One of the ways people can interact with astronauts on the International Space Station is via amateur radio. The ISS is equipped with several transceivers in the VHF (2m) and UHF (70cm) bands. The various equipment facilitates voice and data communication, with the latter including a mode called “Slow Scan TV”. To celebrate twenty years of amateur radio communications from the ISS, the Amateur Radio on the International Space Station organization has organized a SSTV transmission event which occurred this weekend. After seeing a number of people posting copies online of the SSTV images they received, I thought I would see if I could receive the images myself.
To receive the signals from the ISS, I used a RTL-SDR USB dongle and the vertical antenna that came with the rtl-sdr.com kit. This is certainly not an optimal set-up, and a better antenna would improve things, but one has to make do. I did not have any filter on the system, so there’s likely a ton of out-of-band leakage. Being in the center of a large (6.5 million people) city does not help, nor does the fact that I am within sight of a large telecom tower. But we will see how it goes!
Since the transmissions can only be received when you have a line-of-sight to the ISS, determining when overhead passes are is important. I am using the GPredict software to determine when ISS passes will occur and how long they will last.
Once a pass has been identified, I use gqrx to tune the SDR dongle and record audio within the band of the SSTV transmission. The audio is then processed by qsstv to assemble the image from the radio transmission.
Assumed Orbital Elements
I used the following two-line elements for the ISS orbit:
1 25544U 98067A 17203.86281652 .00004407 00000-0 73541-4 0 9990 2 25544 51.6406 220.4890 0006189 62.7487 356.6347 15.54220120 67309
This TLE was obtained from the Celestrak NORAD TLE page.
For each attempt to receive the SSTV signals I provide the orbit number and predicted acquisition of signal time for the Santiago airport – the orbit numbers are taken from
gpredict where the orbital epoch is 2017-07-23 00:42:27, corresponding to an orbit number of 6730.
Attempt 1: Orbit 6740 (2017-07-23 12:44 UTC)
My first try at receiving the signals was a morning pass on 23 July, with the pass starting at 12:44 UTC.
gpredict, this corresponded to orbit 6740.
The pass was reasonably high, reaching a maximum elevation of 48°.
Even better, the pass was to the northeast of my location, which is the optimal direction for my location, in terms of blockage by buildings.
In order to maximize my chances of capturing a signal, I started recording about 90 seconds prior to the ISS coming over the horizon. Per some advice I had seen online, I tuned to 145.800 MHz with a 16 kHz filter.
Unfortunately I ended up not recording any actual data because I had set the squelch too high! I was overly optimistic about the signal strength, and it never reached a strong enough level to trigger the audio. If you look closely at the waterfall plot, you can see a small pattern at the desired observing frequency. I think that is the SSTV signal peaking slightly above the noise – if I had set a lower squelch threshold, I might have gotten that and at least been able to reconstruct a partial image. Oh well, a lesson for the next pass!
Attempt 2: Orbit 6742 (2017-07-23 14:21 UTC)
I had a half-hearted second attempt, for a pass starting at 14:21. The maximum elevation was 15° with an azimuth that would have signals arriving through the other side of my building. As expected, no signal was received.
Attempt 3: Orbit 6746 (2017-07-23 20:51 UTC)
My third attempt was for a more ideal pass, with a maximum elevation of close to 80°. But alas, despite the improved pass configuration, I did not receive any SSTV signal that I could tell (or decode).
As is evident from the waterfall image of the first attempt, there is a lot of noise in the band. I spent some time unplugging things in my apartment to see if it was a local contribution, but I was unable to find it. Scanning through, the noise seems fairly broadband, but I was unable to get any clues as to its origin. It could be interference from electronics in another apartment (there are many around me) or it could be a strong out of band signal that is overwhelming the receiver. (Thanks to LA3PNA for some thoughts on the noise source). Unfortunately I do not have any filters on hand to try and rule out the latter possibility. And beyond the noise issues, my antenna situation could certainly use improvement.
That was the last ISS pass within the window of the SSTV transmission event, so it looks like I am out of luck for this one. But it was nice to chase the reception and learn some about SSTV. Better luck next time, hopefully!