In the case of MH370, this is why I always found the simulator history of the pilot the most relevant. It doesn't require this long chain of relationships like some other things. It is the most impactful person recently doing something related to the event that is extremely weird behavior.
This was also relevant in that Airbus crash from maybe 15 years ago between South America and Africa.
This is not a problem on the main to/from North America routes because there are enough populated islands along the way (Iceland, Greenland, Hawaii, Guam, etc)
Radar requires line-of-sight, and oceanic air traffic control has to make due without that, which means operating either via ADS-C (i.e. planes self-reporting their position, autonomously determined via GPS or inertial navigation, via satellite communications) or effectively dead reckoning, augmented by occasional position reports via HF radio, which is called procedural control:
https://www.navcanada.ca/en/air-traffic/space-based-ads-b.as...
It's a very exciting technology, sure, but I think it's many years of testing and flawless operation as a secondary data source away from replacing ADS-C on oceanic routes.
Most of the South Pacific / South Atlantic / Southern Ocean (Antarctic); a lot of the North Pacific / North Atlantic / Indian / Arctic outside of the major islands and direct routes has nearly no coverage.
(And this is if you're a jet flying at high altitude. If you're down where smaller aircraft fly, I can point to areas on the US west coast that don't have radar or radio coverage. Heck, there's parts of the mainland US that don't even have NEXRAD weather radar coverage.)
Is that a good comparison, though? It seems like NOAA funding for the NEXRAD sites is perpetually inadequate. The one in Portland occasionally has outages that last a while. I would have guessed that civilian air control radar is better funded than NEXRAD, and military radar probably so well funded that every square inch of CONUS is covered by at least one.
The limiting factor is geography. Radar coverage in the US is excellent, but the mountains out west make universal coverage hard, and there's some areas that just aren't cost effective to cover.
At 40,000 MSL, yes, everything in the US is covered. Below 10,000 MSL though, there are definite gaps - especially in the mountainous west. It's just physics, the mountains block the signal, and filling in all the gaps is cost prohibitive. Instead, they just tell aircraft to fly higher if they want radar service.
As for outages... ATC radar goes out all the time. You just never hear about it because they either switch to an adjacent, (slower) en-route radar site as part of CENRAP [2], or fall back to procedural control (voice position reports on the radio). Redundancy is baked into the system at many levels, both technical and human.
[1]: https://en.wikipedia.org/wiki/Joint_Surveillance_System
[2]: https://aviation.stackexchange.com/questions/9464/what-is-ce...
ATC radar is line of sight. At an altitude of 40,000 feet, you're beyond the radio horizon and lose coverage after about 380 miles - and that's the theoretical best case.
Oceanic air traffic control is primarily a non-radar environment, using procedural separation. Even over land, most of Canada (outside major cities and the airways between them) doesn't have radar coverage. There's a map showing the gaps here: http://ilanreich.com/Public_Pics/Alaska/Section%20One.htm
(The military does have over-the-horizon radar, but that's mostly used for missile defense. OTH radar isn't used for air traffic control.)
They are not used for air traffic control or other safety-critical purposes.
So it's not going to help at all in a MH370 scenario.
True, but that is ultimately the case for all flights outside of primary radar coverage.
These absolutely can be used for air-traffic control. ADS-B has better temporal resolution than radar, and ADS-C is at least as good as manual position reports over the radio (used for procedural control over oceanic/non-radar airspace).
However, it's worth noting that these aren't really "transponders" (in aviation, that usually refers to beacon used for Mode-A/C secondary radar). Also, these are all active systems, dependent on the aircraft's electrical bus and easily disabled. In the case of MH370 the transponder _was_ powered down for some reason.
The only system that the pilot can't turn off is primary radar, but again, coverage for this system is limited once you leave populated areas.
ADS-C is a two-way communications protocol; ATC can request a certain update interval and I believe also boundary conditions requiring extra location transmissions. If that communication fails, both the ATC and pilot are informed about the fact, and they can reliably switch to other means of communications (e.g. voice position reports over HF).
> These absolutely can be used for air-traffic control.
ADS-B is (at least for now) mostly an augmentation of primary and secondary surveillance radar, as well as something to give other aircraft better situational awareness (via ADS-B in), as I understand it. I don't think either ATC nor pilots are currently relying on it for safety-critical decisions.
The big problem is that it's always possible for some plane's ADS-B transmitter to fail (maliciously or otherwise). On land, you have secondary or primary radar to fall back to.
This applies doubly so to third-party (i.e. non-ATC/non-AOC) satellite-based relaying services, since not only the ADS-B transmitter can fail in that scenario (and ADS-B was not originally designed to be received by satellites), but the relay service could fail as well, and the failure mode would be much less visible to both parties than in ADS-C.
This is not to say MH370 was without incident, only that apparent contradiction may be unavoidable if you look closely enough at anything.