Don Brown runs a nice blog on Air Traffic Control issues called Get the Flick, but unfortunately he doesn’t accept comments. So instead of responding to his piece on ADS-B Intel as a comment, I’m going to have to do it here.
Here’s the problem: how do we automate the tracking of airliners on oceanic routes? The technology in use today is the same as it’s been since time immemorial: pilots are required to check in via HF radio and make a verbal position report at specified waypoints (e.g. every 10 degrees of longitude). HF radio is unreliable and noisy, and the low frequency of position updates means that the controllers have a very crude and approximate view of what’s going on.
Over land, things are much better. Every airliner carries a “Mode S” transponder which for ADS-B is enhanced so that it “broadcasts the position of the airplane (and some other stuff) to the rest of the world 60 times a minute.” (This is called an “ES” message.) But Mode S is line-of-sight only, so once an airliner is over the ocean, the signal gets lost.
Don considers the satellite option:
If we could get the data relayed through satellites then we would have universal coverage. That would be a huge boost over the oceans where we currently run wide spacing between aircraft because we don’t have radar coverage. But […] it turns out that sending data via satellites is expensive. Prohibitively so. […] That data can be sent through a satellite but it won’t be (very often) because it is so expensive. I understand it’s about $2 per position report to run it through a satellite. That would be $120 per minute if it matched the refresh rates of the GBTs. Oh, and the big catch is that the airlines pay for the satellite transmission. In other words, we won’t get a lot more position reports than we do now (via voice) over the radios.
Bummer. But then he notes that because the signal is line-of-sight:
… any other airplane that is within range will receive the signal. In other words, over the middle of the ocean, controllers won’t know where airplanes are with any great accuracy — they won’t get frequent updates from ADS-B over satellites — but the pilots will know where the other planes in range are with the same once-per-second accuracy. I see an attempt at “pilot-based separation†in the future. How about you?
Well, no, I won’t expect “pilot-based separation” (and don’t we already have that, called TCAS?) But if there are enough aircraft in the air at a time, why not use a mesh network? In the simplest approach, each aircraft could simply rebroadcast the ES messages from other aircraft in its vicinity. If that’s too chaotic, it would be relatively simple (and cheap) to add a smart mesh network appliance to each aircraft, and route the ES data over this network. The FAA has deployed ADS-B transceivers on oil rigs to extend coverage in the Gulf of Mexico, so hybrid ADS-B networks are already possible.
Having said all this, I expect satellite networking will become the preferred approach. As more and more airlines offer wireless Internet access, the cost of satellite data transfer is going to drop. (See my piece First blog entry from 34,000 ft, posted nearly three years ago. How long before the rule is “Make position report via HF or email”?) But I’m a big believer in using redundant technologies which don’t share failure modes, so mesh networked ADS may well have a future.
