by
Darryl Phillips
It's the dinner hour and the phone rings. The familiar voice on the other end says "This is Dan Rather, and here is the news". For the next half hour he reads you the happenings of the day. Then he hurriedly signs off, with the remark that he needs to call the guy next door and repeat the performance! You are left wondering how long it'll take to cover 20 million households at a half-hour each. Has broadcasting come to this?
Well, not quite. But if the FAA has it's way, we're in for a situation that's remarkably similar, and certainly as inefficient. They want to load everything but the galley sink onto a single frequency. It's happening already, and our flying is being affected now. And it's going to get worse.
The frequency is 1090 megahertz. Quick now, glance around your instrument panel and see if you can determine what common piece of equipment uses this frequency. While the Nav/Com, ADF, and DME each have hundreds of frequencies from which to choose, the transponder has only one. 1090 Mhz. Remember that number, it may be the reason ATC is vectoring you all over the sky.
Of course the transponder isn't the only piece of avionics that works on a single fixed frequency. We have the marker beacon at 75 Mhz. The ELT uses one frequency in the communications band, 121.5 Mhz. (It also uses 243 Mhz in the military band.) Loran works at 100 Khz, that's a tenth of a single megahertz. More recently we have GPS, most of the civilian models use just one frequency near 1575 Mhz. There isn't anything wrong with using just one frequency, as long as we're trying to do just one thing.
It's a different story when a number of aircraft systems attempt to use the same frequency at the same time, with signals competing and conflicting and colliding. If we listened to 1090 the way we listen to Unicom, the result of all this activity would be obvious. Bedlam.
What is all this stuff on 1090? For starters, we find the Mode A signals from every transponder. The squawk code, from airliners and Ercoupes. Bonanzas and B-1s. Every transponder squawk, regardless of the code set by the pilot, is on this single frequency.
Next, all the Mode C altitude information has been piled on 1090 too. Remember the Northeast Reorganization Plan? It coincided with the time most of us were required to add Mode C. Suddenly, with the same number of airplanes, the sky was twice as full of signals and the FAA found they couldn't cope. So they reorganized, moved flights further apart, not to keep aluminum from bumping but to keep the signals from colliding. The result: it takes longer to get from here to there. More time aloft, more chance of something going amiss, more fuel consumed, more dollars spent.
Next came TCAS. An airborne collision avoidance system that doesn't rely on the ground is an excellent idea. Unfortunately, the wisdom of Washington prevailed again, and TCAS was put on 1090 Mhz. Every time an airliner interrogates your transponder, it replies on that frequency. All the other planes for miles around reply also. If there are numerous airliners, the sky thickens with replies. As TCAS came into use, controllers began complaining that they were losing signals. Whole sectors, or even the entire TRACON, would overload for minutes at a time. Sometimes the scopes show no targets at all, other times the targets jitter along, delayed seconds behind the true aircraft position. It is scary that a system intended to improve flying safety would do this instead. All because of one badly overloaded frequency.
And of course there is Mode S. That subject deserves a column all it's own, but since Mode S is on the same frequency, it can't be ignored here. Mode S signals are longer than Mode A or Mode C. A lot longer. If microseconds were dollars, a single VFR 1200 squawk would equal $1.80. The same 1200 squawk in Mode S is $30.00 in the basic transponder, or $58.00 in a full-featured Mode S installation like the airliners use. Can you imagine how much interference $58.00 can buy, compared with a measly $1.80?
We don't have to equip General Aviation aircraft with Mode S, at least not for awhile. There aren't any Mode S ground stations yet. But when one airliner TCAS talks to another, they talk in Mode S today. So the effects of the preposterous bursts of signal are affecting our flying in the same way Mode C did. Flight spacings are again being increased.
Next on the list is DataLink. One of the professed advantages of Mode S was the promise of DataLink, a means of transmitting all sorts of information from the controller to the cockpit. Clearances would be magically displayed, or even printed out. Pilots wouldn't have to listen to all the verbal chatter. Individual broadcasts made to individual aircraft, sending each plane only the data it needs. At a few selected airline hubs, an early form of DataLink has been used for some time, operating on VHF. It works fine in that limited role, on it's own frequency. But the FAA wants DataLink to be part of Mode S, adding to the congestion on 1090 Mhz.
And what sort of data do they intend to link? One of their prime candidates is ATIS. Imagine that. The FAA equipment calls your aircraft, it replies that it's ready, and the FAA sends a copy of the ATIS information. Then it hangs up, calls the next plane, and reads the same data again. Doesn't this sound a lot like the baloney you read in the first paragraph? We've spent the past 75 years learning to efficiently move information to many recipients simultaneously via broadcasting. It works. Even if there were unlimited spectrum space, it makes no sense to read the news to one person at a time. And putting that bad idea on a frequency that's already overloaded is something that really qualifies as Washington wisdom.
But wait, there's more! One of the many wonders of Oshkosh is the FAA building, where new ideas are unveiled. This year they were touting the advantages of Traffic Information Service, or TIS. In beautiful full color, TIS shows the pilot all the traffic. The display could be used alone or combined with a moving map or color weather radar. Each plane is shown and the concept is stunning. They would lead you to believe it's about ready for production. Fortunately it's not, and with any luck it never will be. To move all that data from the ground to each individual aircraft the FAA intends to use - you guessed it - the same 1090 Mhz DataLink.
As the controllers complain that the increased signal density is wiping out their view of the traffic, the FAA is hiring consultants to swear it isn't happening. The alphabet organizations don't know who to believe, and there isn't much push yet from their members to address the question. Unfortunately, ignoring the problem won't make it go away.
Next time, we'll discuss a system that provides more benefits than TIS, TCAS, and Mode S, at a tiny fraction of the cost. Stay tuned.
If you're still wondering how long it'll take Dan Rather to work his way around to you again, the answer is about 1140 years. If the FAA doesn't fiddle with the phone system.