Thursday, 31 March 2016

USA 186 rising, and revisiting the KH-11 constellation

click image to enlarge

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:

USA 128 186 flaring near 19:17:55 UT (March 28, 2016)
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:

click to enlarge

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.

Tuesday, 29 March 2016

Hitomi (ASTRO-H) scientific X-ray satellite suffered an orbit-altering event

On 26 March 2016 near 1:42 UT, the Japanese Hitomi/ASTRO-H satellite, a scientific Röntgen space telescope launched February 17, suffered a clearly energetic event that changed its orbit and resulted in a communications loss. JSpOC reported the release of at least 5 fragments from the spacecraft. The existence of these fragments was first detected by US tracking stations near 8:20 UT. Observations by amateur satellite observers reported on Seesat-L (here, here, here) suggest that the spacecraft is tumbling with a periodicity of about 23.5 seconds (flashes each ~11.75 seconds) according to a preliminary analysis by Ted Molczan here.

click to enlarge map

Analysis of the pre- and post-event orbital elements suggests that the event occurred near 1:40-1:50 UT on March 26 (see note at end of post (*). JSpOC estimates the event occured at 1:42 +/- 11 m UT. The satellite was just past perigee and had just passed the ascending node of its orbit at that moment, moving over Meso-America (see map above). The nominal time for the event, 1:42 UT, is about 12 minutes past nodal crossing.

The Japanese Space Agency JAXA reports that since noting loss of communications at 7:40 UT on March 26, they have managed to receive signals from the spacecraft twice, near 13h and 15:30 UT on March 28.

The event was energetic enough to alter the orbit of the satellite, slightly increasing its eccentricity and dropping slightly in altitude: it's semi-major axis changed by 2 km, with perigee dropping by 4.5 km. The inclination was changed too, by about 0.0045 degrees.

(click diagrams to enlarge)
data source: Space-Track

Based on the change in semi-major axis and inclination, and if my calculations are correct, the orbit change of ~2 km in semi-major axis and ~0.0045 degree in inclination is the result of a velocity impulse with delta V of about 1.2 m/s. The most likely source is a sudden venting or a small explosion of some sort. An explosion of some sort also explains the discharge of five fragments reported.

The time of the disruptive event is almost exactly 6 hours before the satellite was to turn operational.

How serious the event is, is unclear at the moment. The fact that the spacecraft still appears to be sending signals is a positive aspect. If JAXA can re-establish contact and stop the tumbling, perhaps the mission can be saved. Or perhaps not, depending on the damage to the spacecraft.

* Note: my initial analysis of pre- and post event orbital elements on March 28 yielded times near 1:40 UT (March 26), close to JSpOC's nominal 1:42 UT time for the event. Annoyingly, I had some odd brain malfunction next and tweeted "4:40 UT" instead. 

It was probably induced by a mix-up in my mind of the established 1:40 UT event time with the epoch time of the first abberative TLE, which was16086.196059 = 26 Mar 4:42 UT. I had jotted both times down and then mixed them up apparently...such things happen when you spent too much time coding and processing data in an excel spreadsheet.

The way in which I analyzed the probable time of the event, is by taking pairs of pre- and post-event elsets for the satellite and treating them as if they concerned two separate objects. I then used Rob Matson's COLA software to establish close encounters between the "two" objects: near the time of the event, the orbital positions from both elsets should nearly coincide. Depending on which elsets you pair, this yields times scattering between 1:30 and 2:06 UT . My initial test of four elset pairs had three of them yielding times near ~1:40 UT.

Saturday, 12 March 2016

MUOS 4 recovered at 75 E

MUOS 4 at 74.8 E on 8 March 2016
image (c) by Greg Roberts, South Africa
(click to enlarge)

In a recent post I wrote that MUOS 4 (2015-044A) had left it's check-out position at 172 W near Hawaii late January, drifting westwards. I presumed that it was being moved to its assigned operational spot at 75 E, over the Indian Ocean just south of India.

This is now confirmed. On 8 March 2016, Greg Roberts from Cape Town, South Africa, recovered MUOS 4 at 74.8 E. Greg's recovery image (used with his permission) is above. As I wrote before, it probably arrived there on 29 February 2016, after a 37-day drift westwards at a rate of ~3 degrees/day.

