Saturday 27 July 2019

The Mating Call of the CUCU [updated]

The ISS is seeing busy times. On July 20, Soyuz MS-13 was launched from Baikonur bringing a new crew to the ISS. Then, on July 25, SpaceX launched the Dragon CRS-18 cargoship to the ISS from Cape Canaveral, docking today (July 27). And it will get even busier: in a few days, currently slated for July 31,  a Progress cargoship will be launched from Baikonur towards the ISS as well.

Soyuz MS-13

As is usual these days, the Soyuz MS-13 launch from Baikonur on 20 July 2019 was a fast-track mission, launching at 16:28:21 UT (20 July) and docking at 22:48 UT, a mere 6 hours 20 minutes later.

One orbit before docking, near 21:05 UT, the Soyus-ISS pair was visible chasing each other in a still bright twilight sky over Leiden, the Netherlands, the two objects being some 20 degrees apart. In the image below, the leading bright streak is the ISS, the fainter trailing streak near the clouds is the Soyuz (enlarge the image to see it). Visually, the Soyuz was about magnitude +1 and easy to see:

click to enlarge

During the next pass, near 22:40 UT , they already were too close to visually separate, but I could hear the kosmonauts onboard the Soyuz talk (in Russian) at 121.75 MHz FM during this pass, only minutes before docking to the ISS at 22:48 UT. Here is a recording of the best part received:


The Mating Call of the CUCU

Only 5 days after Soyuz MS-13, on 25 July 2019, the SpaceX Dragon CRS-18 launched from SLC-40 at Cape Canaveral. The timing of the launch, 22:01:56 UT, was unfavourable for initial sightings from the European mainland (Ireland and western UK did have sighting opportunities) as it already was in earth shadow while passing over mainland Europe 20 minutes after launch.

The next night did see visible passes, that unfortunately for me in Leiden were clouded out. I did however detect related telemetry signals at 400.5 MHz during two passes (19:22 UT, in daylight; and again during the clouded out 20:59 UT pass).

The three peaks in the frequency diagram and broad yellow bands in the spectrogram below (from the 19:22 UT pass) are the CUCU signal. CUCU stands for the "COTS UHF Communication Unit":

CUCU signal on 400.5 MHz

CUCU is a duplex telemetry broadcast that allows the ISS to communicate with the Dragon and vice versa, homing it in for berthing. It is what you could call the 'mating call' of the pair. CUCU was not active right after launch during the first Dragon revolution (I listened), but was notably active the next day, as Dragon CRS-18 was slowly approaching and climbing towards the ISS.

The CUCU signal sounds like a humming noise and a regular sharp "Beep! Beep! Beep!". Below is an audio recording of the CUCU signal, from the 19:22 UT pass, roughly corresponding to the spectrum shown above:

Initially I thought this was the CUCU of DRAGON CRS-18 itself, but looking at the Doppler curve of the signal, it was actually the CUCU signal of the ISS calling out to the fledgling Dragon (HT to Cees Bassa for noting it corresponded to the ISS rather than DRAGON).

The spectrogram below shows the signal as received during the second pass, near 20:59 UT, with the characteristic Doppler S-curve. The diagram below it shows how this Doppler curve matches with the Doppler curve for the ISS at that time:

click to enlarge
click diagram to enlarge

This was the first time I have heard the CUCU mating call, and I was surprised by how strong the signal was. The reception was made with a homebrew 120-deg V-dipole antenna with ground plane reflector, optimized for 400 MHz, and an SDR dongle.

UPDATE 28 July 2019

Dragon CRS-18 docked to the ISS earlier today, near 14:00 UT. During the 18:33 UT and 20:09 UT passes (I did not monitor the third pass at 21:46 UT), there was again radio activity around 400.5 MHz connected to the ISS/Dragon. It was different in character than when the Dragon was still free-flying. Compare the spectrogram below, from the 20:09 UT pass, with thatfrom the previous day  above (note: the fuzzy band in this case is interference - the ISS/Dragon signals are the s-shaped lines):

click to enlarge

Thursday 25 July 2019

X-37B fact and fiction

X-37B. Photo: USAF

If there is one classified space object that speaks to the public's imagination, then it is the US Air Force's  X-37B robottic space plane, also known as Orbital Test Vehicle (OTV).  These 9 meter long uncrewed spacecraft have wings, with a wingspan of 4.5 meter, and look like a mini Space Shuttle. They are launched on a rocket like a normal satellite, but return to earth by landing like an airplane (or indeed like the Space Shuttles did). They have a payload bay of 2.1 by 1.2 meter in which they carry experiments and from which they could perhaps also release and retrieve small satellites. They are launched in very low orbits, between 250 and 450 km orbital altitude (i.e. generally below the orbit of the ISS).

The US Air Force has two X-37B's and is currently flying it's 5th OTV mission with one of them, with 685 days on orbit on the day of writing.

The winged design and the coloquial 'space plane' lead many people to think the X-37B flies and banks like an airplane or a Star Wars X-wing fighter while in space - its infamous purported "manoeuverability", a notion recently fuelled again by remarks of former SecAF Heather Wilson (see below).

This is mostly a misunderstanding and part of the mythos that surrounds the X-37B: in space, the wings of the X-37B are useless and it behaves and orbits the earth like any other satellite. The X-37B does not change its orbital plane at a whim - or at least not generally. That is quite clear from amateur monitoring of the five OTV missions so far.

