The Ins and Outs of ADS-B

Chris Gall

As you've doubtless heard, Automatic Dependent Surveillance-Broadcast, or ADS-B, long spoken of as the surveillance system of the future, is officially here. In late May the FAA published the final rule, though, fortunately, it will be years before you need to do anything about it.

If you think of ADS-B as being like a transponder, only with possible benefits, you aren't far off. Our ADS-B transmissions will let ATC (air traffic control) and other similarly ADS-B equipped airplanes know where we are, and they will provide additional information, such as altitude, N-number and vertical speed.

It's not just technology for its own sake. There are benefits coming down the road, according to the FAA. Eventually, the agency says, ADS-B will offer services that will benefit every user, but especially the airlines and some business airplanes. Enhancements for more efficient approaches, expedited departures and closer in-trail separation will all, to hear the FAA tell it, increase capacity. For the general aviation community, the traffic management benefits are less glamorous. Eventually, we might see expedited approaches to nonradar airports in low IFR, conditions under which currently only one airplane at a time is cleared for the approach, the so-called "one-in, one-out" rule.

The most basic form of ADS-B will be mandatory by 2020 for all of us who want to fly in the places where we now need a transponder. In the final rule, the FAA decided to mandate just ADS-B Out, which transmits only. (ADS-B In, which requires a display of some kind, allows you to see traffic and to get data-linked graphical weather for free.)

The FAA considered making ADS-B optional for all but the very busiest airspace, a move supported by several alphabet groups, including AOPA (Aircraft Owners and Pilots Association), but the agency rejected that idea in the name of near universal equipage, a key tenet of the plan.

The decision was understandable. The system's most basic and critical function is to keep aircraft separated. ATC's current radar can see primary targets even when their transponders are inoperable or turned off. ADS-B, on the other hand, is completely dependent on an airplane being ADS-B equipped and on that gear being operational. You might know that the requirement for transponders was born of a couple of tragic and horrifying midair collisions between light GA airplanes and airliners, a fact that no one at the FAA has forgotten. So in the end, the FAA erred on the side of requiring ADS-B in as much airspace as it felt it could while still allowing non-ADS-B airplanes to have some airspace to fly in.

As you probably know, the national ADS-B system is already being developed, and ADS-B is in use in a few places in North America, including the Gulf of Mexico, where controllers are using the system, with ground stations installed on oil rigs, for separation of traffic over the Gulf.

The nationwide system is already largely in place along the East Coast, and it will be phased in nationwide over the next several years. As I said, even after the system is complete, operators will have until 2020 to equip their airplanes with ADS-B gear.

The almost decade-long lead time looks to be a good thing, as the architecture of ADS-B, for a variety of technological and nontechnological reasons, has become far more complex than anyone originally envisioned.

If you're confused about what exactly ADS-B is, what it does and what it means to you, don't feel alone. The final rule has created a convoluted, multitier, multitechnology system with primary features that are fundamentally different from the international standard, with other features that have yet to be developed, and with regulatory issues that still need to be ironed out.

While the final rule presented few surprises to insiders, for the rest of us, it provided a better picture of what this new technology is and what it will mean to our flying. It also made clear just how much the vision of ADS-B has changed since its launch 15 years ago.

What exactly ADS-B will mean to our checkbooks is a harder question to answer, though we have some ideas on that subject too.

A Winding Road
The FAA has been talking about ADS-B, or something resembling it, for nearly 40 years, and experimental systems have been under development for close to 15 years. The new surveillance scheme is a critical part of the FAA's NextGen air traffic management system, which seeks to eek more capacity out of the national airspace system, in large part through modernized equipment.

Indeed, the idea that grew into ADS-B was compelling. Instead of radar dishes, the FAA would make use of simple ground stations that would receive automatically generated aircraft position reports and compose them into a near-real-time picture that would allow ATC to do two main things: improve coverage so that airplanes operating even in very remote areas would be under surveillance and improve accuracy to the point that separation could be reduced, allowing more airspace capacity. This latter idea was and is at the center of much of the FAA's NextGen air traffic management plans.

As a bonus, the new system would allow users to get rid of their old transponders, and it would allow them to see traffic and to get weather in the cockpit. Such value-added benefits would motivate owners to equip their airplanes with ADS-B.

