by
Darryl Phillips
Believe it or not, a lot has happened since the first introduction of Mode T in this column almost a year ago. In case you missed it the first time around, here's what Mode T is, and what it means for aviation.
See and be seen. Some pilots prefer the term "see and avoid". That is the basic idea. If we flew slow, and never encountered poor visibility or traffic coming from somewhere we don't have windows, see and be seen would suffice. We wouldn't need Air Traffic Control or transponders or radar or ILS or much of anything at all. We would just go out and enjoy flying.
But we sometimes fly too fast for the human eye, and we often fly in haze or glare or when we can't see anything at all. And we have traffic coming from places we can't see, like directly above and below. See and be seen isn't enough, so we evolved ATC and IFR and all the electronic gee-whiz boxes. And with each "improvement", we gave up more control. As time passed, the cumulative loss of freedom resulted in older pilots deciding it just wasn't so much fun anymore. And the young crowd who used to pump gas and wash planes in exchange for lessons are taking up skydiving or hang gliding or bungie jumping. Same thrill, less hassle.
Mode T is a way to reverse this trend, to get more bang for the buck and to put more utility and enjoyment back in flying. Mode T is being developed by users, General Aviation pilots who are spending their own money for development.
Technically, Mode T simply consists of a tiny box that receives GPS position data in three dimensions, and continually relays that data on a single frequency with a range of about 15 miles. Mode Ts go anywhere a warning beacon would go. Towers, mountaintops, high buildings, obstructions near the airport, and at the airport itself. And of course on planes. Anyone, aloft or on the ground, can receive this data and see the big picture. Since the system is differential, the precision is a few feet.
Why isn't FAA using some of their thousands of millions of dollars to develop such a system? Unfortunately, they are! We now find ourselves in a race between Mode T and what FAA terms Automatic Dependent Surveillance.
The problem is, we have three considerations here, but the feds think we only have two. Communication, Navigation, and Avoidance. In VFR operations they are obviously three distinct subjects. Just as we use com and nav when flying VFR, we also use avoidance. We use it to miss other aircraft, and also to miss the hills and towers and everything else. We just fly around them, in the same way drivers avoid other cars. It's so simple and obvious we don't think about it, but avoidance is not communication, and it's not navigation either. It's a distinct third channel.
In the world of instrument flight and Air Traffic Control, avoidance has been stirred and slurred into the other two. We avoid by communicating, and by navigating. But the black boxes are imprecise. And it takes extra time to route information through a third party. The net effect is to deny ourselves the utility of aviation. We've given up much of the joy of flight, and we've given up most of the transportation capability too, in exchange for this thing we call safety.
Safety isn't the real culprit, it just gets the blame. The real reason we're denied so much capability is the loss of the third channel.
Compare it to people. The most important human senses are sight, hearing, and touch. Three independent channels. Each is incredibly valuable, and each supplies input that cannot be duplicated by the others.
Hearing has an obvious analogy in the flying world, communication. Navigation compares most closely with touch, a person robbed of both sight and hearing could still navigate through familiar surroundings by touch alone (we do this if we experience communications failure while flying IFR).
Avoidance equates with sight, the most valuable channel of all. What a shame that we lost that ability in the IFR world. And FAA doesn't even know it's gone!
Consider how we avoid colliding with something in IFR today. We use two methods, navigation and communication. We talk with the controller. And we navigate where the clearance specifies. In human terms, this is a blind person using a combination of touch and verbal instructions to get around. It works, but it's an unsatisfactory substitute for seeing where you are going.
Let's give IFR sight! And let's give VFR sight where it doesn't now exist, improving the pilot's ability to see and avoid.
That's where Mode T comes in. By electronically extending the ability to see and be seen, we can have the same capability at high speed and in clouds that the slower VFR pilot has enjoyed all along. Plus a whole lot more.
Some small parts of what Mode T offers already exist today. A few planes have Ground Proximity Warning Systems or some other form of radar altimeter. This gives a limited ability to see the surface. Air carriers have TCAS, giving them the means to see other aircraft as long as everyone is equipped with a working transponder. But TCAS is proving to be unworkable in heavy traffic situations, exactly where it is needed most.
Each TCAS interrogates the planes around, and each plane responds to each interrogation. If there are 10 TCAS planes, each of the 10 interrogations results in 9 replies (a plane does not respond to it's own request). That's 90 replies. If there were 100 TCAS aircraft, 9900 replies would result, and 1000 planes would make nearly a million replies. A classic case of geometric progression.
