ADS-B Is Coming. What Is It?

ADS-B? Sounds complicated. But Mac explains that it?s really a lot simpler than the acronym makes it sound.


The FAA has awarded an initial contract to purchase ADS-B ground-based equipment, and set a bunch of dates for implementation of the system, but what is this new air traffic control technology, and what will it mean for airplane owners and pilots? A place to start is with the letters that make up the abbreviation, ADS-B, and what they stand for. The letters are the initials of automatic dependent surveillance-broadcast. Sounds obscure, but the concept is actually easy to understand, though the full FAA implementation will be anything but simple.

The automatic in ADS means that avionics equipment onboard each airplane will perform all tasks with no input from either pilots or controllers. Dependent means that each airplane depends on the other to supply necessary, accurate position information. Surveillance is the function of keeping track of the position of airplanes in the system. And the broadcast-following a dash-means that information will be broadcast to all, not just selected nearby airplanes. The "B" follows a dash because broadcasting wasn't part of the early ADS concept.

ADS became possible when the GPS navigation system was devised. For the first time GPS provided a common global grid for all airplanes to fix, and report, their position automatically. The original ADS concept was that each airplane automatically sends its GPS-derived position to all other airplanes within range. The dependent airplanes all respond with their positions, and pilots see a display of all traffic around them.

The great part of ADS is that each airplane not only sends the others its position, but also altitude and velocity, and even flight plan information. With this data all airplanes in range can use their avionics computers to not only calculate the relative position of other aircraft, but also plot where those airplanes will be in the next several seconds. With this knowledge the ADS can show the pilot the location and track of all nearby airplanes, and can also predict conflicts and advise how to resolve them.

ADS was a leading contender for a traffic warning system, what came to be called TCAS, because it offers excellent accuracy and reliability. But ADS was shelved in favor of TCAS because of the "D," for dependent. ADS is dependent on all airplanes having compatible equipment to automatically send their position and velocity information. TCAS, instead, relies only on airplanes being equipped with transponders. A TCAS measures range to nearby airplanes by timing the round-trip signal from an interrogation of the transponder and then calculates bearing using an ADF-type of antenna. As soon as you install a TCAS, or the less sophisticated TAS, you have traffic information about any transponder-equipped airplane that is nearby.

So it is the "D" in ADS that is the rub. All airplanes in any block of airspace need to have cooperating equipment or the system has blind spots and can't see those that don't. An ADS system is great for confined or remote airspace where it is possible to quickly equip the limited number of airplanes that use that airspace. For example, parts of Alaska are so remote that an ADS system can be installed in all airplanes that fly IFR there and everybody can then see everybody else. Or in a place like Australia where only small parts of the continent are served by conventional air traffic control radar, installing ADS on all airplanes is a much cheaper and quicker solution than covering the enormous empty spaces with ATC radar.

The initial ADS thinking was to keep the information in the cockpit and supply it to pilots who could avoid traffic when necessary, or use the data to separate themselves from other airplanes just like we do in VFR conditions. That's how UPS, the big package hauler, uses ADS to smooth traffic flow into its Louisville hub late at night. The entire UPS fleet of big jets is equipped with ADS equipment and the crews can see their colleagues and space themselves into the runway because there is virtually no other traffic in the area late at night.

But for ADS to work on a wide scale there needs to be regulation demanding that all airplanes in the airspace be equipped, and that's where the FAA came in, and where the system added the "broadcast" capability. In fact, the FAA has bet the farm on ADS-B and insists that it will be the sole means of air traffic control and separation sometime after 2020 when its "NextGen" ATC system is in place.

What the "B" added to ADS means is that data will now be broadcast from airplanes to the ground, as well as to other nearby traffic, and ground stations will broadcast data back to airplanes. The FAA plans a network of ground stations that will listen to the ADS-B signals of position and velocity from airplanes. Computers on the ground will plot the position, altitude and velocity of all equipped airplanes and show a controller a plot of the traffic situation very much as a conventional radar does today. Airborne ADS-B equipment can also use the data to independently plot the relative position of other airplanes nearby, but it is the broadcast of position data to the ground that allows the FAA to use ADS-B as a radar replacement.

The ground stations will also broadcast data up to all airplanes, including the ground-based plot of traffic, plus weather information such as Nexrad or airmets and sigmets, and eventually air traffic control data.

Though the concept is easy to visualize, and all of the tools are available to make it work, there are nothing but questions surrounding the final form that ADS-B and NextGen will take. For example, what is the minimum airborne equipment necessary to allow an airplane to operate in NextGen airspace? Initially it looks like a burst of data tacked on to the end of a transponder reply can provide the necessary position and velocity data for an airplane to be accurately plotted by ADS-B-equipped airplanes and the NextGen system. This is called "extended squitter" and Garmin panel-mounted transponders can already do it.

But what is the integrity required of ADS-B equipment, particularly the most compact and least expensive? If your equipment fails and no extended squitter is being transmitted, the entire system is suddenly blind to your presence. Or, what if the system broadcasts an erroneous position and velocity? You can imagine the risks that would create. Of course, our present day transponders can quit working, or send a wrong code or altitude, but the primary radar should still see our basic position. We're also talking to controllers and reporting our altitude initially, and agreeing on our approximate position. With ADS-B, remember, the "A" is for automatic, so there needs to be a high level of integrity monitoring built in so pilots and controllers could be warned of unreliable data.

Apparently there will be a transition phase from the current system to NextGen during which not all airplanes flying IFR will be required to carry ADS-B equipment. Obviously, high altitude and congested airspace will be first to demand universal equipage, but there will still be a period of years between 2010 and complete implementation of NextGen in 2020 during which there will be a hybrid system of radars, or some other type of ground equipment identifying the position of non-ADS-B-equipped airplanes. Think back 10 years and consider all of the avionics developments that have occurred, and then try to project what improvements could be made in capability and cost of ADS-B equipment in the decade between launch and full implementation. Buying early may be risky because you could easily pay too much for too little.

There is no question that ADS-B has the potential to improve the position tracking accuracy of airplanes in the ATC system. ADS-B will send out the essential information once each second with greater precision than current radars that require many seconds to complete a sweep to update aircraft positions. Also, ADS-B should be more reliable and less costly to maintain because the huge rotating antennas with their high-power transmitters will be eliminated.

But equally fundamental to the NextGen plan is to move costs up from the FAA on the ground and into the airplane. The cost of equipment that can compute, broadcast, receive and display all NextGen information in the cockpit will be much higher than the price of the transponders that perform the equivalent job today. Yes, all of us, both in the ATC system on the ground and in the cockpit, will have much more useful data available, but the cost equation shifts toward the airplane.

There is, however, one thing about ADS-B that I am willing to bet my house on-it will be late. The FAA has never, ever stayed on a schedule to introduce new technology, and it has never tried to make such wholesale changes in the fundamental ATC system before. My best guess is that there will be several interim steps, and most of us will install equipment that will become obsolete before the FAA completes the move to NextGen. You can call me cynical, but if you want to bet on a horse that has lost every one of its races in a lifetime, I'll call you a sucker. The operative word in NextGen is "Next" and we really can't know what that is, or when it will be here.