan 30, 2014. [Update 4 Feb 2014: it was not!]

A de-orbit fits into expectations. In September and October, I published a number of analytical posts on the past and future of the KH-11 KeyHole/CRYSTAL constellation. They detail how I think/thought the constellation of KH-11 satellites will be re-arranged following the lauch of a new satellite, USA 245 (2013-043A, NROL-65), into the primary West plane of the constellation on August 28, 2013. The two most pertinent of these posts are the ones here and here.

So far, my predictions seem to have been quite in line with what consequently actually happened. I suggest that this week will see the closing overture of this spatial spy satellite ballet.

I earlier predicted that after a few months of checkout of the newly launched USA 245, the older USA 186 would be moved from the primary West plane to the secondary West plane, by shifting the RAAN of its orbit 10 degrees more westward. In doing so it would take up the position formerly filled by USA 129 during its extended mission. I also predicted that USA 186 will at some point drastically lower apogee, slightly raise it perigee, and circularize it's orbit. I in addition expected USA 129, which was over 17 years old, to be de-orbitted near the moment those goals were attained.

The latter (the de-orbit of USA 129) seems to have happened in the past few days.

So far USA 186 has also been keeping to the plan. In mid-November 2013 (on or near 12 November), USA 186 made a manoeuvre that changed its inclination by 1 degree (see my post here), causing the satellite to temporarily lose sun-synchronisity and causing it to gradually drift in RAAN from the primary West plane towards the secondary West plane. It is nearly there now. At the current drift-rate (delta 0.12 deg/day relative to the sun-synchronous drift value of the other KH-11 satellites), it will reach the former orbital plane of USA 129 and a 10-degree separation in RAAN from USA 245 (now the sole satellite in the primary West plane) within a week from now, on February 6, 2014.

click image to enlarge

The image above shows how as a result of the Nov 12 manoeuvre, the orbital plane of USA 186 gradually drifted (and as of this writing on Feb 1 still drifts) from the primary West plane to the secondary West plane between November 12, 2013 and February 6, 2014. This is exactly what I predicted to happen back in September and October.

(in the images above, the grey line is the orbit of USA 245, the white that of USA 186, and the red that of USA 129)

The next step is that I expect a large manoeuvre by USA 186 on Thursday February 6th, in which it lowers it's apogee (currently at 975 km) to ~390-400 km, and slightly raises its perigee (currently at 260 km) to ~380-390 km, attaining a much more circular and on average lower orbit with eccentricity close to 0.00055 (currently 0.05) and Mean Motion near 15.59 revolutions/day.

The current orbital inclination of  96.91 degrees is already very close to the 96.99 degree value with which such a 390 x 400 km orbit is sun-synchronous. Lowering apogee and perigee to these values hence would restore a sun-synchronous orbit and stop the drift in RAAN relative to the other satellites in the constellation. As such, the 1 degree inclination change in the satellite's orbit introduced mid-November might be a strong clue that indeed a 390 x 400 km orbit (similar to USA 161, in the secondary East plane) is intended.

click image to enlarge

The image above shows the KH-11/CRYSTAL constellation as of 28 January 2014, and excluding USA 129 which was de-orbitted on or very shortly after that date. The small yellow arrow perpendicular to the orbital plane of USA 186 indicates that I expect it to shift by an extra 0.6 degrees over the coming week.

The image below shows how the constellation will look like after the apogee-perigee changing manoeuvre which I expect USA 186 to make on Feb 6. Note the lower, more circular orbit of the latter compared to the image above:

click image to enlarge

click image to enlarge

As a caveat, there is a very small, alternative possibility that USA 186 will not manoeuvre on Feb 6th. In that case, it will keep drifting another 2.5 months untill the RAAN (orbital plane) difference with USA 245 reaches 20 degrees (and the RAAN separation of the outermost, secondary E and W planes 90 degrees). My hunch is however that this will not happen, and USA 186 will manoeuvre on Feb 6th into a sun-synchronous orbit with RAAN 10 degrees from the RAAN of USA 245, as explained above.

Note: many thanks to Greg Roberts, South Africa, for keeping an eye on the KH-11 satellites during the Northern hemisphere winter blackout!

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).

Past and future of the KH-11 Keyhole/Evolved Enhanced CRYSTAL constellation (part 4)

In a number of previous posts from the last month (this one being the most pertinent one), I probed the changes to the KH-11 Keyhole/CRYSTAL optical reconnaissance satellite constellation over the past 8 years, aiming to predict what will happen now USA 245  has been added to the constellation on 28 August 2013 (launch NROL-65).

The previous analysis was focussed on the orbital planes of the satellites. In this fourth post in this series, I will take a look at other orbital parameters, such as apogee and perigee heights, eccentricity and mean motion.

West plane KeyHole/CRYSTAL satellites:

 USA 129: launched in 1996,
now in secondary West plane, 
probably soon to be de-orbited?

(imaged 28 Sep 2013)

 USA 186: launched in 2005,
soon to switch from primary West plane to 
secondary West plane?