But ten planes only need to make ten reports. Likewise with a hundred planes, or a thousand, one report per plane is sufficient. Sending out one copy of position per second is enough, everyone who is interested can receive it and it's not necessary to send a separate copy of the same data to each aircraft. Why can't the feds understand this?
There have been occasions when the controller ordered all the traffic to turn off their TCAS, and when they did the overload ceased and the controller got his picture back. TCAS works fine when there are only a few aircraft, but that's not good enough and at $200,000 per copy we can't afford it anyway.
Neither TCAS nor GPWS is the answer, as they don't provide separation from obstacles such as TV towers. Nor do they help general aviation planes see their traffic. We need a way to see, at all times, everything we see in VFR conditions. This is not to suggest that we'll "see" a farmer plowing his field or a beautiful sunset. Technology hasn't come that far yet. But we can see everything we need to see to fly safely.
Let's back up for a moment to ADS. Automatic Dependent Surveillance. What is the difference between ADS and Mode T? Technically, they aren't that far apart. Both rely on each aircraft transmitting it's position about once per second. ADS position could come from GPS, or Inertial Navigation, or whatever. It might be picked up directly by ATC, or relayed to ATC by satellite. ADS would help ATC see the traffic. It has been in development for many years.
Mode T is much more. It transmits GPS three-dimensional position like ADS, but the similarity ends there. The plan initially was to transmit an ID signal like Mode S or ADS, so those who receive it can compute speed and track, which are necessary to determine the collision potential. But that's a dumb idea. Suppose there were a hundred planes in an airspace. Each would receive the data from the other 99, and each would have to compute a hundred vectors, their own and everyone else's. That's a lot of intense high speed math. On the other hand, if each plane transmits it's own vector (ground speed and direction, plus climb and turn signals) then the computing job is incredibly simpler and less expensive. And no ID is needed. Let's face it, if a collision occurs the other plane's ID isn't one of your more pressing problems!
Most collisions happen in the pattern at a non-radar field in VFR conditions. Mode T is the answer here, because it doesn't need FAA cooperation. But the feds can receive Mode T, and benefit from position data that is much more accurate and up to date than anything provided by the radar and transponders of today or tomorrow. And incredibly cheaper too.
With Mode T, you can see more than the other aircraft. By placing a small unit at TV towers or mountaintops or anywhere else you might place a red light, the pilot can "see" these obstructions.
Putting one on each ramp vehicle lets ground control see all the traffic for the price of a PC. Every runway, taxiway, intersection, and ramp gate shown in full detail. "United 405, pull ahead 3 feet to clear the fuel truck!"
Next, putting Mode T on the airport itself gives pilots DGPS capability, permitting landing minimums to be cut substantially. The DGPS demonstration at Oshkosh '93 worked. Some pilots reported that there were bugs yet to be solved, but the idea is sound. What isn't so sound is the cost, estimated in the range of $250,000 to $500,000 per runway. Mode T can do it for one percent of that amount. And not only at big fields. Anyone with a private strip can put in his own DGPS approach. Pilots are doing it today with GPS and loran, Mode T does the same thing at 100 times the accuracy.
Along with accuracy to a couple of feet, Mode T has the ability to simultaneously relay surface wind, visibility, ceiling, baro setting, and more. Up to the second ATIS. Try that with anything from the feds!
Like MLS, Mode T approaches will permit curves and various glide slopes. Unlike MLS, Mode T approaches can be winding. Imagine recording an approach to your mountain strip in VFR conditions, complete with every turn and descent that works best for that location. That same approach could be played back later, in fair weather or foul, with the cross pointers keeping you exactly on the same track. Missed approaches and departures can be recorded as well. A database of approaches can be acquired, tailored to your aircraft type and your skill level.
Mode T benefits the pilot. ADS benefits the system. Mode T is inexpensive, ADS is a typical FAA program. But those are small differences, the big one is that Mode T gives aviation the ability to see!
What is the status of Mode T today? Well, for one thing we're looking for a better name. Mode T isn't very descriptive, and after it was introduced in this column last fall others began using the term to describe something different. Other names have been suggested, such as SABS for See And Be Seen, or SAA for See And Avoid. Plus ESABS and ESAA, adding the word "Electronically" at the beginning. Or the end. But none of these really have the zip and pizazz that this dynamic system deserves.
Actually, very little time is being wasted looking for the best name. A working group composed of pilots from all over the country is busy moving the concept toward reality. If you have talents to contribute, please join in, you are welcome and we need you. It is an open standard, available to all avionics developers. Whether you call it ESAA, or SABS, or Mode T, it is happening. Without tax dollars, without government involvement, without foreign interference. You know the old saying. If you want it done right, do it yourself. So that's what we're doing.
What should it be called? Let me know.