In this post I will show that the only significant manoeuvers the OTV's make are frequent alterations of their orbital altitude: they do not significantly change orbital plane during a mission. Periodically changing orbital altitude is something other satellites do too, so the X-37B is not special in this either, except that during recent OTV missions the X-37B's have done this more often than ordinary satellites typically do. And let me add, so you understand me well: you don't need (or indeed use) wings for that. These orbital altitude changes are done with an engine burn, just like 'normal' satellites do.

The X-37B OTV 5 filmed by the author on 26 June 2019

The wings of the X-37B are not for manoeuvering in space, but primarily for use in the lower atmosphere upon its return to earth, when it lands like an aircraft (as the Space Shuttle did). Yet every now and then, the myth of the supposed wing-supported "manoeuverability" pops up again, and connected to it is a whole ecosystem of suspicions and theories about the potential "function" of the X-37B - most notoriously the (almost certainly incorrect) notion that it is some kind of "Space Bomber" ready to be flown to any target on earth within 90 minutes to drop a destructive weapon. The Space Treaty, to which the USA is a signatory, prohibits to deploy weapons from space, and it is really unlikely that the X-37B is such a 'space bomber'.

The X37-B instead likely is a testbed for new space hardware, testing new technologies under real space conditions and then returning them to earth for inspection. We know for example that during the OTV 4 mission, a XR-5a Hall-effect thruster was tested. The frequent changes in orbital altitude are part of this: testing space hardware under various drag regimes.

So what about that "manoeuverability" then? New fuel was fanned on the idea of extraordinarily "manoeuverability" recently by intriguing statements made by former SecAF Heather Wilson. She claimed that the X-37B:

"Can do an orbit that looks like an egg and, when it's close to the Earth, it's close enough to the atmosphere to turn where it is. [...] Which means our adversaries don't know -- and that happens on the far side of the Earth from our adversaries -- where it's going to come up next. And we know that that drives them nuts."

Two things are apparently being claimed here:

(1)  The X-37B can manoeuvre by briefly dipping into the upper atmosphere;

(2)  This makes the X-37B difficult to track.

The wording of the statement is wonderfully opaque, but Wilson seems to suggest that the X-37B can seriously change its orbital inclination by briefly dipping into the upper atmosphere and using its wings to manoeuvre.

I have two problems with this. One is that bringing the X-37B down into the upper atmosphere by an engine burn (there is no other way), have it change orbital plane by using the wings, and then do a burn to get back to orbital altitude again, probably costs as much fuel as a more regular on-orbit engine burn to change orbital plane. So where is the gain in using this dip-and-wing-manoeuvre?

The other problem I have, is that I do not see the claimed behaviour in our tracking data. Contrary to the impression that Wilson is trying to give us, i.e. that the X-37B's are difficult to track due to the tricks they perform, the X-37B OTV missions have been regularly tracked by our amateur network. And we do not see significant changes in the orbital plane during a given OTV mission.

The X-37B OTV 5 imaged by the author in April 2018 (click to enlarge)

Looking at the tracking data we have for these X-37B missions, they show only very minor changes in orbital inclination during a given mission. There is no evidence for sudden, significant changes in the orbital plane, as is illustrated by these diagrams that for each OTV mission plots the orbital inclination against time (the data are from observations by the satobs amateur network):

The only exception appears to be mission OTV 4, which does show a temporary change in orbital inclination and then back again in the last quarter of 2016. The orbital plane change is of little significance however (only 0.6 degrees) and could have been done by a normal engine burn. So if the X-37B indeed can use a drop into the upper atmosphere to make use of it wings to significantly change orbital plane, they so far do not seem to have clearly demonstrated this capability.

(the changes in orbital inclination at the end of the OTV 2 and OTV 3 missions, probably are in preparation for landing).

What the X-37B missions in contrast do have demonstrated, especially during the last two missions, are repeated changes in orbital altitude and orbital eccentricity (in Wilsons words: it "can do an orbit like an egg"). This is illustrated by these plots of the apogee and perigee altitudes against time for the five OTV missions so far:

As I already mentioned this is something other satellites do too, so the X-37B is not particularly special in this either, except that during recent OTV missions the X-37B's do this more often than ordinary satellites typically do. The changes in orbital altitude probably are related to testing equipment under different drag, gravity and irradiation regimes.

So the X-37B missions so far set themselves apart from regular satellite missions by their low orbital altitudes and frequent changes in orbital altitude (in which the wings play no role at all). They can do so because their missions are relatively short compared to a typical satellite mission. Unlike a regular satellite, at one point they will land and be refuelled, and then relaunched after a while.

But as intriguing as the suggestions are, the orbital history of the five X-37B OTV missions so far do not evidence the alledged manoeuverability in orbital plane.

Nor of course, are the X-37B that difficult to track as is claimed. Our amateur network regularly observed and observes the OTV missions. We might lose the OTV for a (usually brief) moment when it has made a manoeuvre to a higher or lower orbit, but a plane scan is enough to relocate it (and as the diagrams above show, they do not manoeuvre daily or even weekly).

So Wilson's remarks appear to be just part of the myth-making around the X-37B.