Almost none of these things have come to pass, at least not as originally envisioned. We will be required to keep, maintain and turn on our Mode-C or Mode-S transponders. The FAA will have to keep more of its radar dishes than anticipated. And separation benefits will be less than imagined and slower in coming. Perhaps most disappointing is that the broad "radar" coverage we were led to believe ADS-B would provide won't be there. ATC coverage will be comparable to what we have today.

In order to test the technology, the FAA conducted arguably the most radical, large-scale, expensive and successful tests of a technology in FAA history with the grand Alaska Capstone experiment. Capstone began modestly in 1999 in the Ketchikan Peninsula of southeast Alaska and eventually expanded throughout the state. During that time the FAA paid to equip hundreds of publicly and privately owned airplanes with fast-track-certified ADS-B gear and installed an extensive ground station system linked to Alaska air traffic control facilities. The end result was that the FAA was able to provide, for the first time ever in many places, radarlike coverage for airplanes operating in the mountainous Alaska environment. The safety gains, the FAA credibly claims, were impressive.

There is little disagreement that Capstone proved that ADS-B can work, but it also showed the thorny technological and political problems that the FAA would face in defining the nuts and bolts of the system.

Understanding the Two Protocols
In order for ADS-B to work, airplanes need two things: a very accurate navigational source - GPS takes care of that for most of us - and a way to transmit the aircraft's position and other data to the FAA's ground stations. It sounds simple enough, but the selection of the frequency to be used by ADS-B was by far the thorniest problem the FAA faced in developing the system.

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.

The third reason our transponders won't be going anywhere soon is the existence of TCAS and TCAS II traffic and collision avoidance gear, not to mention the wide variety of general aviation traffic advisory equipment, such as L3's popular SkyWatch product, all of which rely on transponder returns to function. While traffic utilities are advisory in nature and optional for GA, TCAS is mandated for air carriers. The cost of making all existing TCAS and traffic advisory devices compatible with ADS-B technology, even if doable, would be very high.

Costs, Collisions and Coverage
As program details have emerged during the last few years, the general industry mood about ADS-B has changed from optimism to widespread skepticism, for several very good reasons.

Perhaps the biggest problem with ADS-B is that we're going to have to pay for it. For years the FAA seriously considered underwriting the cost of installing a basic ADS-B box in all of our airplanes but at some point rejected the idea for budgetary reasons. The idea, according to a couple of insiders with whom I spoke, is not dead. Even in the wake of the final rule being published, the FAA was still floating the idea of subsidizing equipage. As it stands today, though, we'll have to buy our own ADS-B hardware and pay to have it installed.

That wouldn't be such a bitter pill to swallow if ADS-B did more for us, but at this point, it offers few benefits.

The main benefits that the FAA touts for ADS-B Out, including improved search and rescue and enhanced flight following, are hardly the kinds of features that will cause owners to rush out to equip their airplanes with expensive ADS-B equipment.

There's not much ADS-B equipment available right now, and what is out there is quite expensive. But the FAA hopes that, over time, a competitive marketplace will emerge with much lower cost ADS-B equipment available. Today, owners with the most advanced equipment, specifically Mode-S transponders, will likely have the cheapest road to being ADS-B compliant.

Another concern is security. Unlike existing radar sites (which, by the way, will not all be going away under the newly adopted regulation), ADS-B doesn't see any airplanes at all unless they're broadcasting, so in areas without conventional radar, non-ADS-B airplanes will be invisible to ATC. Without primary radar detection (which requires a spinning dish), there would be nothing preventing an intruder from simply turning off his ADS-B gear and flying invisibly through the airspace.

And the threat of midair collisions, due to the dual-link architecture, continues to be a concern. AOPA has asked the FAA to rebroadcast ADS-B at all general aviation airports to mitigate that risk, though the FAA appears to have neither the funding nor the will to do that.

ADS-B Hopes
While the challenges and costs of ADS-B loom large right now, there is hope that the technology will provide more benefits than might seem likely today. And there's little doubt that rule will create new markets and products.

