this post was submitted on 12 Jul 2023
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https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202346374 Here is the paper, it has free access which is nice. Most of this went over my head but the conclusions are well written. Basically they were able to detect broadband and narrow band emissions from the satellites as they passed by that were outside of their assigned range. Some of the frequencies they attributed (or at least speculated) to ground based transmissions reflected off them and they are asking regulators to consider these reflected emissions in addition to EMI coming from the craft itself. Cool stuff, but my favorite part is that they think there is a 25MHz oscillator on them because they detected a harmonic series with 25MHz as the base. Are you serious??!? how cool is that? 25MHz isn't even in the range they were scanning and they figured it out anyway. So from earth, they were able to detect the faintest signal from some onboard microcontroller just doing its job, not even being a radio device, just ticking away at its designed clock speed. I use arch linux btw.
Oscillators for computers are quite easy to detect because they create an enormous series of harmonics, and relatively strong ones. This is because a typical oscillator for this application is square wave, and an ideal square wave is the sum of sine waves starting at the fundamental frequency, with each 3rd harmonic 1/3rd of the power as the previous (so if you have a 25MHz oscillator, you'll have the fundamental frequency of 25MHz at amplitude 1, 75MHz at amplitude 0.33, 125MHz at amplitude 0.11 and so on ad infinitum). Eventually you may end up with a PCB trace or wire that just accidentally happens to be the right length to be resonant, and that harmonic will radiate quite strongly.
When your intended signal is minuscule, it doesn't take much to have it swamped by some unintentional radiator - at the very least it's extra noise you need to deal with.
A good example of unintentional radiators swamping a signal, a problem with some aircraft VHF com radios when receiving is that something like the 11th harmonic of the local oscillator (used to tune the radio) on certain VHF channels will completely swamp the GPS L1 band as it leaks out the VHF radio. The signal strength of this harmonic is absolutely minuscule, but it's huge compared to the GPS L1 signal, and it's the LO in the receiver rather than a spurious emission from the transmitter (part of the procedure of fitting a new VHF radio to an aircraft is making sure it doesn't interfere with a GPS receiver, and if it does, a stub filter usually needs to be fitted to the antenna port of the radio).
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