Maybe the article just excluded this detail, but is there any reason for a submersible to not have a black box and transponder similar to an aircraft? Do they not work under water after a certain depth?
I feel like the company in charge of all this should at least know where the submersible is, if not what happened to it.
Radio waves don't work well through salt water. Even VLF signals have trouble going past like 30 meters. There's some super duper low frequency radios but it's still a one-way proposition - build a huge facility on land to communicate with subs, and if the sub needs to reply it has to surface. And even that limited setup is only doable by the military industrial complex of a small handful of nations.
ELF (Extremely low frequency) requires finding a section of earth's crust with very low conductivity that can be repurposed as an antenna. These antennas are very large, the US had one that was 45km long, the Soviet antenna is 60km long.
The resulting signal has a wavelength of 3500-4500 KM, about a quarter of the earth's diameter.
Because of this huge wavelength, the bandwidth is extremely low. The US system took 15 minutes to transmit a single 3 letter message. The system was only really useful for transmitting pre-agreed signals (which were usually along the lines of "surface and contact us for the actual orders").
But they also acted as a dead-mans switch, they continually transmitted an idle signal. If they stopped transmitting (which would still take several minutes to notice) subs would establish communication to check if a nuclear war had started.
They could communicate out via a buoy and really long cable, and in fact that is how a lot of scientific expeditions work. They communicate with a support vessel via cable. However, once the "wreck factor" is introduced all bets are off. Underwater objects being extremely difficult to locate is kind of why they are popular with militaries.
"It was near the titanic at X time and date" is probably the best data you can ever hope for, and that's covered by the dive plan, no tech needed.
Acoustic beacons exist for underwater position [0]. They usually connect to a transponder on the surface. Not sure this thing had one. Reading this: "This experimental vessel has not been approved or certified by any regulatory body, and could result in physical injury, emotional trauma, or death."[1] makes me think it was a disaster waiting to happen.
There are technical limitations there, the only really viable way to signal from great depths would be sonar. A radio beacon would be useless under 4000m+ of water, and there's no realistic way to ensure that a small bit of wreckage ever surfaces to make a radio beacon work. There's also no legal requirement, there is no FAA for submersibles. Now something like an airplane's black box has an underwater locator that works for a while, but there's no particular reason to expect that a meaningful and timely search can be made at the sort of depths a submarine like the one in the article works at.
tl;dr Rescue and recovery at 4000m+ is pretty much never going to happen, although it is technically possible, and there's no reason to worry about safety standards for something this niche, or limited to militaries.
Well, my main question was mostly _why_ it doesn't work (I may not have worded that very well). That said, presuming there was a way to make it work, I'd have it transmit all the available information about what's happening on the sub (respecting any technical limits).
Then if the transponder stops sending data, you know something is wrong. Even if you don't know the exact location, knowing the last depth/speed the sub was at before losing comms, for example, is valuable.
The caveat to all this is, like other comments said, radio doesn't work so well in salt water (which I didn't know).
Due to an indeterminate crush depth of various electronic media, I would implement it as stone cylinders with chiseled data. The chisel would also act as a sonar signature.
I feel like the company in charge of all this should at least know where the submersible is, if not what happened to it.