(imaged  5 October 2013)

USA 245: launched 28 August 2013
into the primary West plane

(imaged 5 October 2013)

Let me first briefly summarize the previous analysis. In these I showed that the KH-11 constellation consists of two primary orbital planes separated by 48-50 degrees in RAAN. In addition, each primary orbital plane has an accompanying secondary orbital plane, 10 degrees more west for the West plane and 20 degrees more East for the East plane.

Satellites are initially launched into one of the primary planes, in their primary mission: after a couple of years, and after a replacement has been launched into the same orbital plane, they shift to the accompanying secondary plane, going from primary mission into secondary extended mission.

For example, USA 129 did this in 2006 after the launch of USA 186; and USA 161 did this in 2011 after the launch of USA 224. I pointed out that I expect USA 186 to do the same early 2014 following the recent launch of USA 245 into the West plane. I also expect USA 129 to be de-orbitted.

The graphic summaries given in that previous post, were these two images (see previous post for discussions):

Shifting from primary to secondary orbital planes is however not the only thing that happens. When we look at various orbital parameters, we can see other, accompanying patterns, notably in the apogee and perigee heights:

(click diagrams to enlarge)

(note: all the orbital parameters used in the diagrams above have been determined by Mike McCants from amateur observations, including mine).


New plane, lower apogee altitudes, and more circular orbit

For example: in the previous post on this topic it was discussed how USA 161 (2001-044A) in the East plane manoeuvred from the primary East plane to the secondary East plane late 2011 by changing its RAAN by 20 degrees (i.e., by rotating its line of apsides). This followed the launch of USA 224 (2011-002A) into the primary East plane, as a replacement for USA 161.

In the diagrams above, we can see that other orbital changes took effect as a result of the same series of manoeuvres. In addition to its orbital plane, USA 161 (blue dots in the diagrams) also changed its orbital eccentricity and its apogee and perigee heights. The apogee height was significantly lowered (which initially confused analysts at the time), from about 960 km to eventually about 390 km altitude. The perigee height was raised somewhat, from 310 km to 390 km altitude. The result is a much more circular orbit.

The inclination of the orbit was also changed, by about one degree. The reason for this can be seen in the lowermost diagram: with the changes in apogee and perigee altitudes, the orbital inclination had to be changed to make the resulting orbit sun-synchronous again.

In all, although much of this was accomplished within 6 months after the massive manoeuvre of late August 2011, it took USA 161 about a year to settle in its new orbit.


A repeat of an earlier case

Earlier, in 2006-2007, changes in the orbit of USA 129 (1996-072A) in the West plane can be seen to follow a somewhat similar pattern.

After the launch of USA 186 (2005-042A) into the primary West plane in 2005, USA 129, by that time already 10 years old and hence quite of age, moved to the secondary West plane by changing its RAAN by 10 degrees. Accompanying this move, is a change in perigee and apogee altitudes. The perigee is gently raised from about 280 km to eventually 310 km altitude. The apogee is lowered from about 1020-1030 km to eventually about 770 km altitude. The orbit becomes much more circular as a result.

With USA 129, this process took much longer than with USA 161 and the changes are less drastic. Yet the ideas behind them are clearly similar to what USA 161 did five years later: change orbital plane from primary to secondary plane, lower apogee significantly, raise perigee gently, and circularize the orbit (although not to the degree like USA 161 later did).

The more gentle approach taken by USA 129 in 2006-2007 compared to USA 161 in 2011-2012 might implicate either of these two scenarios:

(a) USA 129 had less fuel reserves left in 2006 than USA 161 had in 2011;

... or (and I prefer this explanation):

(b) it was anticipated in 2006 that the lifetime of  USA 129 needed to be prolonged untill well after the initial lifetime estimates, putting restrictions on fuel use for manoeuvres.

Remember: this is around the time the KH-11/CRYSTAL follow-up program, the FIA Optical program, entered delays and was next cancelled. So option (b) could well be the case.


What to expect?

Based on these past patterns, I expect USA 186 to do the following things by means of  a series of manoeuvres starting the first months of 2014:

1) change RAAN by 10 degrees (i.e. rotating its line of apsides), moving itself from the primary West plane into the secondary West plane (see previous post here);

2) drastically lower apogee (currently at about 1020 km) to about 390 km altitude;

3) gently raise perigee (currently at 260 km) to about 390 km altitude.;

4) circularize its orbit as a result of (2) and (3);

5) change inclination by about one degree to re-attain sun-synchronicity after the altered apogee and perigee altitudes.

These changes should take a few months and be completed towards the end of 2014. They will likely be initiated by a large manoeuvre early 2014 (in February or March likely).

As mentioned earlier I expect USA 129 to be de-orbited this winter or spring.


Why the apogee and perigee changes?

One question pertaining is: why these changes in perigee and notably apogee? Is a circular ~390 x 390 km orbit easier to maintain? Is there instead some operational reason behind this change in altitudes, in terms of desired track-repeat intervals or equipment performance (e.g. demands of image resolution)? If  so, why are similar changes not made to the orbits of the primary plane objects but only to the secondary plane, extended mission objects? I have no answers, and at best I can speculate from a few ideas I have. That is not for this blog, however.


This post benefitted from discussions with Ted Molczan and Cees Bassa. Interpretations and any errors theirin are mine.