Thursday, December 04, 2008

ADS-B Intel



Keeping up with technology is not my strong suit. Ask anybody. As a matter of fact, I’m typing this blog on a laptop that is over four years old. (Oh, woe is me.) Some of the stuff I’ve been reading about NextGen just didn’t compute so I thought I’d brush up on the subject a little.

Specifically, I was having trouble understanding the role of the Ground Based Transceivers in ADS-B and how that tied in with a “satellite-based system.” ADS-B is a “line-of-sight” system. It’s refresh cycle is once per second. That means that the airplane’s ADS-B system broadcasts the position of the airplane (and some other stuff) to the rest of the world 60 times a minute. The FAA thinks the greater accuracy that allows (greater than radar) will help run airplanes with less spacing between them, thereby, increasing system capacity.

Where I was getting confused was in the use of this technology over the oceans. 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 if we have satellites then why do we need Ground Based Transceivers (GBTs) ?

It turns out that sending data via satellites is expensive. Prohibitively so. (It’s probably a slow process too.) We need the line-of-site GBTs to get that greater accuracy (once per second) that the FAA is counting on. 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. It’s better than not having it but it isn’t to be confused with when the FAA is talking about the fact that ADS-B is more accurate than radar.

Here’s something that is really important to note though. ADS-B is line-of-sight so 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 ? (I won’t go off on a tangent here but let me just say I’d pay good money to watch that goat rope.)


Here’s another tidbit I’ve learned about ADS-B. There are currently no examples of reduced separation using ADS-B over land with GBTs. The standard separation being used is five miles -- the same as radar. And there aren’t any known studies to demonstrate that the separation can be safely reduced. That’s according to the guys involved in the Capstone program in Alaska.

According to the guys involved in the experiment being run by UPS at Louisville, the controllers don’t even use ADS-B targets. In other words, the targets they use for separation on their radar scopes are the same old radar targets they’ve always used. The only ones using the ADS-B targets are the pilots, and the last one of those I talked to (at Communicating for Safety) said they are still “a long way” from using them to separate themselves.

Now keep in mind, the FAA’s whole rationale for spending $20-50 billion on NextGen is that by running airplanes closer together we can increase the capacity of the system. I still say it won’t matter because I believe the limiting factor is the runways. My point today is that the more I find out about NextGen, the less there is to like. If there is some grand vision -- some future ATC nirvana -- I don’t see it. And it’s guys like me that will have to make it work. What I see -- what I still see -- is a sales job. There are some really interesting individual components of NextGen. But a system that will significantly increase the capacity of the National Airspace System? I don’t see it.

The two-dollar-position reports (above) reminded me of a story. Way back before radar, the FAA did not have a lot of direct radio communications in the enroute environment. Position reports were made through a company called ARINC that had a nation-wide radio communications system. The FAA had very specific guidelines as to when controllers were allowed to use the service because it cost a nickel every time a controller used it to send a message to a pilot.

A controller noticed he had two airplanes due over the same fix at exactly the same time. They were also last reported at the same altitude. One was IFR but the other was VFR. The controller didn’t call the pilots and warn them of each other because he was not responsible for separating the two aircraft (the VFR flight was operating under “see and avoid”) and the FAA’s rule prohibited using the radio system for traffic advisories because they cost a nickel apiece. It was simply too expensive. Those two airplanes hit each other over the Grand Canyon in 1956. I wonder how expensive that was ?

Don Brown
December 4, 2008

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