ADS-B Progress and the Proposed Rule

Two steps forward and one step back. That’s the pace for many technologies and appears to be the story for automatic dependent surveillance-broadcast (ADS-B), as well. The Federal Aviation Administration (FAA) seems to be accelerating its work on ADS-B, as part of Safe Flight 21. It plans to announce several milestones regarding the technology this year, and its Safe Flight 21 office is pursuing nine enabling technologies with ADS-B.

But the agency also has issued a notice of proposed rulemaking (NPRM) that ADS-B proponents believe is ill-conceived and potentially damaging to the technology’s advancement (see sidebar, page 24). They believe the NPRM, which requires cargo aircraft to be equipped with a collision avoidance system by the end of this year, precludes ADS-B as an option to the traffic alert and collision avoidance systems (TCAS II) and fear it thus will lessen interest in the new technology.

ADS-B has been in development for more than a decade and has been a component of Safe Flight 21 since 1998. It is a multipurpose tool that determines the aircraft’s exact position, using an onboard GPS receiver, and broadcasts that position at a high rate to other aircraft, ground stations and air traffic controllers within about a 150-mile radius via data link. The positioning data is converted to a digital code and is combined with other information, such as aircraft identification, ground velocity, vertical rate and intent information. Since the ADS-B link transmits and receives digital data, the technoloogy also can be used to uplink textual and graphical weather, NOTAMS, NEXRAD and traffic information from ground radar, although these are not specifically ADS-B functions.

The Safe Flight 21 office sees ADS-B as an "enabler of Free Flight." The general aviation (GA) community champions the technology because it performs multiple tasks at relatively little cost; for a base price of about $15,000, a GA pilot can acquire the GPS receiver, display and transceiver. ADS-B can present positioning data when aircraft are in all phases of flight; unlike ground radar, it tracks aircraft at low altitudes and on the ground. ADS-B is viewed as both a surveillance tool and a prime provider of pilot situational awareness.

Envisioning 23 applications of ADS-B within nine enabling technologies, the FAA is testing and evaluating the technology primarily at two locations: in the Ohio River Valley, working with the Cargo Airline Association (CAA); and in Alaska, as part of Safe Flight 21’s Capstone project. The two efforts are complementary. The CAA participates in the evaluation of ADS-B applications with large commercial jets, while Capstone does so primarily with GA and other smaller aircraft. Also, according to John Hallinan, Capstone program manager, "The CAA is primarily chasing the efficiencies of ADS-B, while we’re trying to chase the safety capabilities."

The Europeans, too, are working on ADS-B. Eurocontrol will provide a status report of the technology in Europe this month (March 12-14) at its ADS-B symposium in Rome (see www.eurocontrol.be).

The pace of ADS-B development appears to be quickening. "Our role at Safe Flight 21 has been migrated from simply testing the feasibility of ADS-B to testing it for applications and maturing the technology for implementation," says Paul Fontaine, Safe Flight 21 product lead. Fontaine and Rocky Stone, manager of flight systems technology for United Airlines, cochair RTCA’s special committee (SC) -186, which is developing standards for ADS-B.

For some in commercial aviation, the quicker pace couldn’t come too soon. They want to accelerate the arduous task of establishing minimum operational performance standards (MOPS) and minimum aviation system performance standards (MASPS) for ADS-B and to more quickly resolve the selection of a standard data link.

Two data link technologies are being proposed in the United States: the 1090-MHz extended squitter (1090ES), which is based on Mode S technology, and the Universal Access Transceiver (UAT) technology, which operates at 978 MHz.

(A third technology–VHF data link [VDL] Mode 4, which operates on multiple 25-KHz channels in the VHF radio navigation band–also could be used as an ADS-B data link. It is favored by some European organizations; however, the U.S. prefers the 1090ES and UAT.)

Both 1090ES and UAT have benefits and drawbacks. UAT was developed by Mitre Corp. specifically for ADS-B. Since the 1-megabit/sec system is based on technology similar to that used by mobile phone operators, its wide public use would make it economical for the aviation community. "[UAT] is very promising," says a member of SC-186, "but its drawback is that it’s not compatible with other systems."

1090ES is compatible with Mode S, but as a pulse-emitting system, it requires costly amplifiers, which are not required for a "gentler" waveform like UAT. Airlines tend to favor 1090ES. Since their aircraft already are equipped with Mode S, they forgo the expense of installing another antenna and additional wiring for another data link. GA operators, whose aircraft generally are not Mode S-equipped, prefer UAT.

