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


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