Data takes flight

Today's commercial airliners come packed with computers smart enough to land the planes, but when it comes to air-ground communications, much of the airline industry uses voice radio systems that haven't changed much since the era of propeller-driven DC3s.

Vital communications about weather or route changes between pilots and company dispatch centers, as well as air traffic controllers, run over narrow-band and often overcrowded voice radio circuits that resemble an old-fashioned party-line telephone.

When pilots can transmit on one of these VHF air-ground circuits, the poor quality of the connections and ambient noise often require them to repeat themselves. Even with repeats, these voice communications are prone to errors.

Some airlines do use air-to-ground data communications systems, but current technology provides a connection with a data rate equal to that of an early-1980s dial-up modem, 2.4Kbit/sec. Although it provides clearer and more error-free communications, this slow data service doesn't support advanced needs such as automatic updates of avionics software. The VHF radio channels that support this data service are also becoming crowded and harder to access.

Communications companies that serve the airline industry, such as Arinc Inc. in Annapolis, Md., and Societe Internationale de Teiecommunications Aeronautiques (SITA) in Geneva, have developed a new data service called VHF Digital Link Mode 2 (VDLM2), which more than quadruples the air-ground data rate while using relatively uncrowded portions of the VHF aviation radio band.

But major airlines, still cash-strapped in the wake of 9/11, have been slow to adopt this new service fleetwide. One exception is low-fare pioneer Southwest Airlines Co. in Dallas. Southwest plans to equip its 388 planes with VDLM2 by the end of this year and will deploy the technology on half of them by this summer, according to Brian Gleason, the carrier's director of flight operations, technical.

When Southwest started operations in 1988, it served just three Texas cities with a handful of planes. At that time, it built its own network of ground stations operating in the upper end of the 108/137-KHz VHF aviation band, which allowed pilots to place calls to the company's dispatch center using a touch-tone keypad. Even though Southwest expanded the network of ground stations, it still could accommodate only one voice call at a time, frustrating pilots who needed to contact the dispatch center for weather updates or route changes.

To break that logjam, Southwest in January reached an agreement under which Arinc will provide the airline with VDLM2 service over an Arinc-owned network of 148 VHF ground stations that cover North America, says Gleason. Southwest first considered using the Aircraft Communications Addressing and Reporting System data service that Arinc introduced in the 1980s, but it passed because of the service's low bandwidth and congestion in the ACARS frequency band, which runs from 129.125 to 131.475 KHz.

Dan Pendergast, director of Arinc's GlobaLink Services division, says the global VDLM2 frequency of 136.975 KHz is relatively uncrowded today compared with the ACARS frequencies and offers roughly 15 times the data rate of ACARS. But since VHF channels are only 25 KHz wide, the data rate is far lower than wideband services such as Wi-Fi, whose channels are 20 MHz wide, and the VDLM2 bandwidth is slightly less than 1,000 times narrower than a Wi-Fi channel.

Although VDLM2's data rate of 31.5Kbit/sec. (vs. 2.4Kbit/sec. for ACARS) seems low compared with Wi-Fi's throughput of 11Mbit/sec., Southwest can pack a lot of information into such a narrow channel, says Gleason.

Southwest uses the VDLM2 service to handle communications between its dispatchers in Dallas and aircraft captains, both of whom the Federal Aviation Administration considers responsible for the operation of flights.

Storm warnings

Southwest primarily uses VDLM2 for short text messages between dispatchers and flight crews. Most of those messages are weather-related, such as instructions on how to route a flight around a storm. The flight crew also uses the data link to obtain weather reports for cities they are heading to, saving the crew from listening to an audio broadcast of weather conditions.

Instead of flight crews waiting until they are in range of an airport's Automatic Terminal Information weather-service broadcast on a VHF air-band frequency, crews can use VDLM2 to request weather information en route, Gleason says. Southwest also plans to use VDLM2 to transmit high-resolution weather images from NexRad radars operated by the National Weather Service, he says.

Dispatchers prepare and receive messages using the Hermes Ground Data Link System developed by the Reading, England-based Rockwell Collins (U.K.) Ltd. division of Rockwell Collins Inc., Gleason says. Hermes acts as a specialized e-mail system, parsing and routing messages from the dispatch center to a cockpit terminal through an aircraft data-link radio provided by Honeywell International Inc. in Morristown, N.J.

Messages are sent from the Southwest dispatch center to an Arinc gateway that then sends them out over the network of VHF ground stations. The network is spaced across the U.S. to allow communications from stations to aircraft operating at and above 18,000 feet, Pendergast says. Arinc also operates VHF ground stations at the 59 airports in 58 cities served by Southwest to provide communications when planes are on the ground, says Pendergast.

