ATM Modernization, Business & GA, Commercial

Safety: Disrupting the Harmony Aloft

By David Evans | August 1, 2005
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The unrestricted use of cellular telephones in flight is fraught with social and technical questions, of which the technical issues may be the more intractable.

This is the picture that emerges from more than 7,000 comments submitted in response to a Dec. 15, 2004, Federal Communications Commission (FCC) proposal to relax the present ban (see Docket 04-435). Current FCC rules require that cellular handsets be turned off once an airplane leaves the ground to avoid interfering with terrestrial cellular systems. In addition, FAA regulations restrict the use of mobile telephones and other portable electronic devices on aircraft to prevent interference with onboard avionics.

Let’s deal with the social nightmare first, the specter of a passenger chatting away on cell phones with no regard for those sitting immediately adjacent, which is the subject of most comments in the docket. The Association of Flight Attendants, the union representing many of the cabin staff that will have to deal with this problem, says bluntly in its submission: "Allowing airborne use of cellular telephones is a prescription for bedlam in the skies."

Now to the potential interference problem. Graham Strauss submitted a copy of his doctoral thesis, pointing out that some respondents erroneously pointed to the air phone (seat back phones) as justification for allowing cell phone use. Such responses fail to recognize that air phones are permanently installed and use externally mounted antennas, whereas cellular phones "have only recently been selectively tested on aircraft for compatibility, and this has mostly been limited to ground testing."

The Department of Justice (DoJ), including the Federal Bureau of Investigation (FBI) and the Department of Homeland Security (DHS), raised the prospect of terrorists using cell phones: "The uniqueness of service to and from an aircraft in flight presents the possibility that terrorists and other criminals could use air-to-ground communications systems to coordinate an attack (e.g., a hijacking)."

For example, persons in the air could coordinate their attack with persons on the ground, with persons traveling on different aircraft, or persons traveling on the same aircraft located in different sections of the cabin. This being the case, the capability to at least record the origin and destination of wireless calls is essential, the DoJ argues.

FCC has proposed lifting the ban on cell phones and other devices, as long as they operate under the control of a pico cell located on the aircraft. The pico cell would basically restrict the power of mobile phones, which would work through a cell base on the aircraft to keep the amount of energy emitted in the cabin quite low. Without a pico cell, cell phones are unable to easily contact a ground station, so they boost to high power in hopes of finding one.

Of the many cell phone companies submitting comments, this one from Sprint is typical: "Sprint agrees that the theory of a pico cell architecture appears to be `promising;’ however, it is clear that major technical and operational issues must be addressed, tested and resolved before pico cell systems can be authorized.

"There are two additional facts that the commission needs to understand about airborne use of cell phones not controlled by an onboard pico cell. First, airborne cell phone emissions will transmit at far greater distances than handset emissions when located on the ground (because ground signals are `attenuated’ by terrain, buildings, etc.). For example, signals received from an airborne cell phone at 100 miles (161 km) from a terrestrial base station (assuming line of sight) will generally be 100 to 10,000 times stronger than signals received from a ground-based handset 100 miles away. The point is that airborne cell phones can cause considerable harmful interference at considerable distances.

"Second, thousands of base stations can be in view and line-of-sight of a single airplane.

"A pico cell needs to be all-inclusive by including all CMRS [commercial mobile radio services] air interfaces and…spectrum bands. But it is important for the commission to understand that the identical interference problem will occur even with an operational onboard pico cell if the pico cell cannot communicate with a handset because it is not designed to use the handset’s assigned spectrum band or the handset’s particular air interface.

"Thus, if a passenger with an iDEN phone … sees a passenger in the next seat using his Sprint PCS handset, the passenger will understandably assume that she can use her service as well. But if the onboard pico cell does not support the iDEN interface…the pico cell cannot control the handset. And if the pico cell cannot control the handset, the handset will attempt to communicate with full power with the carrier’s terrestrial network and thereby generate interference into the network."

United Airlines recommends that systems be "preprogrammed to automatically shut off if they fail to connect with an aircraft’s pico cell system within a reasonable period of time." The problem here is that not all devices can be assured to have been retrofitted with this feature.

The more serious problem may be interference with onboard avionics. Consider this commentary submitted by David Sullivan-Nightengale, a former Honeywell employee: "Honeywell has mapped almost every conceivable failure mode based upon distinct types of outputs to the aircraft system. These failure modes include incorrect airspeed, altitude, attitude, position, external temperature and so on.

"Adequate shielding from radio frequency [RF] radiation is not always achieved, whether among these systems or even within a box such as the ADIRU [air data inertial reference unit] itself, simply because either it was never required or not seen as needed…It is important to note that many Honeywell navigation systems are more than 25 years old, and none were required to be tolerant to cell phone interference.

"I expect aircraft subsystems to receive interference from cell phones based upon years of experience of measuring signal strength of low power (1- to 3-watt) transmitters inside aircraft. I also believe that the number of cell phones (800 or more on the A380) on some aircraft will cause interference with safety critical systems. Furthermore, how will the Transportation Security Administration and the crew be able to distinguish between a cell phone and a jamming device? While jamming can be currently achieved by other means, such as hiding a jamming device in a computer, microelectronic technology has improved to spoof navigation systems into telling a crew a wrong location. This may be an invitation to terrorists to bring jamming equipment on an aircraft."

Although Sullivan-Nightengale does not represent the official Honeywell position, he nevertheless sums up many problems. The official Honeywell position supports the development of pico cell technology. "We do not support the use of mobile phones without pico cell control due to the risks posed by interference to safety critical communications, navigation and surveillance equipment that is crucial to the safe and regular operation of the aircraft," the Honeywell submission states. "Our support…is conditional on additional FCC rulemaking on the matter of spurious and out-of-band emissions by the portable devices."

Separate from the comments in the docket, the European Organization for the Safety of Air Navigation issued a report in December 2004 on certifying cellular telephones on airliners. In terms of system design considerations, it included a safety function that "monitors the RF field strength within the aircraft." It continues: "The function may include monitoring of call throughput as a measure of passenger use of the system.

"An alert may be used in conjunction with cabin crew procedures…to control passenger usage, and/or as an input that ensures a safe termination of the telephone service to the cabin. Consideration should be given to the need for an alert on the flight deck and related crew procedures."

FCC is now digesting various comments. Retaining the current ban on cell phone use in flight may be prudent given the technical concerns and uncertainties, and citing the "bedlam in the skies" envisioned by the Association of Flight Attendants may be sufficient public justification. Disrupting the tranquility of the cabin is one thing, but disrupting the harmony of avionics is quite another.

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