With four satellites at their operational positions, the MUOS constellation is now complete.

MUOS 1   2012-09A       177 W  Pacific
MUOS 2   2013-036A      100 W  CONUS
MUOS 3   2015-002A     15.8 W  Atlantic

MUOS 4   2015-044A       75 E  Indian

However, one more MUOS satellite will be launched. This fifth satellite will be parked at 72 E and will function as an on-orbit spare, in case one of the other four MUOS satellites malfunctions on-orbit.

(click to enlarge)

As can be seen in the illustrations above, the MUOS satellites are separated by ~90 degrees in longitude, but with a slightly bigger gap (~108 degrees) between MUOS 1 and MUOS 4, a gap representing the Pacific. The latter is probably in order to assure access to/from at least two ground facilities, with Hawaii and California serving MUOS 1. The latter would not have been possible with MUOS 1 at ~90 rather than 108 degrees from MUOS 4. MUOS ground facilities are indicated by yellow squares in the map above.

Friday, 11 March 2016

Imaging a "UFO" (Ultra High Frequency Follow-On)

UFO F2 on 3 March 2016
(click image to enlarge)

The image above is my first image of a UFO...

(* cue X-Files tune *)

No need to call in Mulder, however. The object in the image is a geosynchronous satellite, UFO F2 (1993-056A).

The truth is out there

The acronym 'UFO' in this case does not stand for the classic Unidentified Flying Object. It stands for Ultra High Frequency (UHF) Follow-On, the name of a class of US Navy communication satellites.

The UFO satellite constellation consists of 11 satellites (not all of them operational) in geosynchronous orbit, launched between 1993 and 2003. It serves fleet-wide communication needs for the US Navy (including its submarines, but also Marine units on land). The system is currently being replaced by the newer MUOS constellation (see a previous post) and will gradually be phased out.

UFO satellite constellation on 9 March 2016
(click image to enlarge)

The first launch in the series, the launch of UFO F1 on 25 March 1993 with an Atlas 1 from Cape Canaveral, resulted in a partial failure to reach the intended geosynchronous orbit due to the failure of one of the rocket engines. The second UFO launch, UFO F2, the one imaged above, was the first truely successful launch of this satellite class.

USA 236 on 28 February 2016
(click image to enlarge)

I imaged more geosynchronous objects the past week, taking advantage of clear moonless evenings. The image above shows a star field in Orion in the evening of 28 February 2016, with USA 236 (SDS 3 F7, 2012-033A), an SDS data communications satellite in geosynchronous orbit. These satellites relay data from other US military satellites, optical and radar reconnaissance satellites in Low Earth Orbits such as the KH-11 'Keyhole'/CRYSTAL, Lacrosse (ONYX) and FIA (TOPAZ), to the US.

PAN on 28 February 2016
(click image to enlarge)

I also did my periodic revisit of the enigmatic SIGINT satellite PAN (2009-047A) as well (see image above). PAN is still stable at 47.7 E (see my long-term analysis here), near Yahsat 1B. The image above shows it near that satellite and a number of other commercial communications satellites in an image taken on 28 February 2016.

Mercury 1 r on 3 March 2016
(click image to enlarge)

On Feb 28 and March 3, I recovered Mercury 1 r (1994-054B), the upper stage from the launch of the Mercury 1 SIGINT satellite. We had lost this object for a while, it had not been seen for 153 days when I recovered it. The image above shows it in Hydra on 3 March 2016.

USA 186 on 5 March 2016
(click image to enlarge)

As spring is approaching, the visibility of satellites in Low Earth Orbit is gradually coming back for northern hemisphere observers.  This means we can take over from our lone southern hemisphere observer, Greg. The image above shows the KH-11 'Keyhole'/CRYSTAL optical reconnaissance satellite USA 186 (2005-042A) imaged on 5 March 2016.

Sunday, 6 March 2016

The tumble period of the UNHA-3 upper stage from the recent North Korean launch is slowly changing

click image to enlarge

The image above, taken in the evening of 5 March 2016,  is a 10-second exposure showing several flashes of the tumbling UNHA-3/Kwangmyŏngsŏng rb 2016-009B, the upper stage from North Korea's recent Kwangmyŏngsŏng-4 launch. It was taken during a very favourable 67-degree elevation pass, using my Canon EOS 60D and a SamYang 1.4/85mm lens (set at F2.0). The sky had cleared just in time for this pass (a last wisp of clouds is still visible in the image).