Strangely enough, one potential bright market is with handheld and portable devices. Since the ADS-B signal is public, it stands to reason that manufacturers should be able to design panel-mount or portable devices that would receive the signal and display its free traffic and weather information on a portable display. Indeed, a couple of small companies are already doing just that. But it's not that simple, unfortunately. The FAA has decided, apparently in order to promote participation in ADS-B, to remove some of that value. Currently it is broadcasting traffic to receivers only within a certain distance of participating aircraft. So even if an airplane had an ADS-B receiver, unless it had ADS-B Out, the pilot would not necessarily see traffic near it, making the traffic function of ADS-B all but useless. The matter is far from settled. It's not clear that the FAA has the legal right to pull the plug, effectively speaking, on this traffic data, and alphabet groups continue to pressure the FAA to free up the data.

It's also likely that a number of combination-technology ADS-B products will emerge during the next few years. One can imagine the demand for combination UAT/1090ES boxes that would receive data from all participating ADS-B aircraft even when not in the vicinity of a rebroadcasting ground station. And combination ADS-B/GPS and even ADS-B/GPS/air-data devices are a natural too, especially for aircraft not yet equipped with approved GPS.

Our advice at this point with ADS-B is to wait and see what kinds of products become available over time. There's simply no compelling reason to equip right now. Indeed, those operators who wait even a few years are likely to be rewarded with better, more capable and cheaper ADS-B solutions.

ADS-B at a Glance There are several areas of confusion with the ADS-B final rule, and I'll attempt to clarify some of the most noteworthy.What It Is: ADS-B is simply a new-technology transponder-like device. It tells ATC (and ADS-B airplanes) where we are.What It Does: With ADS-B Out, we send our position and flight data to a ground station that relays that data to ATC's "radar" screens.ADS-B Out: The mandated ADS-B is called "ADS-B Out" because it sends its signal out to ATC. This is what we're all going to be required to have by 2020.ADS-B In: ADS-B In is what we'll get with an ADS-B receiver and display (either a dedicated display or, more likely, an MFD with an ADS-B function). With ADS-B In, we'll be able to get traffic through the CDTI (cockpit display of traffic information), weather and, potentially, more.How It Works: The ADS-B transmitter in our airplane broadcasts (that's the "B" in ADS-B) our GPS position and more, which ATC receives though a series of ground receivers. ADS-B receivers are hooked up to ATC to give controllers the big picture. Because the FAA's ADS-B system relies on dual technologies, UAT and 1090ES, there are four systems at work, UAT, 1090ES and rebroadcasts of both of those systems. As pilots, we get the information directly from airplanes using the same kind of technology as we have and from all airplanes when we're in range of a ground station.What the FAA Gets: The feds get hundreds of "radar" sites that are a lot less expensive for the FAA to install and maintain than radar dishes are. The FAA stands to save a fortune in the deal, probably billions over time, by gradually shutting down more than 300 radar sites. The feds also get greatly improved position information on all of us. Make no mistake: ADS-B makes sense for the FAA.What the Airlines Get: The hope is that commercial operators will get reduced separation, especially on oceanic routes, fuel savings due to better routings, better traffic advisories and, ultimately, more arrivals. It's potentially a huge cost benefit to the airlines. While many in the airline industry are skeptical or still on the fence about the benefits, the FAA seems convinced that the potential cost savings are both real and substantial.What It's Going to Cost: This is a bit of a guess, as there are no compliant products right now, but based on what equipment is out there, it will likely cost $5,000 for a basic ADS-B Out box with a single antenna to as much as $30,000 or more for a dedicated ADS-B In box with a display and with dual antennas. If you've already got a Mode-S transponder, as the airlines do, you might be able to convert your Mode-S transponder for as little as $1,000, though, ironically, it's going to cost the airlines a lot more than that to make the transition. Over time it's almost certain that the costs will come down, but for those without Mode S, it's unlikely that equipping will cost any less than a few thousand dollars in equipment and installation costs.How Compatible It Will Be: While the airlines and their 1090ES system will likely play well worldwide, unfortunately it looks as though operators with UAT-based ADS-B systems won't be compliant with ADS-B requirements in either Canada or Mexico, or through much of the Caribbean. There remain some sticky international problems to be ironed out there.

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