Future Flight: Links to Tomorrow

Part 2 of 12

Pilots of the future can look forward to more and better on-board safety equipment, thanks in large part to new services that are just now in the first stages of evaluation. In just a few years, it’s very likely that the multiple navigation, weather detection, and traffic avoidance boxes common in many modern airplanes will be replaced by single units capable of combining and displaying all those functions on one screen—and boosting pilot situational awareness by orders of magnitude.

What will enable these information systems of the future? In a word, datalink. What’s datalink technology? Simply put, it’s a method of digital communication involving ground stations, satellites and aircraft—and between aircraft and other aircraft—using special transceivers. These transceivers are analogous to the modems we use to send and receive data through land lines.

With datalinks, aircraft can broadcast their positions to each other—and to air traffic controllers on the ground—via special transceivers and ground stations. By the same token, air traffic painted on ground radar can be uplinked to aircraft displays. So can Doppler and other weather radar imagery, as well as text messages such as ATC clearances and weather reports. Even e-mail messaging is possible, as evidenced by Echo Flight Inc.’s StratoCheetah. Its current satellite up- and downlinking services offer two-way messaging and ground-based weather radar imagery.

The goal: ADS-B

ADS-B stands for Automatic Dependent Surveillance-Broadcast. Under this scheme, aircraft with GPS receivers (although RNAV equipment will work, too) and special two-way digital datalink equipment (the system is "dependent" on GPS position information) automatically (that’s the "automatic" part) transmit (that’s the "broadcast" part) their positions, altitudes, groundspeeds, tracks, and other information (such as aircraft type and/or N-number) to other aircraft equipped with datalink equipment—and to ground-based antennas connected to ATC.

In the ideal world of the future, pilots and controllers would see the same targets and the same information on a single display. (This is the "surveillance" component of the acronym.) Pilots could see potentially conflicting targets as far away as 100 nautical miles, and alter their courses and altitudes so as to avoid the chance of midair collisions. For more immediate traffic threats in heavily traveled airspace, ADS-B can work equally well, although ATC could issue traffic advisories, or TCAS-equipped airplanes could follow any traffic or resolution advisories issued by their own on-board equipment. For that matter, ADS-B could also be configured to give the same traffic and resolution advisories that TCAS II currently does.

The whole idea behind ADS-B is to expand system capacity and enable the Free Flight concept. Under the Free Flight proposal, aircraft would be free to fly more direct routes, using GPS; pilots could see virtually all of the traffic around them, and do more to safely separate themselves; and ATC could be freed of much of their en route controlling work load, letting controllers focus more on the efficient management of the entire airspace system, and to concentrate their energies on sequencing and separation in terminal areas. ADS-B has particular relevance to transoceanic traffic, where escalating numbers of airliners are held to 60-nm in-trail separation. With ADS-B, that distance could be narrowed, allowing more flights per route.

With the high closing speeds experienced by jets and other fast-moving, conflicting traffic, ADS-B’s ability to see at long ranges should, in theory, mean much more advance warning of a potential midair collision.

First things first

All this warm and fuzzy talk of an interlinked, omniscient traffic-avoidance and information system is nice, but first something very important has to be hashed out—what system and frequency will the datalinks use?

Some advocate using 1090 MHz, the same frequency used by Mode S transponders. The airlines are especially enamored of Mode S, because their fleets already have it as part of their TCAS systems. To be players in a Mode S world, general aviation aircraft would have to be upgraded with datalink-capable Mode S transponders (there’s only one out there now—the $3,500 Honeywell/Bendix-King KT-73) and the necessary displays and controls, which could bump the ultimate cost to approximately $6,000 or more.

On the downside, Mode S wasn’t really designed to pass huge amounts of data. Update rates of changing traffic situations may drop to critically slow speeds in areas of dense traffic. Also, Mode S doesn’t have a sufficiently wide bandwidth to permit the exchange of weather graphics, text data (e.g. clearances, weather reports) and display traffic. Mode S, you see, already operates on a very crowded frequency—the one that supports TCAS and ground-based surveillance radars. With all those radar sweeps and TCAS interrogations and replies, it could be difficult to use the frequency to display all traffic.