(Some vendors have indicated that UAT and 1090 could be combined into a single box with little difficulty and could even share the same antenna. There also is a move afoot within the airline community to put all L-band avionics in one box, which would accommodate UAT, as well.)

Both 1090ES and UAT have been used for ADS-B. "We use UAT for Capstone," says Hallinan, "and at Ohio River Valley, they use both UAT and 1090."

RTCA completed the MOPS for 1090ES about a year ago, according to Fontaine. "By June, we probably will approve 1090, revision A, which takes into account [the data link’s] use with specific ADS-B applications," he adds. SC-186 also plans to publish the MOPS for UAT by June.

Meanwhile, the FAA’s Systems Architecture Division reportedly plans to put its long-awaited recommendation for an ADS-B data link standard on the administrator’s desk this spring. The decision probably will be to use both standards: the UAT data link at lower altitudes (and primarily by GA aircraft) and 1090ES by the air transport aircraft at higher altitudes.

How would the two data link frequencies work together? "First, the only time the frequencies would be mixed is in the terminal area, where GA aircraft and the big carriers come together," Fontaine explains. "There, in the dual-link world, TIS-B [traffic information service-broadcast] would become the key to interoperability. Alternatively, GA could equip with the UAT only, while air carriers equipped with both links, thus ensuring interoperability."

TIS-B broadcasts surveillance information from ground stations via the ADS-B data link. The information can be derived from primary and secondary radar and/or a multilateration system. Using a function called "multilink gateway," TIS-B also can rebroadcast data, taking in a UAT signal and delivering a 1090 signal and vice versa. "All the pilot sees is an ADS-B message and the signal source, UAT or 1090, becomes invisible," says Fontaine. He emphasizes that "the dual-frequency approach is not released FAA policy yet." RTCA expects to complete its MASPS for TIS-B in late 2002.

FAA plans to begin transmitting TIS-B messages from a ground station at Memphis airport in May 2002 for test and evaluation. The ground station is part of the airport surface detection equipment-X (ASDE-X) system, which has X-band radar and multilateration ground stations as its prime components. The TIS-B ground stations transmit at 1090 MHz but can be upgraded to accommodate the UAT frequency. In addition to the Memphis facility, prototype multilateration ground stations have been installed in Louisville, Ky., Detroit and Dallas-Fort Worth. Milwaukee and Orlando, which are the ASDE-X key test site locations, will receive ground stations soon, according to Fontaine.

The Safe Flight 21 product lead reports that two complementary advisory circulars (ACs) on ADS-B from FAA’s Flight Standards Service and Aircraft Certification Service are being issued to "familiarize our folks in the field with this new technology." The agency approved ADS-B on April 9, 1999, and has issued three supplemental type certificates (STCs), based on the technology’s applications. United Parcel Service (UPS), has received two STCs for ADS-B applications: enhanced see-and-avoid and enhanced visual approach. For the Capstone project, "radar-like separation certification" was issued, granting ADS-B’s use as a substitute for ground radar. In parts of Alaska, air traffic controllers now "can use ADS-B data to direct traffic just as if there is radar," says Fontaine. The system is ready for use in Bethel, which has no primary or secondary radar. Fontaine expects FAA will receive another STC application this year, for ADS-B use with a surface moving map.

"We measure the progress of ADS-B technology one STC at a time," Fontaine tells Avionics Magazine.

Mitre has been conducting studies for Safe Flight 21 to indicate the best direction for continued ADS-B development. The studies expand on Mitre’s role as test and operations lead for SafeFlight 21’s Operation Evaluation 2 (OpEval 2), conducted in October 2000 in Louisville, says Edward Hahn, principal communications engineer with Mitre. Mitre’s suggestions to the Safe Flight 21 office took into account pilot acceptance of ADS-B applications.

"Generally, we found good applications for final approach and runway occupancy awareness," says Andrew Zeitlin, a principal engineer for air traffic management/avionics with Mitre. "Those applications are advanced enough to involve developing avionics for verification in the very near future."

Three applications are "being worked on intensely" by Safe Flight 21 says Fontaine. They are:

Use of ADS-B with a surface moving map–FAA plans to equip its Boeing 727 with moving map displays of airport surfaces, which will use ADS-B to show surface traffic. The aircraft will accommodate displays from several vendors. "Between May and July [2002], the aircraft is scheduled to go to Chicago, Washington National, Dallas-Fort Worth and Atlanta," says Fontaine, adding, "we want to expand the awareness of ADS-B beyond Memphis and Louisville."
Cockpit display of traffic information (CDTI) -enabled flight rules–FAA will test to see if pilots can use ADS-B to safely acquire aircraft in front of them while encountering a temporary loss of visual contact during a visual approach. "Can the pilot safely punch through the clouds and continue his visual approach?" Fontaine asks, rhetorically.
Uplinking of other situational data–FAA wants to see if ADS-B can be used to uplink graphical weather and other data to improve situational awareness in the cockpit.

