Slow Scan TV (SSTV) signals from the International Space Station (ISS) are usually transmitted on 145 MHz and on FM in an analogue format. On the 20th of February 2022, there was an experiment whereby a digital version of a SSTV signal was sent on 437 MHz instead.
The experiment was for five passes of the ISS over Europe between 05:12 UTC and 11:51 UTC.
The above image from Riccardo, IU4APB in Italy shows a montage of images that he managed to decode. Like any good experiment, there are a number of lessons to take away from it.
Path Loss... Some people noted that the digital SSTV signals on 437 MHz were a LOT weaker than the usual analogue SSTV signals on 145 MHz. This is to be expected as the path loss at 437 MHz is about 9dB higher than at 145 MHz.
The images above captured by IU4APB were with a directional Yagi on 437 MHz. Many people are using just simple vertical antennas on 145 MHz to receive the usual SSTV signals and may well be using similar low gain antennas on 437 MHz with disappointing results. Lesson - Use a directional antenna with more gain to compensate for the extra path loss.
Repeater interference... From my understanding, the SSTV signals on 145 MHz are usually 'broadcast' from the ISS. For this experiment however, the digital SSTV signals were sent via the repeater on the ISS.
While the experiment was announced beforehand, not everyone may have heard about this. Some stations may use the ISS repeater as per normal without knowing that they were causing interference to the SSTV experiment.
A lot of the Black lines in the image above and below were probably be due to this as this interference was reported.
Digital - All or Nothing... As with most digital signals, either the signal is perfect or it's not. Contrast the digital SSTV signal above and the analogue SSTV signal below...
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| Example of an analogue SSTV signal |
With the analogue signal, noise appears as small dots and lines. With the equivalent digital signals, the small dot is replaced with a large square when the signal is too weak.
Doppler... As the signal was on 437 MHz, the doppler shift of the downlink signal was three times that experienced on 145 MHz. It's another parameter to control.
In conclusion... While the usual 145 MHz analogue SSTV transmissions have the greatest reach in terms of people being able to decode the signals, it's also a well worn path. It's good to try something different and hopefully the experiment will be repeated.
It would be good if the digital SSTV signal could be 'broadcast' on 145 MHz as opposed to via the ISS repeater. I'm not sure how feasible this is as it may require some input from the onboard astronauts.
Update: Miro, SP5GNI in Poland also reports reception of the experimental digital transmissions from the ISS and has some images at the bottom of this post HERE