Other frequency options are in the DME spectrum, and the equipment to use these frequencies is called the Universal Access Transceiver (UAT). UAT, developed by the Mitre Corporation’s Center for Advanced Aviation System Development (CAASD), runs in the 966- to 981-MHz frequency range, and can exchange lots of data at a faster clip than Mode S. Experts guess that a UAT transceiver might cost around $2,000 (excluding display), so its lower cost and higher capacity make UAT the current favorite choice of most general aviation advocates.

The other ADS-B frequency option is in a very narrow (0.25 MHz) VHF bandwidth. It’s called VDL-4, the "VDL" standing for VHF Data Link. Its strength is its ability to see far (about 300 nm), but it needs multiple frequencies and multiple receivers to display enough traffic to be optimally useful. In testimony to its ability to carry a big dataload, UAT has been able to display 700 targets at a time—as demonstrated in a test conducted in the Los Angeles area. With VDL, only 200 or so can be shown simultaneously. After that, targets may drop off the screen. If those targets pose an immediate threat, this bandwidth limitation could obviously have serious safety implications.

VDL research began some 10 years ago in Sweden, and it remains the ADS-B vehicle of choice among the European nations.

Traffic Information Services-Broadcast (TIS-B, for short) is another method of providing traffic—and weather—information. TIS-B can operate in a wide range of frequencies, making it compatible with both UAT and VHF equipment. With TIS-B, pilots would see the same traffic information that controllers do. That’s good, because there’s the promise of better coordination of traffic information between pilot and controller. The drawback? Detection ranges are limited to the radius of the ground-based surveillance radars being used. Also, the system will work only where ground-based radars exist, making it comparatively useless on transoceanic routes.

Unfortunately, the choice of equipment, systems, and frequencies is a political as well as technological issue. Obviously, all aircraft must use the same datalink equipment if the system is to work worldwide. For that to happen, there must be an intellectual as well as a political reckoning. Right now, that consensus seems a long way off. Mode S, UAT, and VDL advocates don’t seem to be conceding much at this time.

Testing, testing

Under the FAA’s Safe Flight 21 program, ADS-B and its candidate systems and frequencies have been undergoing a series of operational tests. These tests include the uplinking not just of traffic information, but also of ground-based Doppler and other weather radar imagery, and text messaging. At the same time, terrain databases resident in on board GPSs portray the nearby geography.

The first test was conducted with the help of the Cargo Airline Association (CAA), and took place in the Ohio River Valley region. There, four airplanes operated by Airborne Express, FedEx, and UPS successfully used all three frequencies—but the apparent favorite was UAT.

Recently, Safe Flight 21 embarked on the Capstone initiative, a three-year-long program to test ADS-B using TIS-B (with a terrain database) in the Yukon-Kuskokwim Delta region of Alaska. This region was chosen because of its paucity of ATC radars, its hostile terrain, its famously rotten weather, and its communities’ dependence on regional commuter and Part 135 flights.

Under Capstone, ground-based traffic and weather radar imagery is beamed up to 132 aircraft fitted out with UPS Aviation Technologies (UPSAT, formerly known as II Morrow) avionics suites. These include UPSAT’s Apollo MX-20 display screen, plus a GPS/com, a nav/com radio, and a Mode C transponder.

Under this setup, the MX-20 receives—via TIS-B uplink—the same traffic information seen on ATC radars (be they Mode C, Mode A, or Mode S targets), plus any participating ADS-B targets via a UAT radio. There’s an uplink of precipitation echoes, and pilots can watch their progress on a moving map that shows both terrain and navaids.

Capstone’s nine big goals foreshadow a rosy future wherein pilots will avoid midair collisions, controlled flight into terrain, and weather-related accidents by observing all these threats on one large screen, such as the MX-20.

