Tooting and Waving

What's ratcheted up the current interest in ADS-B is the FAA's announcement that ADS-B will be the cornerstone of the NexGen (next generation) air traffic control system (described as "aircraft-centric") and its Notice of Proposed Rule Making (NPRM) mandating that by 2020 all airplanes operating in Class A, B and C airspace over the 48 contiguous states, and in Class E airspace above 10,000-feet msl, will be required to be equipped with ADS-B-Out. The rule would also require airplanes flying within 30 nm of FAA specified "busy" airports to be ADS-B-Out equipped. (More about the "Out" in ADS-B-Out later.)

It looks as if ADS-B, like the Volkswagen Beetle, will become ubiquitous. In the interest of full disclosure, I have to admit I'm in favor of the implementation of ADS-B, and in fact, my Cessna Cardinal has been equipped with ADS-B for more than three years.

Unfortunately, there seems to be a great deal of confusion about exactly what ADS-B is and what it might do for pilots and controllers. The existence of the confusion was driven home to me by a reader poll on flyingmag.com in April (the results of which appeared in the July issue of the magazine); the question asked was: "How would you rate your knowledge of WAAS approach procedures?"

According to the poll, 70 percent of the pilots who responded to the question indicated a "high degree of uncertainty" about WAAS approaches, but even more concerning was that 45 percent, more than half of the 75 percent, voiced "a complete lack of familiarity with the technology and 25 percent admitted having only a passing knowledge of it."

Since WAAS, (the wide area augmentation system) which transmits its data once every second, is an apparent prerequisite for ADS-B, a lack of understanding of WAAS would obviously fuzzy up the pilot population's perception of ADS-B.

Put simply, WAAS is a system of satellites and approximately 25 ground stations positioned across the country that monitor the data being downlinked from GPS satellites. Two master stations on the coasts collect the data from the reference stations and generate a differential correction message that is then broadcast through one of two geostationary satellites. The corrections account for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere.

With the WAAS corrections GPS navigators are able to provide position accuracy that's as much as five times better than without it and at an update rate of five times per second. A WAAS-capable receiver can report a position accuracy of better than three meters 95 percent of the time.

So, if an airplane equipped with a WAAS GPS knows where it is in space to such a high degree of accuracy, and if its position could be depicted on controllers' displays and on multifunction displays in airplane panels, it would work better than radar for traffic information. In fact, with ADS-B, both pilots and controllers can see radar-like displays with highly accurate traffic data that update in real time.

And that's only some of what ADS-B can provide. But I'm getting ahead of myself. Back to basics: ADS-B stands for Automatic Dependent Surveillance-Broadcast. "Automatic" means it's always on; it's "Dependent" because it depends on accurate position data; it provides radar-like "Surveillance"; and it continuously "Broadcasts" its data to any airplane or ground station equipped to receive ADS-B.

In my airplane, the ADS-B data is processed by Garmin's GDL 90 UAT (universal access transceiver) at 978 MHz for presentation on the GMX 200 multifunction display. In addition to providing traffic information the UAT bandwidth is also capable of uplinking real-time Flight Information Services (FIS-B) that provide weather and other data.

The FAA is promoting ADS-B because it offers some real advantages compared to the current ATC radar system. Unlike radar, ADS-B accuracy does not seriously degrade with range, terrain or atmospheric conditions, and the altitude and update intervals of targets do not depend on the rotation time of a radar sweep, which varies from three to 12 seconds. The ground infrastructure to provide and support ADS-B is as little as one-tenth the cost of equivalent radar equipment. And unlike radar, an ADS-B ground station requires little power and can more easily be installed in remote areas.

ADS-B-equipped airplanes combine their position data with other information such as speed, heading, altitude and flight number and simultaneously broadcast it directly to other ADS-B capable aircraft and to ground-based or satellite communications transceivers, which then relay the aircraft's position and additional information to Air Traffic Control centers in real time.

For ADS-B aircraft to display airplanes that aren't equipped with ADS-B they have to be within line-of-sight of a ground-based transceiver (GBT). The GBTs (there are currently some 45, most of which are installed along the Eastern Seaboard) receive and process transponder signals from aircraft within line-of-sight of the station and uplink the position and altitude information to cockpit displays of traffic information (CDTI) in ADS-B-equipped airplanes. The presentation of the target data of non-ADS-B aircraft is known as TIS-B (Traffic Information Service-Broadcast).

So, what's this "ADS-B-Out," mandated by the FAA's NPRM? The "Out" indicates that it's a system that only broadcasts its position and other information out to other ADS-B equipped airplanes and to ground-based transceivers. It doesn't allow target information to be displayed on the host airplane's CDTI. For traffic to be displayed on a CDTI, the airplane has to be equipped with an ADS-B-In system that displays targets of all ADS-B-Out-equipped airplanes and non-ADS-B targets processed and uplinked by GBTs.

More confusion was thrown into the discussion when the FAA selected two datalink systems for ADS-B; the UAT at 978 MHz and a 1090 MHz system. The 1090 MHz transmitter supports a message type known as an extended squitter (ES) message that periodically provides position, velocity, heading, time and, in the future, intent. The 1090 MHz ADS-B link is intended for airline aircraft and operators of high-performance general aviation aircraft that operate above FL 240; the UAT ADS-B link is for typical general aviation airplanes. (The GBTs also translate signals received in either the 978 or 1090 format and uplink them in both formats to ensure everyone gets the big picture.)

