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Would it be possible to use this as the infrastructure for a clandestine, low-bandwidth, text-based mesh network? An internet for nerds kind of thing.
by rv-de 7y ago
Would it be possible to use this as the infrastructure for a clandestine, low-bandwidth, text-based mesh network? An internet for nerds kind of thing.
- bobowzki 7y agoYes.
- LfLxfxxLxfxx 7y agoShh... don't tell anyone!
- jdietrich 7y agoYes, for a definition of "low-bandwidth" that you'll probably find disappointing. The WSJT digital modes are capable of long-range communication at well below the noise floor, but we're talking about single-digit bits per second in typical use. https://www.physics.princeton.edu/pulsar/K1JT/ https://www.physics.princeton.edu/pulsar/K1JT/
- pmoriarty 7y agoWhat about short, rare bursts of high bandwidth transmissions from a mobile platform? Depending on how creative one was about the locations and operation times of the transmitter, I'd expect it'd be pretty difficult to find.
- superkuh 7y agoThose transmissions won't go more than the next slight rise in the ground or valley. High data rate requires line of sight which requires height above terrain which requires either money or rare luck.
- pmoriarty 7y ago"High data rate requires line of sight which requires height above terrain which requires either money or rare luck." What about a transmitter temporarily sent aloft on a balloon, model rocket, or drone, then making its brief high-bandwidth transmission and then parachuting or landing, being retrieved by its operator who then disappears until the next transmission from another location? It might even be disposable, and potentially either deliberately sink itself in to an ocean or other difficult to access place, and/or self-destruct to make recovery and forensics more difficult.
- ac29 7y ago> High data rate requires line of sight This isnt true. High data rate requires large channel size, which is certainly more commonly found in >1GHz portions of radio spectrum that require line of sight. But, a 10MHz channel is just as fast at 400MHz as it is a 4GHz. HDTV doesnt require line of sight to gets tens of megabits on VHF/UHF channels.
- superkuh 7y agoIn what legal jurisdiction can you use a 10 MHz bandwidth channel under <30 MHz (where you actually get ionospheric bounce (sometimes))? And even worse than the answer to that (nowhere) is that in the USA the data rate limit is actually in terms of symbol rate, not bandwidth used, to a mere 300 baud for the bands that have regular non-line of sight propagation.
- ac29 7y ago> In what legal jurisdiction can you use a 10 MHz bandwidth channel under <30 MHz (where you actually get ionospheric bounce (sometimes))? I never implied you could? Its certainly possible in VHF/UHF, though (for licencees). > And even worse than the answer to that (nowhere) is that in the USA the data rate limit is actually in terms of symbol rate, not bandwidth used Sure, but those limits only apply for HF and longer-wavelength frequencies. You're welcome to do 256QAM in the microwave range, for example.
- superkuh 7y ago> Its certainly possible in VHF/UHF, though (for licencees). >You're welcome to do 256QAM in the microwave range, for example. Which gets back to line of sight propagation (tropo is not common) and everything I said in my first post. I've tried the home to car omnidirectional link thing on 902-928 MHz. All that matters is line of sight. I tried a 25w bidirectional amp (ham general license) and that didn't help. I tried bringing the ubiquiti transceivers w/broadband hamnet firmware all the way down to QPSK modulation and the narrowest channel I could. I even tried using narrowband 56k telemetry dongles. I customed designed and built my own microstrip based filters for the front ends. The only thing that really helps, and it's not enough, is height above terrain. When I go park my car on the hills outside of town megabits are possible. But the slightest hill or valley losing LOS kills it.
- KirinDave 7y agoYes, but also no. Firstly, it's not a new idea. Folks use JS8call (formerly FT8call). It's quite sophisticated and can work even below the noise floor. Folks have even gotten it to work in pretty ridiculous conditions. It has comically low bandwidth but works at comically low power. That said, the only place I think this would be "clandestine" is in urban environments where there is a lot of reflection. Modern electronics and antennas get the benefit of very high fidelity simulation, so finding a signal with professional gear (especially if it's cutting edge and understands modern maps) is pretty wild. I saw a demo of a very expensive system that used Google Earth as its UX a few years ago, and it could do some amazing things. Even in cluttered environments with low power transmitters, it still isn't very secretive. Amateur radio does this thing called "fox hunting" where they use a variety of handheld elements to quickly track down low power signals. Some of these contests get pretty technical, with the target transmitter being highly directional on some axis and deliberately casting false reflections, and lately they're entirely solar/lifepo4 so they're power constrainted. But folks with a bit of training and practice can find them really quick. If you want to be clandestine, then actually what you want to do is overlay your network over existing networks so that your traffic is difficult to isolate from a computationally infeasible flood.