There’s another big goal, remind AOPA and other general aviation advocates. That’s to make sure that the chosen system provides—free of charge—enough basic services so that general aviation owners and pilots will voluntarily equip their aircraft with the new technology. Private vendors would then, as always, be able to provide more enhanced, tailor-made services for those who want to pay for them.

State of the art

Thanks to a new generation of avionics from the private sector, some of those threats can be managed today. The former Bendix/King’s (now Honeywell’s, after those two companies merged) Integrated Hazard Avoidance System models IHAS 8000 and 5000 use the KMD 550 and 850 large multifunction displays, show terrain derived from GPS databases, and are already capable of uplinking Mode S-datalinked traffic information, lightning detection information, and text messages.

The Apollo MX-20 and other moving-map avionics come with terrain databases, so you don’t have to be a Capstone participant to have a moving map with terrain highlights.

As for uplinking of weather radar, weather graphics, and textual weather such as METARs, Arnav already licenses weather data and imagery to manufacturers such as Archangel, Eventide, Garmin, Honeywell, and, soon, Avidyne—all for display on those companies’ large, easy-to-read display screens. Weather data will be fed to Honeywell’s IHAS units via a network of VDL stations that came with the company’s purchase of NavRadio last summer.

Echo Flight’s StratoCheetah can bring Nexrad radar imagery to the general aviation cockpit via satellite. And let’s not forget the scads of lightning-detection units and airborne weather radars that have been available to general aviation for decades.

As for traffic avoidance, Ryan’s TCAD, BFGoodrich’s Skywatch and TCAS I, and TCAS IIs built by all the major avionics manufacturers also have given good service and had enhanced features added over the past few years.

Ground Proximity Warning Systems (GPWSs) are also becoming more popular. Honeywell’s Mark VI EGPWS—the E is for "enhanced"—includes voice commands such as "pull up, pull up" when an aircraft flies too low to the ground. Less expensive (about $15,000) GPWS units paint approaching terrain in pixellated, symbolic shades: red for dangerously close terrain, and yellow and green for less threatening elevations.

Parallel worlds?

Given these capabilities that already exist, how will ADS-B make the pilot’s job any different or safer? First, there will be more traffic information in the cockpit—although pilots will still be able to "declutter" distant targets to concentrate on closer-in threats. Because GPS information from other targets will contain flight path, altitude, rate of ascent or descent, and next waypoint—even N-number or type of aircraft—it will be easier for pilots of aircraft on conflicting tracks to share evasive maneuvering responsibilities with ATC. Second, the cockpit equipment will be able to cover longer ranges. Third, all information—traffic, weather, terrain—will appear via a single box. So once ADS-B is standardized, pilots will find it easier to go from aircraft to aircraft and from jurisdiction to jurisdiction without having to endure a potentially dangerous learning curve.

Some are more pessimistic about ADS-B, and say that it will be a decade or more, if ever, before the system becomes operational. In the meantime, pilots may wonder what will happen to the weather radar, TCAS, and other collision-avoidance equipment they’ve just purchased. Will ADS-B make them obsolete overnight? Air traffic controllers fear that the shared separation burden envisioned under the ADS-B scheme will erode their job security and make things more, not less, dangerous.

What’s most likely is that ADS-B and traditional cockpit equipment will coexist through a lengthy transition period, and that controllers will still be vital to safety. Those with TCAS, TCAD, and weather radar will still be able to use it in an ADS-B world. In fact, for close-in work, where avoidance distances can be on the order of a mile or less, TCAS, TCAD, and airborne radar can continue to serve as necessary augmentations to ADS-B traffic and weather information services. Controllers? You bet we’ll need them. When a dozen flights make a direct GPS "Free Flight" beeline for the same outer marker, we’ll need some world-class people to make sure everyone doesn’t arrive at the same time, and plays by the rules.

Links to additional information can be found on AOPA Online ( www.aopa.org/pilot/links/links0002.shtml ). E-mail the author at [email protected] .