Image: SSTV image that was downloaded by ZS1Q on the 5 May 2025 during the "strange" phenomena.
During a fly-past of the ISS on the 5 May 2025 two SSTV Images were broadcast over South Africa as part of the SSTV Experiment 73 Series 27. A "strange" phenomena was observed by Francois ZS1Q in George who participated in the experiment. During the pass timeline 17h03 - 17h13 Francois observed a strange "event". During the predicted pass moving from SW to SE instead of following the ISS track as predicted he received a better signal tracking the ISS from SE to SW. He was using a portable 3 element Yagi antenna at the time. Now one can argue that the Yagi received the signal "backwards" which I doubt because the front to back ratio of the antenna is excellent. When he told me about what he experienced I was curious to know what caused this "phenomena". Maybe two reasons: 1. A reflected signal from a building. 2. Airplane scatter - Airplane scattering (or most often reflection) refers to the reflection of a radio signal by the metal structure of a passing aircraft. It is observed on radio waves from high frequency to VHF through to microwaves and, besides back-scattering, yields momentary propagation up to 800 kilometers even in mountainous terrain.
Now we have large mountains to the North West of George and also a provincial airport to the South of the town. I decided to look at flights taking of and landing at the time of the ISS pass. Low and behold FlightRadar24 indicated that ChemAir Flight 5Z451 departed from George to Johannesburg. The flight was sceduled to depart at 16h55 SAST, actual departure was 17h05 SAST and arrival in Johannesburg was at 18h50. The 17h05 actual departure of the aircraft coincided with the ISS broadcasting SSTV images in the George tracking area. What is interesting is that the ChemAir aircraft was flying in a northerly direction while the ISS moved towards a north easterly direction and the QTH of Francois ZS1Q was in between the ChemAir aircraft and the ISS. This scenario makes it possible that a reflected signal from the ISS was bounced of the ChemAir aircraft and back to the station of ZS1Q. Was this indeed the case? Very difficult to tell as there was no upfront setup to work Aircraft Scatter. (Note: ZS1Q is currently looking to see if he can find the RAW SSTV audio file to listen to. Unfortunately Robot36 does not save the audio file, only the image. )
Image: ISS Track
However the scatter could have played a roll in the "strange" phenomena that Francois ZS1Q experienced while downloading SSTV images from the ISS on the 5 May 2025. Never before has he experienced "reverse" tracking while downloading images. We will most probably never know for sure but one thing stands out above all. If you do not experiment and participate in events you will never experience the magic of the amateur radio bands, modes, propagation and other strange phenomena. There is still many unknowns that need to be discovered out there and in space. Be ready for such events and participate in events and experiments.
More on aircraft scatter:
Radio amateur usage Compared to a single trail of meteor scatter, the time of opening caused by a single airplane is longer – it can be from 30 seconds up to a few minutes. Slight asymmetry can sometimes be encountered both in terms of exact timing and the gain of the path. Range and possible contact lengths are affected by the size of the plane, flight level and the approximate crossing time of the central part of the straight line between the two communicating stations. Both the onset and decay of the channel is rapid, so to improve success rate, a short contact procedure is used. Contact opportunities can be predicted based on the precise flight paths of surrounding planes. There exists software for prediction using local (Mode S and ADS-B) or online sources. Microwave access using high gain antennas needs to consider precise aiming that takes into account elevation angles as well.Links to Aircraft Scatter Web-Pages:
7 October 2024 – Aircraft Scatter, the Hitch-Hikers Guide by Ian White, GM3SEK
SM6FHZ's aircraft scatter page: He has an aircraft scatter path-loss Excel Spreadsheet.
SM0DFP has a page describing his use of ADS-B signals to assist with airplane scatter contacts
A PowerPoint talk by DF9IC on airplane scatter
Images: Click on images for larger view.
Sources: Flightradar23, Orbitron, ISS and ARISS
