[wglb]

Keep in mind that over-the-horizon radio communication is subject to the vagaries of the ionosphere and is not totally reliable. Further, there is a day/night difference, and a dependence on latitude. Line of sight at 30000 feet is about 200 miles, so there are huge areas where everything is over your horizon.

[fnordfnordfnord]

But don't most actual aircraft flight paths tend to stay near land? For those flights that must go over that horizon, couldn't they fail over to a lower freq / lower bandwidth channel, when needed; while still keeping near real-time status updates?

Or connect via another aircraft who is further behind them but on a similar flight plan.

Here is probably one of the worst flight paths for this type of potential comms http://flightaware.com/live/flight/UAL888/history/20140310/0...

I had intended to use that to support my notion that aircraft tend to stay near land. I've flown that route several times and each time we flew an arc that kept us near or overland. Like this, but never in Russian airspace. http://flightaware.com/live/flight/CCA985/history/20140309/0...

Man, these guys are all over the place. I may have to reform my ideas about flight paths.

[wglb]

Well, for the ones that travel within sight of land, we don't have much of a problem. And that may be "most". But "most" flights don't end in disaster. It is the ones that spend long hours over the horizon that are of the most concern.

A significant number spend lots of time more than 200 miles from land. And "land" here means some land with a data link endpoint.

With respect to airplanes forming some sort of mesh network, consider the bandwidth requirements throughout this link.

[fnordfnordfnord]

>With respect to airplanes forming some sort of mesh network, consider the bandwidth requirements throughout this link.

Right, but all that is really necessary is groundspeed, altitude, and equipment number. That gets you enough information to find a crash / forced landing site quickly, and to know where to search for wreckage. So, even for those parts of the flight path that are "dark" It still may be potentially superior to low-bandwidth satellite links.

[sliverstorm]

You can't just "fail over to a lower freq". Propagation beyond the radio horizon at all is at the whim of the ionosphere, which is the only reason long-distance radio works in the first place (it reflects back down, when the conditions are right)

[fnordfnordfnord]

With full duplex communication you can.

[sliverstorm]

I... what? What does that have to do with radio wave propagation & the ionosphere?

[wglb]

How would that work?

[fnordfnordfnord]

When the aircraft stops receiving signal from the ground on one channel, it searches for a beacon on another channel.

[wglb]

And what I am saying is that in bad HF conditions, it won't hear anything, and thus won't say anything. Putting the likelihood of a missed communication higher.

Duplex doesn't fix no propagation.