The flashes had a brightness of about mag. +3.5 and were visible by the naked eye. The resulting brightness variation curve is this one:

click diagram to enlarge

I have briefly mentioned the tumbling behaviour of this rocket stage in an earlier post. Over the past week I have been following this rocket when weather allowed, obtaining observations in the evenings of Feb 28, Feb 29, March  3 and March 5. This now allows a first look at how the tumble rate is (very) slowly changing.

The theory behind tumbling rocket stages and why their tumble rate changes over time, is briefly discussed here on the site. After the payload and the upper stage separate, usually by means of exploding bolts, the upper stage gets a momentum from this separation.

Over time, the resulting tumble is influenced by interaction of the rocket stage body with the earth's magnetic field. Spent upper stages are basically hollow metal tubes, and the Earth's magnetic field causes induction in it, leading to the tube getting an electric charge. Basically, the rocket stage becomes a dynamo. The Earth's magnetic field then further interacts with this electrically charged rocket stage, by means of the Lorentz force exerting a magnetic torque on the rocket stage's spinning motion. It is the latter effect which by "tugging" on the tumbling stage, changes its momentum, with a changing tumble period as a result. The resulting change is one towards a slower tumble rate, and eventually the stage might stop tumbling altogether.

I earlier established a peak-to-peak period of 2.39 seconds for 2009-009B from observations on Feb 28 and 29. Analysis of the new data obtained on March 3 and 5 show that the period is changing: I get 2.43 seconds for March 3 and 2.45 seconds for March 5.

I re-analyzed the Feb 28 and 29 data as well, this time using a fit to a running 5-point average on the raw data, which leads to somewhat better refined peaks. I also found that the initial autofit made by PAST is actually not the best fit, based on the r-square values of the fit. re-analysis leads to a 0.01 second revision to 2.38 seconds of the Feb 28 period, while the Feb 29 period stays at 2.39 seconds as initially established.

So the sequence is:

peak-to-peak periods
Date        TLE date    Period(sec)
Feb 28.81   16059.81    2.38 ± 0.01
Feb 29.79   16060.79    2.39 ± 0.01
Mar 03.79   16063.79    2.43 ± 0.01
Mar 05.82   16065.82    2.45 ± 0.01

(NB: the listed uncertainty is an estimate)

Even though the differences are very small, there appears to be an increasing trend to the periodicity, at the rate of about 0.01 second per day. As the difference is systematic, it is probably real and not just scatter due to measuring uncertainty (time will tell if this indeed holds).

[edit 7 March 2016 19:55]
One caveat: the synodic effect. As the viewing angle changes over the pass, this has some influence on the determined period. For fast tumblers this effect is small, but as we are talking about differences in the order of a few 0.01 seconds, the synodic effect comes into play.
The observations of Feb 28, 29 and March 3 were all made some 30 degrees beyond culmination, so the synodic effect should be about the same. The March 5 observation was done at culmination (I actually have a second image post-culmination as well but have not analysed it yet)
[end of edit]

Below are the brightness curves on which these values are based (click diagrams to enlarge):

click diagrams to enlarge

Appendix: on the construction of these brightness curves

I got a number of questions on how, and with what software, I produce these brightness curves. I will briefly explain below.

(a) calibrate exposure duration
What is first necessary, is that the real duration of the exposure is carefully calibrated. A "10-second" exposure set on your camera is not exactly 10.000 seconds: with my Canon EOS 60D for example, it is 10.05 seconds in reality (this deviation seems to increase exponentially with exposure time: a "15-second" exposure for example is in reality closer to 16 seconds!).

(b) measure pixel values with IRIS
The pixel brightness over the trail on the photograph is measured using the free astrophoto software IRIS.  Load the image, and chose "slice" from the menu option "view". Put the cursor at the start of the trail, and draw a line over the trail to the end of the trail. A window pops up with a diagram. You can save the data behind this diagram as a .txt table.
NB: be aware that Iris always measures from left to right (no matter how you draw the line), so if the satellite moved from right to left, you will later have to invert the obtained data series.

