Mysteries of ADS-B

Tom Benenson

It was just a rumor, but I liked the idea. I'd heard that the FAA was going to equip all the airplanes in the Washington, D.C., area with ADS-B and UAT so it could better monitor their movements. The rumor got legs from the unfortunate, apparently inadvertent, incursion by the Cessna 150 into the Washington ADIZ that resulted in the mad scramble that played repeatedly on the national news.

But I liked the sound of the rumor. I recently equipped my Cessna Cardinal with the Garmin GDL90 ADS-B (Automatic Dependent Surveillance-Broadcast) UAT (Universal Access Transceiver), so, although it's self-serving, I'm in favor of anything that will promote the implementation of ADS-B. For the moment, the coverage area is pretty limited unless you're flying along the East Coast or a couple of isolated locations around the country. So I liked the rumor. But, unfortunately, it appears it's just a rumor.

The advantage of ADS-B to pilots is that equipped airplanes offer air-to-air as well as ground-to-air traffic information. The data available in the cockpit includes the target's unique ID and ICAO identifier, the relative altitude (or barometric altitude) of the target and whether it's climbing or descending. (There are actually two data links that are assigned to ADS-B: 1090 MHz Extended Squitter [1090 ES] for aircraft that typically operate at or above 18,000 feet; and the UAT [978 MHz] for aircraft that primarily operate below FL180.)

As good as it is, there's a major disadvantage to ADS-B. Well, not to ADS-B per se, but to the lack of availability of ground-based transceivers (GBTs). In order to display non-ADS-B-equipped airplanes on a cockpit display of traffic (CDTI), the host airplane has to be within line-of-sight of a GBT that sends up traffic forwarded from ground-based air traffic surveillance sensors, typically radar. The uplink of non-ADS-B traffic is referred to as Traffic Information Service-Broadcast (TIS-B). Because the uplinked radar information is not as accurate as the ADS-B targets, the icon on the cockpit display for "degraded" non-ADS-B traffic looks a bit like a Pacman that's had a bite taken out of its backside; the ADS-B traffic is displayed as a chevron or arrow head pointing in its direction of movement.

In my installation, the traffic and weather are displayed on an MX20 multifunction display. A series of options are available on the MX20 to control the display. The ADS-B traffic, but not the TIS-B radar targets, can show a trend vector that indicates where the target will be in one to 10 minutes (selectable by the pilot). The ADS-B targets also display their unique ID. The UAT system does provide a VFR "privacy" mode for pilots who don't want to receive air traffic services and don't want their ID broadcast to the world. Other airplanes will still see their target but it will be flagged as "VFR."

With the dedicated traffic page displayed I have the option of zooming in or out, or "selecting" any particular target. When a target is selected, its icon changes from blue to green and a data block in the upper right hand corner of the screen shows the target's relative position to my airplane and my position relative to the target, the target's speed, distance from me and its relative altitude. (I can also elect to have the traffic information superimposed on the sectional-style map.) If I haven't opted for traffic to be displayed and a target becomes a potential threat, the MX20 flashes a "TRAF" alert warning me to call up the traffic page.

I can also select a "text" page that lists all the targets within the selected range of the display in order of their distance from my airplane. In addition to the distance, the text lists: host/target relative positions, speed, altitude and for ADS-B targets whether they are small, medium or heavy.

The first chance I got to exercise the ADS-B in my airplane was during a flight from upstate New York to Danville, Virginia. At 8,000 feet, I began to pick up targets near Harrisburg, Pennsylvania, and stayed within sight of GBTs for the rest of the flight.

Whenever a controller called traffic for me, I had already been aware of it from the display. With all traffic selected and the map scale set at a range of 100 miles the map was covered with blue targets. I scaled back down to 20 miles and narrowed the "protected" airspace to 2,000 feet above and below my altitude.

At one point, I saw an airplane on the display at my 12 o'clock position coming straight toward me. According to the target's data block the airplane was 900 feet above me. I waited for the controller to call the traffic. When he didn't, I did.

"There's traffic at my 12 o'clock and two miles, 900 feet above me."

"What's your problem?"

"No problem, just wanted to let you know I have the traffic."

"He's a thousand feet above you; that's legal separation!"

True, it was never a threat, but it was still nice to know it was there.

In addition to the rumor about FAA equipping D.C.-based airplanes, ADS-B is the subject of a couple of other rumors. One is that some 50 military airplanes in the D.C. area have already been equipped and GBTs have been installed so the TSA and DOD can have positive control of their airborne assets.