OpEval 2 reportedly demonstrated that ADS-B increases the efficiency of approach spacing, an application that NASA-Langley also is evaluating. Starting in late January, NASA began work in its B757 flight simulator on a "precise arrival tool," using ADS-B to increase runway throughput, according to NASA senior research engineer Terry Abbott.

The precise position data from ADS-B would improve consistency in aircraft spacing during an approach, Abbott explains. Algorithms were developed by NASA and by Mitre to automatically calculate where an aircraft should be in relation to the aircraft in front of it.

One commercial pilot involved in the NASA simulations reported that issues of workload ("Too much head down") and consistency of the spacing information in relation to aircraft speed needed to be resolved. Also, controllers have expressed concerns of whether the spacing application concept is compatible with current terminal operations. Both pilot and controller concerns must be resolved before spacing can be implemented. The study has just begun; NASA plans to evaluate the spacing capability aboard its B757, starting this summer.

Meanwhile, officials at Capstone expect to accelerate development of ADS-B once the data link standard is made official, the MOPS for UAT is complete, and the Bethel control tower is fitted with a display to show ADS-B data of aircraft positions. All should occur later this year.

ADS-B obviously holds many potential benefits. NASA, for example, in its Airborne Information for Lateral Spacing (AILS) project, has examined the technology’s use to enable closely spaced parallel runway approaches in IMC conditions. FAA requires runway separation of at least 4,300 feet (1,311 meters) for such operations. "Using ADS-B data link and differential GPS, we believe we can get down to 2,500 feet [762 meters]," says Abbott.

The Washington Post has written of ADS-B’s potential use as a security tool. The newspaper notes that, since ADS-B can present aircraft position updates much faster than ground radar, which operates at five- to six-second update intervals, air traffic controllers could detect more quickly an aircraft’s departure from, say, its approach path to a Washington National Airport runway.

The Controversial NPRM

ADS-B would appear to have numerous applications. However, the one application that the Cargo Airline Association has proposed–airborne conflict management, as an alternative to TCAS II–appears to be sidelined by an FAA notice of proposed rulemaking, issued on Oct. 31, 2001.

In response to Public Law 106-181–issued in early 2000 and often called AIR 21–FAA issued the NPRM (see Docket No. FAA-2001-10910), which would require that all large air cargo aircraft be equipped with a collision avoidance system by Dec. 31, 2002. (Air transport aircraft have been required to use TCAS since Dec. 30, 1991.)

The NPRM’s primary purpose is to standardize collision avoidance requirements. It would require equipage based on aircraft weight and performance, as opposed to mission.

Cargo aircraft initially were omitted from the original TCAS equippage requirement because they comprised a relatively small fleet, which flew largely at night, when most passenger-carrying aircraft were in their hangars. In 1987, prior to first TCAS rule, the U.S. air cargo industry operated about 375 aircraft, FAA notes. Today, however, some 1,150 cargo aircraft operate in the United States, and the demand for air cargo services is growing.

FAA also notes a petition for the rulemaking from the Independent Pilots Association (IPA), representing the UPS pilots. Citing TCAS’ "proven track record," the IPA points out that the cargo carriers now operate "numerous daytime flights…and share the same airspace with passenger airplanes."

To further support issuance of its NPRM, the agency cites a Feb. 6, 1999, incident in which a cargo airplane and passenger airliner passed within 1 mile horizontally and 600 feet vertically. And it refers to FAA statistics that show that pilot reported near midair collisions (NMACs) dropped from 454 reports in 1990, before TCAS II was required, to "an all time low of 194 in 1996."

The NPRM would seem to be straightforward and uncontroversial. Not true.

Disagreement over the NPRM begins with the wording that says cargo aircraft must be equipped with a collision avoidance system that is "equivalent and interoperable with TCAS."

"There is no statutory basis for this requirement," says UPS in its comments on the NPRM. "The law requires collision avoidance systems to prove an equivalent level of safety as TCAS II, not to be interoperable with TCAS II. Nine-five percent of the aircraft traffic operating in U.S. airspace today is equipped with only transponders or TCAS I. Neither of these systems is interoperable with TCAS II to the extent required by the NPRM." Steve Alterman, president of the CAA, adds, "The NPRM is such a maze; it leaves no alternative to TCAS."

Regarding equivalency of ADS-B and TCAS II, UPS contends that "there must be a common standard against which both can be measured. The NPRM does not provide this standard."