The FAA's announcement of the mandatory installation of ADS-B-Out by 2020 wasn't met with a great deal of enthusiasm by the industry. For the most part the "alphabet" associations applauded the idea of ADS-B but rejected the FAA's method of implementation.

A major concern was that the proposal mandated general aviation airplane owners to equip their airplanes only with ADS-B-Out, which wouldn't provide the benefits of ADS-B-In. The requirement was also coupled with a mandate to maintain Mode C transponders rather than allowing the ADS-B equipment to replace them.

The plan calls for a new software standard but doesn't allow for the grandfathering of the more than 5,000 airplanes, like mine, currently operating with ADS-B equipment that meets the earlier standard.

For pilots, the only benefit without ADS-B-In is that other pilots who do have it will be able to see them. With it there are a number of features that make ADS-B different from -- and in my opinion better than -- other traffic display systems. Some systems don't indicate the direction a target is moving. That works pretty well for targets that are moving quickly enough so their progress across the display is obvious, but for slow-moving targets it may take a couple of anxious moments for the pilot to visualize their direction of travel. Most traffic warning systems have a protected cylinder of airspace, for example 1,000 feet above and 1,000 feet below the host airplane. So, for example, if a target climbs (or descends) into the protected airspace to the right of your course, it's helpful to know if the target is moving to the left, across your flight path, or to the right, away from it. With ADS-B, targets are depicted as an arrowhead-shaped icon with the point aiming in the direction the targets are moving. An option allows the display of a vector arrow based on time intervals of one to 10 minutes of where the target will be. For non-ADS-B targets, the icon appears as a "bullet" symbol.

The ADS-B system lets the user select two different vertical limits for the protected airspace; either 2,000 feet above and 2,000 feet below the host aircraft or all targets within range from the surface up.

All targets can be shown with either their barometric altitude or their altitude relative to the host airplane. It may be easier, if you're cruising at 6,000 feet, to recognize the separation if it's shown as 62 or 58 rather than +02 or -02. ADS-B also lets the user "select" individual targets to see their flight ID, aircraft category, clock position, GPS altitude and groundspeed. When traffic is called at your 6 o'clock, same altitude, same direction, it's reassuring to be able to see that its groundspeed is slower than yours.

A "text" page displays a series of columns. The columns include all the targets in order of their distance from the host aircraft; their flight ID (if it's being broadcast); the category (Small, Large, Heavy, Rotorcraft, etc); the clock position to the target and the host's position from the target; the speed of the target and its altitude.

Airborne targets are cyan in color; targets within five miles and 2,000 feet vertically are yellow; airplanes on the surface as well as vehicles are colored in tan on the display. The vehicles are shown as rectangular icons. Being able to display surface traffic reduces the potential for runway incursions.

Although the implementation of air traffic control with the primary reliance on ADS-B and WAAS GPS navigators will not eliminate the entire costly radar array, the potential reductions of equipment and maintenance support should result in substantial savings.

Nevertheless, the NPRM did not commit to or support any operational or financial incentives for early ADS-B-Out equipage. At one time I was told that if the FAA decommissioned all the NDBs in Alaska, the savings would approach $100 million over 10 years. Quite a savings! If the FAA equipped every airplane in Alaska with WAAS GPS navigators and ADS-B it would cost approximately $50 million, a savings of $50 million.

I recently got a government-provided $40 coupon to purchase a converter to allow my analog television to display digital broadcasts, which will be required by February 2009. That got me thinking. If the government is willing to pay to upgrade millions of privately owned televisions, why shouldn't it be willing to upgrade privately owned airplanes?

There have been several suggestions, ranging from outright credit for aircraft owners to a sliding scale of rebates depending on how soon pilots upgrade to the system. For example, one suggests that pilots updating within a year of the start of the incentive program would receive the largest rebate with the value of the rebates declining in each successive year.

The Aircraft Owners and Pilots Association (AOPA), in acknowledging its participation on the consortium of 25 industry associations, corporations, universities and government agencies that was awarded the Collier Trophy, called ADS-B "an affordable combination of GPS, moving-map displays and datalinked weather and traffic information [that] could significantly improve both safety and utility, enabling an all-weather transportation system serving virtually every airport in every corner of the nation."

But, after the NPRM was released, Phil Boyer, AOPA president, was not happy. "The suggested rule is not acceptable in its current form. It is a high-cost plan that offers few if any benefits for general aviation. We believe the current proposed rule is so unworkable that the FAA needs to abandon plans to publish a final rule, go back to the drawing board, and issue a supplemental notice of proposed rulemaking," he said.

And the FAA is reportedly doing just that. As a result of more than 300 negative comments received during the NPRM comment period (extended 60 days from January 3, 2008, to March 3, 2008), the ADS-B Program Office has apparently been instructed to consult with industry representatives to come up with a more palatable version of its NPRM. The new version is now expected to be released later this year.

Personally, I'm anxiously awaiting the promised expansion of the array of GBTs so the system in my airplane will be that much more useful. I'm too far north to receive any TIS-B traffic uplinked from the GBTs at the heliports in Manhattan. Unlike Florida, Arizona and North Dakota where flight schools have equipped their fleets, ADS-B targets are few and far between. Nevertheless, the other day, while doing approaches to stay current with Kent Haskin as safety pilot, when I ranged out to 100 miles on the GMX 200 to demonstrate the XM Weather function, I got all excited because there was an ADS-B target on the display. It was a "light" airplane 57 miles away, at 5,400 feet and traveling at 142 knots. I wanted to wave and toot!