Business & GA, Commercial

ADS-B: North to Alaska

By Brian Evans | February 1, 2000
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In May 2000, Alaskan pilots will be at the cutting edge of the largest Free Flight testing initiative planned to date. This is the Capstone project, a three-year, U.S. Federal Aviation Administration (FAA)-sponsored operational evaluation of one of the key airborne technologies of the agency’s future National Airspace System.

Capstone will involve no fewer than 150 aircraft. The technology is Automatic Dependent Surveillance-Broadcast (ADS-B), which is regarded as an essential element of Free Flight. Put simply, ADS-B promises to be the air traffic control transponder technique of the 21st century, and it offers a number of important benefits compared with today’s units.

When interrogated by an air traffic control (ATC) secondary surveillance radar (SSR), current transponders reply with the aircraft’s ident and altitude. It does, that is, provided, first, that they are high enough to be "seen" by the radar’s line-of-sight interrogation transmissions and, second, that an SSR is close enough to track them, even at high altitude.

Typically, an SSR can track transponder-equipped aircraft cruising above 30,000 feet at ranges of up to 250 miles. But, due to the earth’s curvature, the SSR’s interrogation range is significantly reduced when tracking lower level aircraft, and is generally ineffective when aircraft are below 3,000 feet.

As well, SSR coverage is not practical in remote regions and over the oceans. In those areas, the only interrogations likely to be received are from airline aircraft equipped with advanced Traffic Alert and Collision Avoidance Systems (TCAS-2), which transmits investigative signals identical to those from ground radars.

ADS-B is quite different. Here, each aircraft unit transmits—regardless of its altitude or the presence or absence of radar—a burst of data every second. The data includes the aircraft’s ident, GPS position, altitude and intent, i.e., whether it is climbing, descending or in level flight.

This information is received by unmanned ADS-B ground stations, which, being much smaller and cheaper than the complex SSR facilities, can be installed over a much larger geographic area, including remote regions, with their received traffic information being automatically linked to a central air traffic control point. But in addition, the once per second data bursts are also received by every other ADS-B-equipped aircraft within radio range.

The data is presented to the pilots of all these aircraft as clearly identified "targets" on the screens of their multicolor cockpit displays of traffic information (CDTI), which can be selected to cover from 10 to as far as 200 miles ahead. In other words, whether in the mid-Pacific or over the Antarctic or in high-density domestic terminal airspace, ADS-B will provide all pilots with a clear picture of the traffic around them at all times: a major step forward in safety.

While the CDTI screen is a large, airline-style display, neighboring aircraft 2,000 feet higher or lower than the aircraft’s own altitude are not shown, to avoid information overload. Of the aircraft shown on the screen, the display uses red, amber, and green symbology to quickly differentiate between conflicting, potentially conflicting and non-conflicting traffic. Each target symbol on the screen also has a "tail" indicating its present track, and is tagged with that aircraft’s ident, groundspeed and altitude, plus climb, descent or level flight status.

Capstone is the second, but very much larger, evaluation of ADS-B carried out under FAA’s Safe Flight 21 program, which has been established to test potential technologies for Free Flight. The first evaluation, a one-day exercise in Ohio in July 1999, involved twelve B727 and DC-9 freighters operated by Cargo Airlines Association members, plus an FAA B727, a NASA B757 and several smaller aircraft. The Ohio test clearly demonstrated the system’s safety potential in the high-density, late night/early morning "rush hours" of overnight freight operations. And the freighter pilots were unanimous in their praise of the greatly enhanced "situational awareness" that ADS-B provided in the crowded terminal airspace.

But it also demonstrated, in cruising airspace, that pilots could themselves monitor the movements of, and generally assess potential conflicts with other aircraft around them. This capability, currently only available in high-priced airline TCAS equipment, will be a vital requirement in the future Free Flight regime, where today’s fixed air routes will become relics of the past.

With ADS-B, pilots will fly user-preferred trajectories (UPTs) along tracks and at altitudes and speeds appropriate to their individual aircraft’s performance, its specific mission, and the prevailing winds at the time. Here, the lower cost ADS-B equipment should be affordable by all aircraft flying in instrument meteorological conditions (IMC). And, while air traffic control in the Free Flight era will become a safety monitoring function—and renamed air traffic management (ATM)—some added measure of responsibility for safe separation is likely to devolve upon the pilots.