UPS and other individuals objecting to the proposed rule also cited "technical inaccuracies" regarding ADS-B as a collision avoidance tool, as well as limitations in TCAS’ capabilities. Here are some of the dissenting comments to the NPRM:

The proposed rule states that ADS-B is effective in collision avoidance only when all aircraft are equipped. "But if you argue that everyone must be equipped, we won’t have anything new," CAA’s Alterman contends. "Furthermore, the rule fails to factor the benefit provided by ground uplink of traffic data, such as that provided by the TIS-B system…," UPS adds. ADS-B proponents also point out that TCAS, which detects traffic from Mode S and Mode C transponder transmissions, is blind to the thousands of general aviation aircraft that are not equipped with transponders or are equipped but have defective transponders. "The effectiveness of either system grows with equipage," a Mitre Corp. study, published in late 2001, states, adding that the ADS-B concept "asserts it is intended for all categories of users." While not endorsing either system, Mitre noted in its study that airborne conflict management (an ADS-B application) "may benefit a far greater number of users than TCAS and thus can address a safety vulnerability (the roughly 18 midair collisions per year between general aviation aircraft) that TCAS cannot."

The NPRM refers to a National Transportation Safety Board (NTSB) recommendation that says "technical and research issues…remain to be solved before ADS-B can provide anti-collision capability." But UPS points out that the NTSB recommendation was drafted in 1999 and, since then, "most if not all the NTSB recommendations have been resolved." The Mitre study points out that airborne conflict management (ACM) has several alternative concepts (including ADS-B) that "still are under consideration, and no standard algorithms have been agreed upon." UPS Aviation Technologies, prime developer of ADS-B, insists the system could be approved for collision avoidance by the NPRM deadline.

The NPRM states that ADS-B safety is compromised because it derives both surveillance and ownership position information from a sole source–GPS. "The FAA fails to acknowledge that TCAS II relies on a similar interdependence on barometric altitude," UPS replies. "TCAS II is more vulnerable to a failure of the barometric altimetry than ADS-B is to a loss of GPS. In an ADS-B collision avoidance system, the failure of the GPS component would not compromise the ability of radar and air traffic controllers to track and separate traffic normally." "The benefit of ADS-B is that, as a pilot, I know where I am; the controllers know where I am; now we can make a better decision," adds Alterman. In future ADS-B systems, when data is available, vertical height data would be provided by both barometric altitude and geometric data, thereby providing dual, independent sources of data, unlike current TCAS II systems. This added integrity goes well beyond most GA aircraft’s "single string" vertical height encoder system. The GA transponder/encoder combination is considered to be the weak link in the TCAS II implementation.

ADS-B proponents believe their technology can improve on collision avoidance and that TCAS has limitations ADS-B can help resolve. "TCAS does not resolve all potential collision encounters," says United Airlines’ Rocky Stone, in a written response to the NPRM. "An ADS-B data link contains more information content (such as target heading and altitude rates), resulting in more effective collision avoidance maneuvers in the vertical planes." According to a study conducted by the FAA Technical Center, some Mode S transponders in the field "have design problems" and therefore cannot be "seen" by either TCAS or secondary radar. The agency also has unearthed incidents in which Mode C transponders reported incorrect altitude readings. Some Mode C transponders can not be seen by TCAS or secondary surveillance radar. And reports exist of near midair collisions between two TCAS-equipped aircraft. For example, on June 28, 1999, two TCAS-equipped B747s narrowly missed a collision over Chinese airspace. In most cases, says an FAA official, the failures were in the information fed to the TCAS, not in the TCAS units themselves.

Comments on the NPRM include many more issues surrounding the two systems (visit http://dms.dot.gov). An FAA official says the agency’s response to the comments "will be published with the rule."

Perhaps the biggest concern among ADS-B proponents is that the proposed rule could deflate interest in advancing a promising technology. "There is nothing wrong with TCAS, but there is something beyond that technology," says Alterman. "We look at ADS-B as the next generation of collision avoidance." "The NPRM neglects to account for the additional safety that would be provided by ADS-B systems…," adds UPS.

Echoing that sentiment, Stone said, "If all large aircraft are required to be equipped with TCAS II, there will be less commercial incentive to invest and develop future systems that will improve safety and efficiency and increase capacity."

Still, ADS-B development appears to be progressing independently of the NPRM. And manufacturers are producing TCAS systems that will interface with ADS-B. One airline official suggests one system could back up the other. Working together, the two systems should provide enhanced situational awareness and much improved protection against midair collisions.

Nine Enabling Technologies