Southwest also plans to use VDLM2 to remotely monitor engines and keep track of things such as how they're performing, what their temperature is and how much fuel they're using, says Gleason. This will help the company manage engine maintenance and ensure that an engine problem doesn't delay flight turnaround, a key factor in the success of Southwest, which has the highest aircraft utilization rate of any carrier in the U.S., says Gleason.

Gleason estimates the cost of installing VDLM2 at about US$50,000 per aircraft, plus a flat fee per month to Arinc, which he declined to disclose, for "x amount of kilobits of data." The VDLM2 project doesn't have a classic return on investment, but it will improve operational efficiency, he adds.

Robert B. Field, a former U.S. Marine Corps pilot and an aviation consultant based in Chesterfield, Mo., says data communications are much more efficient and less prone to confusion for pilots than voice communications are.

Field, who once served as director of the U.S. Navy test pilot school in Patuxent River, Md., adds that by using VDLM2 to transmit NexRad weather information to pilots, Southwest could provide them with a level of detail not available from aircraft radar systems.

Southwest isn't the only carrier to embrace VDLM2, says Pendergast, but it is the first in the world to equip its entire fleet. Others using VDLM2 service include Air France, American Airlines Inc., Continental Airlines Inc., FedEx Corp. and SAS AB. Arinc estimates that more 500 aircraft will use its VDLM2 service by the end of 2004.

SITA has also started to roll out its VDLM2 network, though it's doing so more slowly than Arinc is. Akhil Sharma, director of aircom services development at SITA, says his company has installed 30 VDLM2 ground stations, primarily in Europe, and he agrees with Gleason and Pendergast that congestion on ACARS channels will be a key factor affecting adoption of VDLM2.

Gleason says equipping Southwest's fleet with VDLM2 for internal communications today will help the carrier make an easy transition to a VDLM2-based air traffic control system planned by the FAA.

The FAA kicked off a VDLM2-based pilot program at its Miami Enroute Air Traffic Control Center in October 2002, using the technology to exchange noncritical communications between aircraft and air traffic controllers. The FAA had temporarily shelved the project for budgetary reasons, but the initiative is now back on track, and the system could be used nationwide by 2009 or 2010.

Sharma says that Eurocontrol, the European counterpart of the FAA, will also require VDLM2 for noncritical communications with controllers in the same time frame. When that happens, airplanes equipped with $400,000 flight-control computers will finally move from the party-line voice-call era to the digital data age.

Side bar

Wi-Fi to boost data delivery

In addition to its VDLM2 deployment, Southwest also plans to boost data delivery to its aircraft by using an aviation version of wireless LAN technology called Gatelink, according to Brian Gleason, the airline's director of flight operations, technical.

Gatelink uses Wi-Fi technology based on the 802.11g standard to deliver information at a data rate of 54Mbit/sec. to aircraft when they are parked or taxiing. Southwest plans to have Gatelink installed fleetwide by the end of 2005, well ahead of other carriers, says Gleason.

"Other airlines have dabbled in (Gatelink), but we're the first to jump in with both feet," he says.

Honeywell will supply Southwest with its aircraft Gatelink radios. Other airlines had explored Gatelink before the terrorist attacks of Sept. 11, 2001, but couldn't finance planned Gatelink projects during the subsequent slump in air travel, says Ed Anderson, business development manager for data link products at Honeywell.

FedEx has installed Gatelink systems on its MD10 and MD11 aircraft, according to Traci Barnett, a spokeswoman at the Memphis-based company.

Using Gatelink will save Southwest from paying Arinc a fee for data transmitted over the WLAN network, Gleason says. Besides serving as a no-cost alternative to VDLM2, Gatelink's 54Mbit/sec. data rate will allow Southwest to automatically upload large software updates for onboard computers and avionics systems, he says. Such fat files couldn't be handled by VDLM2 but can easily be transported with the high-bandwidth 802.11g connections.

Southwest plans to use Gatelink to pull more information off its planes than the engine data harvested by VDLM2, Gleason says. He explains that the carrier plans to use Gatelink to tap into the black boxes on its planes to gather information on systems ranging from auxiliary power units (which provide power while a plane is on the ground) to cabin-pressure control units.

Honeywell is modifying the circuitry inside its aircraft Communications Management Unit to accommodate its Gatelink Transceiver Wireless LAN Unit, which is a more cost-effective approach than building a separate avionics unit for Gatelink, says Anderson. Because of the security problems inherent in WLAN systems, Honeywell will build in redundant security, says Anderson.

Southwest also intends to install 802.11g WLAN systems at the airports it serves but hasn't yet selected an equipment supplier, says Gleason.

-- Bob Brewin

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