In-flight use of wireless devices: an evaluation of the risks

Anyone who flies regularly will be pleased that, finally, international transport regulations have been brought up to date: passengers are now allowed greater use of mobile phones and other electronic devices on board commercial aircraft.

Borrill, Jonathan (small)

Jonathan Borrill, director of marketing (EMEA), Anritsu

No longer should we have to go through the ritual of turning off all of our electronic devices at take-off and landing. Not before time.

In reality, the normal operation of laptop computers, games consoles and tablets inside aircraft would have been acceptable for the past several years, had it been allowed. It has simply taken a long time for the rules to catch up with the reality.

To understand why it is right to allow electronics devices to be used inside aircraft, it is worth explaining briefly the technical basis on which a risk could, theoretically, be posed to an aircraft.

The risk in question was of interference impairing or even disabling the aircraft’s communications systems: these include the voice link between the cockpit and control towers, navigation systems and warning beacons. (In no way could consumer devices have posed a risk to the aircraft’s hydraulic or mechanical flight and control systems.)

In the past, both aircraft and electronic devices might have used analogue communications systems, which both generated spurious and harmonic transmissions, and were susceptible to interference from them. Anyone who listens to an analogue AM or FM radio broadcast will be familiar with the poor sensitivity, high noise and susceptibility to interference of analogue radio technology.

All mainstream commercial aircraft in Europe and North America today, however, use RF-shielded high-speed digital or optical technology for internal and external communications.

And the fact is that these are highly immune to interference. It follows that, since an aircraft’s digital communications systems are highly immune to interference, and consumer devices generate interfering signals at no more than very low power levels, the communications on which a commercial airliner’s safety depends are not in reality at any material risk.

Consumer devices’ digital communications systems (since the introduction of GSM, this includes all mobile telephone handsets and modems) today must comply with certification schemes which limit the interference caused by the devices (both in-band and out-of-band), and which ensure that the devices are not susceptible to interference from any neighbouring electronic signals.

In Europe, the requirement is for the ‘CE’ mark; in North America, all radio products require ‘FCC’ (Federal Communications Commission) approval. The CE mark is a general safety certification, but for radio-type products it specifies limits for in-band and out-of-band emissions, and for the receiver’s intermodulation performance.

This ensures that any product with a radio transmitter circuit does not produce excessive RF interference. For mobile phones, the precise test specifications are described in the European Commission’s R&TTE (Radio and Telecommunications Terminal Equipment) directive, and are based on the RF characteristics of mobile phones described in the 3GPP specifications.

In-band interference is prevented by the precise control of a device’s transmission frequency and RF output power. Frequency control is particularly well managed by mobile phones. And the RF output power of Wi-Fi, Bluetooth and similar equipment is limited by law to such low levels that there is no risk of signal degradation to aircraft communication systems.

RF spurious and intermodulation (out-of-band) interference occurs when a source generates harmonic frequencies that create interference at frequencies other than the frequency at which it is designed to transmit.

Typically these are exact integer multiples of a strong intended signal, and they become a problem when the integer multiple of the source frequency is at the receiver frequency of the target, impairing its ability to receive desired signals. Alternatively, the electronic circuits in the target device might create harmonic multiples of the source signal, and these harmonics might multiply together to create interference in the target device itself.

This type of out-of-band interference was commonly experienced in old, analogue communications equipment. The certifications described above prevent digital systems from suffering signal impairment because of out-of-band interference.

The general use of radio transmitter equipment remains banned on board aircraft, due to the danger of an uncontrolled radio transmission directly affecting the aircraft’s communications or flight control electronics. However the use of mobile phones is becoming relaxed, as they are a much more controlled radio transmission, and the behaviour of the phone can be managed to such an extent that it does not cause a problem for the aircraft.

So now, for instance, mobile phones can be used during taxi-ing manoeuvres on the ground: as stated above, the transmissions from the mobile handset can be managed to ensure they do not interfere with aircraft communications. And during taxi-ing manoeuvres, the flight control electronics are not safety -critical, since the pilot can safely take direct manual control of the aircraft. In any case, the aircraft is already exposed to external terrestrial mobile phone signals once on the ground.

There are also regulations which allow the use of mobile phones inside aircraft in-flight for voice calls and SMS. This calls for the requirement to use a special ‘low RF output power’ mode when operating a mobile phone within aircraft.

In this mode, the passenger mobile phones become a controlled radio transmitter: they can be controlled in such a way that the frequency and power of the transmissions do not affect the aircraft’s flight systems. This makes it possible to use a mobile phone during a flight to make and receive calls via a mobile network installed in the aircraft.

There is, however, a risk that airlines do need to consider – but it is not a risk to the operation and safety of the aircraft’s own communications systems. The use of mobile phones connected to the in-flight service creates a problem for mobile networks on the ground because either the aircraft’s base station transmitter or passengers’ handsets can cause in-band interference to terrestrial networks.

Airlines must therefore be careful to turn the in-flight mobile service off when on the ground, so that it does not compete for frequency spectrum with mobile network operators’ fixed terrestrial base stations at or near the airport.

This, of course, is an operational rather than a safety-critical issue. Normally, then, the in-flight systems are not activated until the aircraft is at cruising altitude, and are switched off during taxi-ing, take-off, and landing.

But the next time your mobile phone connection unexpectedly fails or goes slow when you are on terra firma, look above you: if there is an airliner in the sky, it might just be the cause of your problem.

Writer is Jonathan Borrill, director of marketing (EMEA), Anritsu



One comment

  1. Michael J. Linden

    “All mainstream commercial aircraft in Europe and North America today, however, use RF-shielded high-speed digital or optical technology for internal and external communications.”

    Not completely true. Critical Plane-to-ground (control towers, etc.) voice communications still use that “old analog AM” technology you refer to when over land (between 118 and 136 MHz). And, Single Side Band (SSB) voice communications are often still used when over the ocean.


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