In the end, the FAA made the extraordinary choice of not one but two modes of transmission, UAT and 1090 extended squitter, or 1090ES.
UAT, which stands for universal access transceiver, is a two-way protocol that makes use of an existing but unused DME and tacan frequency (978 MHz). UAT has the advantage over 1090 of having more available bandwidth - important if the FAA uses the system to send relatively large amounts of weather and traffic data to airplanes.
The idea was for UAT to be the system of choice for general aviation users, since it has the bandwidth to support the delivery of graphical weather data through the FAA's free FIS-B (Flight Information Services-Broadcast). FIS-B will offer pilots with ADS-B In free graphical weather through the CDTI (cockpit display of traffic information).
But there are problems with and limitations to that plan.
According to the rule, UAT can be used up to only 18,000 feet. This regulatory limitation, practically speaking, makes it suitable only for nonpressurized and even nonturbocharged airplanes.
UAT also limits, so far as anyone can figure, UAT-only-equipped ADS-B aircraft to United States airspace, because Mexico, Canada and much of the Caribbean will almost certainly go with 1090ES as the standard. The restriction won't kick in for close to a decade, and our neighbors don't have much ADS-B infrastructure in place yet, but it's a consideration for owners pondering which link to pick.
UAT's benefits have also become less attractive since the FAA made its decision. During the past several years, weather in the cockpit via commercial satellite services, XM and Sirius in particular, has become ubiquitous. Thanks to increased competition, traffic systems have plummeted in price as well. This has led to the widespread private adoption of the two main safety utilities that the FAA imagined would someday drive voluntary equipage of ADS-B. If the other protocol, 1090ES, sounds familiar, it should. 1090 is the frequency used by our existing Mode-S transponders and those of commercial airliners. A standard Mode-S transponder can be converted at relatively little cost to 1090ES. The extended squitter allows the system to send the additional position and state-of-flight data.
While the goals of providing incentives to GA owners and making the airlines happy make sense, the choice to use multiple data delivery platforms left the FAA with some real-world complications.
First and most alarming, airplanes equipped with UAT systems would not be able to see airplanes equipped with 1090ES systems, and vice versa. To get around this, the FAA has taken another extraordinary step, rebroadcasting 1090ES-derived data to UAT users and rebroadcasting UAT data to 1090ES users in a function known as ADS-R (R standing for rebroadcast).
Critics say the rebroadcast scheme is an expensive and complicated band-aid. Some warn that this function presents a single point of failure for the entire system. Of more immediate concern, the "rebroadcast" data will be available only within range of a ground station. When out of range, an airplane broadcasting UAT position data will still be invisible to an airplane receiving only 1090 data, and, again, vice versa. The safety implications of this are extremely troublesome.
Private industry might have a solution down the line. There is nothing in the regulations and there are no technical hurdles that would prevent a manufacturer from building a combination UAT/1090ES box, one that would eliminate the different-technology blind spots while allowing high-flying aircraft the ability to get FIS-B through UAT.
Multiple Frequencies, Increased Complexity
The decision to use dual links was driven by political and economic considerations, as well as the shortcomings of the available data-link frequencies. At the time the call was made, the FAA believed that if it offered pilots of light GA aircraft value-added traffic awareness and weather utilities, those owners would voluntarily upgrade to ADS-B in order to get the benefits.
Most airliners and business aircraft, on the other hand, already have traffic, through TCAS (traffic and collision avoidance system) and TCAS II, and weather, through onboard radar. Moreover, because of the decades-old TCAS mandate, most already have Mode-S transponders. While Mode S will give many light GA operators a relatively cheap upgrade path to ADS-B, for the airlines, that might not be the case. The problem is that not many airliners have GPS - they typically rely on other forms of navigation, including inertial navigation, that are not easily compatible with ADS-B. Many older airliners, believe it or not, still get by with VOR/DME. For the airlines, ADS-B could be an extremely expensive proposition. Indeed, the FAA and the airline industry agree that the cost will be in the billions, between around $2 billion and $6 billion, in fact.
Like GA pilots, airline insiders aren't sure that the savings derived from ADS-B will cover the installation costs through fuel savings and more tightly packed arrivals, if those savings do indeed come to pass.
In the view of many, the biggest obstacle to increased capacity will continue to be the lack of available runways. The typical three-mile in-trail separation in the terminal area equals approximately 90 seconds of spacing. This is about how long it takes to clear a runway. So even if decreased separation were possible, it wouldn't do much, critics claim, to increase the rate of arrivals during busy times.
So, despite the fact that no one from GA or the airline community seems convinced of the benefits of ADS-B, the FAA still settled on the dual-link scheme.
Keep Your Transponder
ADS-B is a replacement for our current surveillance technology, though strangely enough, we won't be replacing our transponders any time soon. There's no way of knowing what kind of technological progress might come to pass in the next 10 years, but unless something changes, we'll have to keep our transponders installed, checked and turned on even after near-universal equipage in 2020.
I asked Bill Stone, aviation products manager at Garmin International, about the need to keep our transponders, and he explained that there are several reasons to do so.
One, Stone said, is that, as we transition to ADS-B, the air traffic management system has to keep working seamlessly, so even while we're equipping with ADS-B boxes, we need to be squawking a code, at least through 2020.
The second reason is that Mode C and Mode S will serve as a backup to the ADS-B system in case of system disruptions. Without transponders, in case of a systemwide GPS shutdown, all traffic would become effectively invisible to ATC because ADS-B relies on GPS for its position data. We will in essence be double-equipping for ADS-B.
There are possible alternatives, Stone explained, including wide-area multilateration, which would use small, relatively cheap ground stations to triangulate our positions. No such system has been fielded, though some do exist. It would be a cheap alternative to keeping radar dishes as a backup, and it would eliminate the need for our current transponders.