The Capstone project will expand upon the Ohio work to include more than 150 aircraft and will run for three years. But unlike the Ohio test, which involved multi-engine jet transports, Capstone will be a "grass roots" activity centered on the opposite end of the user spectrum. Almost all participating aircraft will be single-engine, air-taxi machines which, day in and day out, carry all sorts of loads to all sorts of places in one of the world’s most challenging environments.

Centered on the small (population less than 5,000) community of Bethel, in the Yukon-Kuskokwim delta of southwestern Alaska, the Capstone operation area has four demanding characteristics:

  • Many uncontrolled landing strips and lakes,

  • Poor communications and navigation facilities,

  • Mountainous terrain, and

  • Rapidly changing, but generally poor, weather conditions.

Obviously, ADS-B will be a boon to pilots as they maneuver around Alaska’s uncontrolled landing areas, where low ceilings and poor visibility occur frequently, and situational awareness with respect to other traffic is vital.

But Capstone will offer participants additional safety enhancing features of the system. These derive from the dedicated data link frequency used in ADS-B, over which a wide assortment of information is passed automatically from aircraft to aircraft, aircraft to ground, and ground to aircraft.

Weather information, for example, will be uplinked periodically and presented on the CDTI in colored graphical or tabular form, along with pilot reports (PIREPs) and related data, as part of the new Flight Information Service (FIS). This is especially important to pilots of single-engine machines, in which weather radar is impractical because of the equipment’s size and its restricted view ahead, due to line-of-sight limitations at low altitude.

With the ADS-B data link and FIS, pilots will be able to "see" the weather as far as 200 miles away. They no need longer will be concerned about what lies on the other side of the high ground ahead or at remote destinations. The FIS also will uplink the latest notices to airmen (NOTAMs), plus clearance changes and other flight-critical information.

In addition, the data link supports a traffic information system (TIS). The TIS supplies information on aircraft in the vicinity that are not ADS-B equipped but whose conventional ATC transponder returns have been detected and tracked by the local SSR. This will be a necessary capability during the trials, as well as during the future transition from current transponders to ADS-B units.

Built by UPS Aviation Technologies (UPS/AT—formerly II Morrow Inc. and now a subsidiary of United Parcel Service) of Salem, Ore., the ADS-B avionics provided for the Capstone project also will incorporate a GPS navigator and a detailed database of the terrain in the operations area, similar to that carried in the new enhanced ground proximity warning systems (EGPWS) (Avionics Magazine, March 1999, p. 16). This system produces a color-coded terrain warning map on the CDTI whenever the ground ahead rises closer than 2,000 feet below the aircraft’s altitude. In mountainous Alaska, where many controlled flight into terrain (CFIT) accidents have occurred, this truly could be a lifesaver.

So, will ADS-B one day replace both TCAS and EGPWS? It’s possible—but not for many years. Testing and certification of such safety-critical avionics is an exacting and time-consuming process. And, in the case of ADS-B, international as well as U.S. standards must be met, and these have not yet been finalized. Even UPS/AT engineers—who have a strong head start in the marketplace, having also supplied the systems for the Ohio test—acknowledge that their current efforts to develop the equipment into a certifiable collision avoidance system, complete with automatic "escape maneuver" guidance instructions, is a challenging task. Still, they are confident it can be done.

Currently, three different data link candidates have been proposed for ADS-B. These are the Mode-S "squitter" technique, the new universal access transponder (UAT) link developed by the Mitre Corp. of McLean, Va., and the Swedish VHF data link 4 (VDL-4).

As it happens, the Europeans are very interested in ADS-B, and have, in fact, conducted significantly more flight tests than has the FAA. Looking ahead, therefore, UPS/AC engineers have designed their avionics so any of the three links can be used. However, FAA has specified the UAT system for Capstone, and ground stations will be set up to cover the operations area, with each having its own separate satellite communications link to the air traffic control center (ATCC) in Anchorage.

FAA has adopted an unusual funding approach for its Alaska project. The avionics equipment will be provided to the 150 aircraft free of charge, and FAA also will cover the cost of its installation and approval certification, plus any required maintenance over the three-year test period. And, after the three-year evaluation, each aircraft owner may keep the installed units, at no charge.

The reasoning behind this unique approach is that FAA wanted as many program participants in the area as possible—ideally, 100% of the local fleet—in order to simulate the total ADS-B environment of the future Free Flight era. Similarly, Bethel and its surrounding area was chosen because very little outside traffic came through this remote part of the state.

With avionics installations underway, Capstone pilots and program officials are looking forward to commencinging ADS-B operations in the spring of the new millennium, which will put tiny Bethel, Alaska, in the spotlight of aviation technology leadership as the next century opens.

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