It doesn't matter much if you only ever want to poke around in a swimming pool or very shallow stream like shown here, but if you get into this hobby and ever want to run in deeper water without a tether (whether autonomously or acoustically) you'll either want it to be positively buoyant or have an independently powered emergency surfacing mechanism (eg. drop a weight).
Otherwise if you lose power or control you will lose your sub.
Very poorly. The 27MHz radio used here won't penetrate more than a meter or so. GPS doesn't work through more than some inches. But like most things, the lower the frequency and higher the amplitude the farther the penetration. Acoustic modems work better under realistic constraints, though with a few exceptions you need the line of communication to be rather vertical if you're talking very long distances (hundreds or thousands of meters) because the speed of sound in water varies by pressure/temperature/salinity so the signal path curves up back toward the surface (from Snell's law).
Sonar, which of course works by sound, has issues because of the speed variability you mention. However, IIRC (and in salt water) the range of an active sonar at the right frequency (lower is better for range) is on the order of 20 thousand meters. That said, a submarine can hide from sonar under a thermocline, assuming the sonar is above the thermocline.
Absolutely could! Some do this. It depends on your chosen operational constraints. Keep in mind that neutrally buoyant cabling isn't particularly compact, so it creates its own set of problems.
> You can't go infinitely deep anyway, because eventually the pressure will penetrate the seals.
This is actually the least useful reason why you can't go infinitely deep. For exploration you will basically always want to be near the bottom of the body of water because mid column is just extremely boring, so you will want to make sure that your pressure tolerance allows for that. We just haven't found any infinitely deep bodies of water yet.
Terrible (for 2.4/5GHz). They seemed to have worked around there not being a lot of hobbyist receivers by buying toy submarines that use lower frequencies like 40MHz and used the boards from there, nifty but seems very limited.
Doesn’t work on any unlicensed frequencies. 2.4Ghz and 5Ghz are absorbed by water. Militeary submarines use VLF frequencies. With a mile long antenna. So tether is the only real practical method.
Please point them out. All the UUVs/AUVs I've found are either tethered to a buoy or completely autonomous and only transmit when the surface. In all practicality there is no completely untethered ROV on the market. Acoustic controlled UUVs are still line of sight.
Radio communication under the sea is not an attractive option for experiment by the radio amateur as it requires the use of very low frequencies, large antenna systems and very high powers.
Fresh water lakes and rivers have much lower electrical conductivity than the sea and underwater transmission distances (or depths) up to 30 metres appear feasible using the [then-not-now] lowest frequency amateur band of 1.8 MHz. Even larger distances (or depths) could be achieved if a lower frequency band allocation were made available.
However, that was written in 1987 and from a cursory historical review of https://en.wikipedia.org/wiki/ISM_radio_band it seems that exactly around the late 1980s time that article was written, the still earlier-suggested lower frequencies within the ISM bands were popularized and confirmed by various national regulators and there are now commercially available ISM-band transceivers available, eg. https://hoperf.com/modules/rf_transceiver/index.html
You can use directional (high dBi / high gain) antennae at one or both ends of the link, however this adds substantial complexity and generally negatively affects the vessel hydrodynamics.
Let's say a sub delivering critical medicines to peoples in need is supposed to cross an ocean, it probably wants GPS at least sporadically (for inertial navigation error correction) and AIS (to avoid collisions with cargo ships in busy shipping channels) which means it's operating at least near-surface most of the time anyway which means a radio comms tower is likely the best option, and the question is moot.
OTOH, if a sub is launched from a research vessel, it's cheaper and easier to use a tether. If it's just for a hobbyist, then the commercially available ISM transponders are likely adequate. So as per the 1987 comment: it seems in-water long distance comm's is really the domain of the military due to their unique requirement for deep-water stealth and the corresponding need for unlicensed low radio frequencies. For the rest of us, use ISM-band, a towed surface relay (also stealthy anyway), a tether, or just surface for comm's.
Seems like it's pretty common to use a tether (like the OpenROV) for yanking it back out in case of failures also - I assume any self-contained failsafe like dropping a weight could itself fail?
Yes, any failsafe could fail, but there are ways to reduce that risk. For example, if you use an “default safe” system where you have to actively keep a weight from falling, then when you lose signal or lose power, the weight drops, you know that in the “default” state, you’re safe. And example would be if you held a weight in place with an electro magnet. As soon as you lose power, the weight would drop, and the vessel would return to the surface.
This is the same idea behind semi truck brakes. The brakes are by default engaged. When the truck is running, there is a build up of pressure to disengage the brakes so that the truck can drive. If something goes wrong (loss of power on the truck), the brakes are engaged by default to stop the truck.
This was basically the first lesson in automation class, which made an impact at the time.
A safety switch should be normally closed, energizing a normally open relay, such that any broken cables or connections would be like hitting the switch, causing the relay to de-energize, subsequently cutting power to the main circuit.
Same in industrial control systems that are used to control machinery where a failure could lead to injury or loss of life (or property). The default is to stop the machine in a state where it is known to be safe, this can lead to some pretty difficult problems if you want to get out of that state to re-start the machinery but the alternative is to continue to move when you really shouldn't be.
Even something as simple as an emergency stop switch and the subsequent re-energizing of the system can be a very hard to solve problem from a control perspective. Especially with Servo systems with incremental rather than absolute encoders.
Common should probably give way to expected. The Navy uses tehtered control on its torpedos for the utmost in stealthy attacks. If the best the Navy and military budgets can come up with is tethered control, not likely something in the hobby world will be better.
I've watched the BrickExperiments YouTube channel, but I always assumed that the channel was drivel, and that much like social media of today, the details of the build would be omitted, and lost forever- but this changes the game for me.
A 10 part post is beyond what I'd consider most 'good' creators to put together. The author is obviously technical, and is wise to the nature of the videos. I'm impressed.
Some might find the system by James H. Rogers, developed in the era of World-War-One of interest. The late T.E.Bearden claimed that Rogers system was rediscovered and 'lost' at least seven times from then until our times.
As an EE for an ROV manufacturer for 20 years, this was really interesting to see how he attacked some of the design issues that plagued my entire career. Great stuff!
Otherwise if you lose power or control you will lose your sub.