Here is a question many pilots probably have asked: "Why don’t they make a flight deck display that combines the short-range warning advisories of the traffic alert collision avoidance system [TCAS] with the longer-range traffic picture provided by the automatic dependent surveillance-broadcast [ADS-B] system?"
The answer: Now there is. A combination TCAS/ADS-B display has been developed and is built by UPS Aviation Technologies Inc. (UPS/AT), the avionics development and manufacturing subsidiary of overnight package giant United Parcel Service Inc.
UPS/AT’s ADS-B equipment already flies in all 196 aircraft in the Federal Aviation Administration’s (FAA’s) pioneering Capstone I evaluation program, centered on Bethel, Alaska. With its TCAS/ADS-B initiative, the Salem, Ore.-based, company now has achieved what many industry officials felt was inevitable but still many years away.
The company’s combined display–called a cockpit display of traffic information (CDTI)–is no engineering prototype, built to test the feasibility of the concept. In February 2003, the equipment received FAA certification for use in the Boeing 757 and 767, and in April, the first 10 installations were to commence in the UPS fleet of 103 B757s and B767s. All 103 aircraft are expected to be equipped by October 2003.
Coincidentally, this also was the date originally set by FAA’s notice of proposed rule making (NPRM) for mandatory TCAS installations in all cargo jets. While this date has now slipped back–possibly to 2005–UPS had long since scheduled fleetwide TCAS installations by October.
So, having already committed to TCAS, some will ask, why would UPS install ADS-B, as well? Don’t they both do almost the same thing?
Not really. The two systems actually are complementary, and each offers benefits not provided by the other.
TCAS is a close-in collision avoidance system which, depending on manufacturer and model, "looks" at and tracks all aircraft within 40 to 80 nautical miles (nm) of one’s own aircraft and within a few thousand feet of one’s own altitude. In busy airspace, however, it will only display those aircraft that it deems to be "proximate traffic"–typically within 6 nm–since traffic beyond that range is not regarded as threatening. This is because TCAS units are designed to focus their attention on those aircraft whose closure rates will bring them within a predetermined closest point of approach (CPA) to one’s own aircraft within about 45 seconds, depending on altitude.
At the 45-second point, TCAS provides the crew a visual and audio traffic advisory (TA). When this time decreases to around 30 seconds, the unit produces–after a lightning fast exchange of data and maneuver coordination with the TCAS unit on the threatening aircraft–a visual and audio resolution advisory (RA), commanding a climb or descent.
Correspondingly, the TCAS design logic also is aimed at reducing display screen clutter by removing all targets that pose no threat. However, this is exactly what freighter pilots don’t want to happen, as they arrive and later depart during the late night and early morning "rush hours" at the freight hubs. At those times, the pilots want to see every aircraft in the sky around them, threatening or not. They are particularly interested in positioning their aircraft in a stream of other freighters in the traffic pattern to create an orderly, efficient flow, while maintaining safe separation from each other. This means that besides being well separated, they will be flying in the same direction and speed as the other aircraft in the pattern. To the TCAS, however, this is a no-threat, no advisory, no-targets-on-the-screen situation.
This is where ADS-B comes in. If all freighters are equipped and transmitting ADS-B’s tracking signal bursts once per second–including their individual ident, GPS position, altitude and climb/descent status–every aircraft has complete situational awareness of all the other traffic in the vicinity and can position itself in the traffic pattern. ADS-B flight tests during a UPS operational evaluation (OpEval) exercise in 2000 clearly demonstrated that traffic pattern station keeping by freighter jets using ADS-B guidance was a perfectly feasible objective. Additionally, use of ADS-B could potentially achieve substantial time savings, compared with spacing all aircraft according to FAA procedural radar separation standards, which would place them much further apart.
But while this is a recognized ADS-B capability, FAA is not yet prepared to allow UPS pilots to space themselves in instrument flight rules (IFR) conditions, although this will be permissible under visual conditions. However, it seems likely that the agency will relax its rule as UPS accumulates ADS-B experience fleet-wide. Company officials hope this will not take long: UPS has, after all, flown more hours in ADS-B evaluations than any other operator in the country–probably, in the whole world.