(c) Excel manipulation
The resulting .txt data file is read into excel. There, if necessary I first invert the series (see remark above). The result is a table with a column with pixel brightness values,  to which I ad an increasing pixel count. I then ad another column, representing the time for each pixel measurement. The value of the first cell is the start time of the image in seconds (I usually take the number of seconds after a whole minute, e.g. if the image started at 19:43:32.25 UT the value in this cell is "32.25". If I have a total number of pixels of say 430 (with 430 corresponding pixel brightness values), and an exposure time of 10.05 seconds, then I type this in the cell below it: "=[cell above it]+(exposure/number of pixels - 1)". In our example: "[cell above it] +(10.05/429)".
Then drag this down to the end of the column: the last value now should correspond to the end time of the exposure (in our example, it should be "42.30", i.e. 32.25 + 10.05).

If the raw data graph shows a lot of scatter, it can be useful to apply a running average to the data.

(Note: this approach assumes that the angular motion of the tumbling satellite or rocket stage was fixed over the exposure time in question. In reality, this is not the case. But for short time spans of a few seconds, this can usually be ignored, certainly if the image was taken near culmination of the object. It does introduce some deviation in the result though. Compensating for this makes the exercise a hell of a lot more complicated).

(d) read into PAST and analyse
I then copy the columns with the times and pixel brightness values, and paste them into PAST v.3 (very neat and free statistical software developed by paleontologists. I like it because it is versatile and able to create publication quality vector-format diagrams - the latter ability is something often lacking in such packages).
Press "shift" and select the two columns. Next, under "model" chose "sum-of-sinusoids". Next, a pop-up screen with a diagram appears.
Select "points" under "graph style". I leave "Phase" on "free". You then check the checkbox "fit periods" and click the "compute" button. It will fit a period.
However, I have noted that for some odd reason, the fitted period is not always the best fitting period! Check this by unchecking the "fit period" box, and in the box with the period result, varying the value from the initial fit slightly, after which you press the button "compute" again (leave the "fit period" box unchecked). Look at the R^2 values, and by trial and error find the best R^2 value. This is your actual period.
If your graph shows clearly skewed rather than sinusoidal peaks, than there is a second period interacting with the main period (for example, complex spin motion over two axis, or weaker secondary peaks present). You can try to model this by chosing "2" under "partials".

If you want a nice publishable diagram, press "graph settings" after you are done and adjust the diagram to your liking. Save it as .svg if you want to edit it further in for example Illustrator (as I do), otherwise use one of the other image formats available.

[UPDATED] MUOS 4 has been moved to its operational position at 75 E

MUOS 4 imaged while still in its check-out position at 172 W on 27 September 2015

Since September 2015, I have been periodically covering MUOS 4 (2015-044A), a newly launched military communications satellite in geostationary orbit. It was launched on 2 September 2015 and initially placed at 172 W, just west of Hawaii. This position was temporary: it is the "check-out position" where the satellite is initially placed, well situated with regard to key monitoring stations, to check if it is working okay. It stays there for a few months until this check-out is complete: then it is moved to its operational position. In the case of MUOS 4, it is known (see my earlier post and this unclassified publication) that the operational position assigned to MUOS 4 is at 75 E, south of India.

At the start of December 2015, it was announced that check-out had been completed and that the satellite would be moved to its operational position in the spring of 2016.

When I checked upon MUOS 4 on 4 December 2015 using the 0.51-m telescope of Warrumbungle obs in Australia, it was still at 172 W. For various reasons I did not get to check that position again until a few days ago. When I imaged that location on March 1, 2016, using the same Warrumbungle telescope, MUOS 4 was no longer there. It had been moved somewhere between December 4 and March 1.

This is confirmed by observations of the Russian ISON network. I received two of their MUOS 4 elsets for mid February 2016, which show the satellite drifting westwards at a rate of about 3 degrees/day. From the drift rate I reconstruct from these elsets, I find that the move from 172 W to 75 E started near 23.0 January 2016. At this drift rate, it should have reached its designated operational slot at 75 E 37 days later, on 29.0 February 2016.

[UPDATE 11 March 2016:  On March 8, Greg Roberts in South Africa recovered MUOS 4 at 74.8 E, very close to the expected position]