I've also heard that all Boeing and Airbus aircraft are delivered with ADS-B installed. And that there's an effort to expand the Capstone I and Capstone II programs in Alaskastatewide. And finally, I've heard that about 10 years ago, the FAA was considering paying to equip all the airplanes in the country with ADS-B and eliminating the costly to build-and maintain-radar system. ADS-B, since it sends position information at least every second is much more accurate than radar that requires some 10 to 15 seconds to complete a sweep. According to one study, the cost savings alone would warrant the transition to ADS-B since one terminal radar with a 60-nautical-mile-surveillance radius costs as much as a network of at least 20 GBTs, each providing surveillance service up to 120 nautical miles, while providing equal or improved performance. Unlike conventional radar, ADS-B works at low altitudes and on the ground so that it can be used to monitor traffic on airport runways. When an airborne ADS-B target lands, its blue icon turns to brown to indicate it's on the ground. And ADS-B is effective in remote or mountainous areas where radar coverage is limited or nonexistent.

If it only worked as a traffic advisory, the ADS-B system would be worthwhile, but in addition to ADS-B and TIS-B traffic, the UAT can also uplink Flight Information Services-Broadcast (FIS-B). At the moment, FIS-B includes text metars, special aviation reports and terminal area forecasts (TAFs) and graphic nexrad maps. Eventually, the FIS-B uplinks will include information about the status of temporary flight restrictions and special-use airspace.

Some accounts aren't rumors. Embry-Riddle Aeronautical University has equipped its fleets in both Daytona Beach, Florida, and Prescott, Arizona, with ADS-B; the University of North Dakota will be equipping some of its airplanes; and Middle Tennessee State Universityis discussing the possibility of equipping its school fleet. Not only is it a safety issue, but it becomes a selling point for the schools when trying to recruit students. And Australia is installing 28 GBTs that will provide coverage all across the continent at 30,000 feet (and because the transmissions are line of sight, at much lower altitudes closer to the GBTs). A decision whether to expand the use of ADS-B by equipping (or at least subsidizing equipping) all the general aviation airplanes in Australia, should be made soon.

In April 2005, the FAA issued a notam entitled, "Automatic Dependent Surveillance-Broadcast (ADS-B) and Other Broadcast Services-Initial Capabilities in the National Airspace System (NAS)." The notam officially announced the "availability of initial Automatic Dependent Surveillance-Broadcast (ADS-B), Traffic Information Service-Broadcast (TIS-B) and Flight Information Services-Broadcast (FIS-B) capability, and the deployment of ground-based transceivers (GBTs) and supporting infrastructure in selected areas of the National Airspace System (NAS)."

At the time of the notam, the FAA had installed some 40 GBTs, a third of which are in Alaska, with another 13 slated to be installed by the end of this year. Another rumor I like is that the FAA has asked for bids for new GBTs for the entire state of Alaska, with an option to buy enough units for the entire United States. But at least at the moment, the FAA's approach to NAS-wide ADS-B coverage has been described as "one of building up 'pockets of implementation' at points where the system can produce immediate user benefits, and then slowly expanding the pockets until nationwide coverage is achieved." Information about ADS-B and coverage area maps are available at

In addition to announcing the official availability of ADS-B, the April notam also suggested that "ADS-B may include enhanced search and rescue operations and advanced air-to-air applications such as spacing, sequencing and merging."

It's that last capability that was recently demonstrated in Danville, Virginia, at a NASA, FAA and NCAM (National Consortium for Aviation Mobility) showcase of the technologies being developed for the SATS (Small Aircraft Transportation System) program. A practical example of HVO (higher volume operations) was conducted with six ADS-B-equipped airplanes that were able to self-separate and conduct instrument approaches to the uncontrolled field within 30 minutes.

Marion Blakey, the FAA Administrator, is a staunch supporter and insists that ADS-B is the enabling technology for the SATS vision of the future intended to increase the transportation options for people living near the thousands of "under-utilized" airports around the country. The low-cost of installing ground-based transceivers relative to the cost of expanding or maintaining the present radar system is also an argument in favor of ADS-B. But to shift from the current entrenched radar system to an ADS-B traffic system is going to take a major commitment. Nevertheless, a growing number of experts think ADS-B is the solution for the ATC system to be capable of handling the expected future growth of air traffic.

ADS-B is coming to a GBT near you-eventually. I just hope it comes while I'm still able to pass my medical! In the meantime, I'm spreading rumors.


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