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System telemetry is as old as spaceflight, probably older. Surely the low-bandwidth data involved could be multiplexed onto any data channel, or on a special admin-only dedicated channel?
Given the low cost of sensors, some key electrical parameters would seem to be a worthwhile fit. For a few dollars more, we could have temperatures (multiple), pressure & humidity too - not a bad idea for any kind of equipment mounted outside the normal domestic environment, or even within.
The family of lines in 6.5-7 MHz you suggest seem to me to be power line related, though this is true of many SMPS noise sources. When I detect in 10 kHz AM and analyze with Audacity I see astrong 50 Hz family/component
This is as expected from an imperfectly filtered SMPS input. Furthermore, the spectral width of the lines is consistent with jitter/phase noise of a switcher's rate and it appears to have stronger even harmonic components, as is common from full-wave rectification.
But as to the actual source and coupling mechanism - that's harder to say. There are so many uses for SMPS supplies these days, some of them for lighting related power that an after-dark correlation is not unlikely.
To put things in perspective, while your noise floor is no doubt raised considerably by this, you are in pretty good company and a lot kiwiSDRs reveal similar and worse results. If you are using a well matched antenna, your noise is about 30 dB above kTB which isn't so dreadful at that frequency. OTOH, it may be higher than that if your system is not well matched to the radiation resistance. You could be seeing CM currents, current in the earth below or other coupling mechanisms. I suggest not first considering it to be inverse-square radiation. That is actually much rarer, in my experience.
Although intended for monitoring mains voltage, I did a somewhat similar thing using the ESP8266, described here:
The source code is linked, and making it read supply voltage rather than mains voltage would be trivial. The '8266 has only a single A/D input, but the addition of a simple analog MUX chip like the 4051 would easily expand this to 8 inputs - although one would probably want to level-convert the MUX drive to 5 volt logic (e.g. an NPN and two diodes).
* * *
If a network connection isn't in the cards (the ESP8266 has 2.4 GHz WiFi only) this also includes a Morse beacon that telemeters the information - in this case, on 10 meters: This was added so that telemetry could be seen by anyone interested, but also so that we could still receive it should the WiFi connection drop for some reason.
* * *
Finally, the same, basic low-power TX could be driven by a PIC or Arduino, of course to produce Morse telemetry.
To get back to an aspect of your question: I suppose that a BBG GPIO pin with an A/D input would be available, but I chose to leave the KiwiSDR unit unmodified - both for practical reasons (a spare could be swapped out) and also to avoid the possibility of conducting undesired signals in/out of the chassis (e.g. static, RFI, lightning, etc.)
I have some FrSky Smart Port voltage and current sensors, they output 3.3V serial data (inverted).
The only thing stopping me using them is the dense bit of wet carbon between the keyboard and chair.
I designed and built an APRS/solar_charger for monitoring the PS at a remote KiWi site. It's not quite what you are asking for but since the Kiwi data is IP over microwave, the use of out-of-band monitoring means I can see the power situation and termperature even if/when the microwave link is down.
You can see the present condition of N6GN/K2 this way
If you are interested, write me.
The Ireland ISP is HEANet who had the interesting note of being the only Irish ISP that was not forced to filter the Piratebay when that court order came through, as it's an academic supplier and not a home supplier. One SSH port forward later from a machine I had access to on an academic network and I once again had unfettered net access via a proxy. That was very useful when I was abroad and using Irish websites that were geo-locked.
Agreed re: DNS as one of the easy ways to block/lose portions of the net. The other interesting way is BGP poisoning - and that's really interesting to see who trusts who!
The Park Service has denied us volunteers access to the KPH facility since the start of Covid last spring. While that is annoying I am sympathetic to their situation. If they were to grant exceptions to one set of citizens, then other would want the same privileges.
As soon as I am allowed, I will go out there and restore operation. Look for KPH WSPR spots on http://jimlill.com/today_int.html to learn when that happens
You would be better off keeping IP addresses of alternate DNS servers than keeping a local offline copy of DNS. The likelihood of global DNS failing will be more remote than large swathes of IP inaccessibility.
Ones I have memorised:
188.8.131.52 - Satlynx german satellite ISP DNS server, I used type this in so often it's in muscle memory at this point.
184.108.40.206 - Google's alternate DNS
220.127.116.11 - Google's main DNS
18.104.22.168 - Ireland's university ISP dns server
22.214.171.124 and 126.96.36.199 - the OpenDNS servers.
@KB8DOA This morning (19 Jan 2021) R&S shows 13 Kiwi kits in stock. Please get on them immediately and make sure your two backordered units from last July get shipped so this awful situation can finally be resolved.
My sincere apologies for this situation. I never got any answers from anyone, including Seeed, about what happened exactly. I have guesses, but that's all. There is simply no excuse for it.