Other New Functions
By 2004 FAA will have installed an ADS-B supplement at UPS’ hub in Louisville. This is a traffic information system-broadcast (TIS-B), which will uplink, from its associated ADS-B ground station, details of aircraft in the area that carry neither TCAS nor ADS-B, and which are presently not displayed on the CDTI. With TIS-B, pilots will be able to see all aircraft in their vicinity.
By then, UPS pilots may also have another ADS-B function available to them, with a CDTI presentation of airport surface maps, showing all traffic, both aircraft and vehicles, moving or stationary, on the airport surface. Such presentations are considered essential if the aircraft industry at large is to reduce the threat of runway incursions, particularly in low visibility.
The installation of the UPS/AT CDTI in the UPS Boeing 757s and 767s is uncluttered and easily viewed by both pilots. Both aircraft have essentially similar flight deck layouts; they were the first commercial airliners to include electronic flight instrument systems (EFIS) as their factory-standard instrumentation. The captain’s and first officer’s instrument panels have electronic attitude director indicators (EADIs) situated above the electronic horizontal situation indicators (EHSIs), while the center panel has two engine indicator and crew alerting system (EICAS) display tubes stacked vertically. UPS installed the CDTI directly below the lower EICAS display, and at the head of the throttle pedestal, between the two flight management systems (FMS) control and display units.
Data Link Options
UPS has also opted to use Mode S as the ADS-B data link for its Boeings, rather than the newer Universal Access Transponder (UAT) developed by the Mitre Corp., which is being used in the FAA’s Capstone ADS-B evaluation project in Alaska. For UPS, this was a practical decision. Mode S already was installed in the fleet, and the aircraft fly throughout the National Airspace System (NAS) and to Europe, both of which are Mode S environments.
Some industry experts feel that Mode S will become the international standard link for ADS-B, with the UAT and Sweden’s VHF data link Mode 4 (VDL-4) becoming regional alternatives. Other experts suggest that eventually ADS-B transceivers will include at least two, and possibly three, data link formats as a standard arrangement.
But to many, the debate over the merits of various data links seemed to assume more importance than the end result. As Captain Bob Hilb, UPS manager of advanced flight systems, put it, "Instead of working on ADS-B implementation, everyone has been arguing about which link should be used."
Another debate that now appears to be resolved is FAA’s view of ADS-B’s role in the future NAS. When FAA launched its Capstone ADS-B evaluation in Alaska in 2000, senior agency officials frequently stated that while the system had great potential for remote area users like Alaska’s air taxi operators, it did not offer sufficient benefits to make operator equipage worthwhile in the continental United States (CONUS).
This view has changed drastically. At a recent RTCA Symposium, John Scardina, FAA director of system architecture and investment analysis, described ADS-B as a key enabling technology for Free Flight. He also discussed FAA’s ongoing cooperation with Eurocontrol in establishing international ADS-B standards for future worldwide implementation.
International standards will be required. Already, UPS’ pioneering work is being supplemented by evaluations in Europe and, more recently, in Australia, where Air Services Australia regards ADS-B as both a safety system and a means of saving substantial sums of money. Increasing air traffic across Australia’s vast and largely uninhabited–and inhospitable–desert interior calls for continuous flight monitoring, but installing a manned radar network would be incredibly expensive. Tests of ADS-B in eastern Australia have demonstrated the system’s potential to provide an unmanned "pseudo-radar" network of ground stations across the interior. Aircraft ADS-B transmissions would be received and retransmitted over satellite links to distant air traffic control centers, as is the case in the Alaska Capstone program.
One intriguing twist: the Australians are considering providing, at no cost, basic ADS-B packages to all of its 10,000-strong general aviation fleet.
In March of this year, aviation officials in China announced that they, too, were commencing a detailed study of ADS-B applications across their vast country. As these and other nations expand their interest in ADS-B, the innovative developers at UPS/AT are likely to be kept very busy in the years ahead.