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Electronic devices in flight

Doug Kerr

Well-known member
I read that in the US the Federal Aviation Administration has relaxed the rules on the use of electronic devices in flight such that many devices can be used "gate-to-gate", and some can still only be used above 10,000 feet elevation.

Cellular telephones, however, still cannot be used in flight at all. Some may feel that this is a strange limit. Often in the past people have said, "I can't believe that there would really be interference between cellular telephone emissions and navigational systems. They each operate in clearly separate frequency bands, and spurious emissions are tightly controlled. This restriction must be some kind of conspiracy against travelers."

Well, the technical part of that observation is actually apt. In fact, the original prohibition against the use of cellular telephones in flight was only in small part based on concern with potential interference with navigational systems, and is origin was with the FCC, not the FAA.

The principal issue as (and still is) that, based on the design concept of cellular networks, an airborne cellular telephone can wreak operational havoc on the network itself.

The original cellular concept was predicated on the fact that when a cellular telephone sends out an origination request message to request a channel over which to place a call, a usable signal would only be received by base stations in one, or at worst several adjacent, systems.

Because all the base stations in one systems are under common management, and there is coordination of one sort or another between adjacent systems, that would result in only one of those base stations responding to the telephone, giving it an initial channel assignment.

But if a telephone at an altitude of 30,000 feet sends an origination request message, it might be received by several base stations in the Cleveland system, and several in the Columbus system, and several elsewhere, and one from each system might respond to the telephone (totally unbeknownst to the other system). The result is not pretty.

Sophisticated aspects of modern cellular system and intersystem protocols mitigate this phenomenon somewhat, but it still is a major consideration.

Best regards,

Doug
 

Michael Nagel

Well-known member
Doug,

such issues in a cellular network can be easily mitigated at (managment) software level.

A cellular phone at a distance that large to the base station (plus the shielding effects of the plane)
will always send at full power which can be 1W peak.
The field strength at 1m distance in this case can be as high as 50V/m.
Now it is not necessarilyany communications or navigation equipment that will be disturbed by this.
A plane has a lot of sensors, these can also be disturbed by RF and report false values.
I think nobody wants that...

Think EMC...

Best regards,
Michael
 

Doug Kerr

Well-known member
Hi, MIchael,

such issues in a cellular network can be easily mitigated at (managment) software level.

A cellular phone at a distance that large to the base station (plus the shielding effects of the plane)
will always send at full power which can be 1W peak.

It can be 1 W (average), 2W in the 800-900 MhZ BAND. Maybe you meant maximum (average), not peak.

The field strength at 1m distance in this case can be as high as 50V/m.

Maybe more like 5.5.

For an isotropic radiator, with transmitter power (average) P, the theoretical field strength (RMS) (V/m) at a distance r (meters) is approximately given by:

E = (sqrt (30 P))/r

Best regards,

Doug
 

Michael Nagel

Well-known member
Doug,

a cellphone antenna is NEVER an isotropic radiator.
Adding a metal surface or better put a large tube lilke the interior of a plane body around it increases field strength.
When you look at the time-sharing protocol of GSM for instance and apply the 1 or 2W mean power you get to higher peak power values. I let you the pleasure to recalculate...

Besides that - the immunity for commercial ITE is tested at 3V/m - I do not know what the limits are for aviation, but I can search them.

Best regards,
Michael
 

Tom dinning

Registrant*
Now that that's cleared up!
Thanks Doug and Michael. I have the formula noted and will do the calculation next time I'm at the airport.
Isn't it just easier to ignore any conspiracy theories and switch the phone off? After all, who's so important that they need contact with ground control (the missus) or their boss when they are on a flight to Bermuda with their mistress? If the plane does go down for some other reason the pilot can make an appropriate announcement:
Ladies and gentlemen, we are about to die. Feel free to turn on your cell phones and say goodbye to your loved ones".
 

Michael Nagel

Well-known member
Tom,

No need for conspiracy theories. There is quite some stuff you can read (until you get bored). Things may have improved now as this report is older, but OTOH there are still many old planes around.
EMC issues can have interesting effects in several places.

Sure it is better to switch off the phone, for me it is common sense, but common sense is not common...

For everything else you can stop worrying :)

Best regards,
Michael
 

Jerome Marot

Well-known member
After all, who's so important that they need contact with ground control (the missus) or their boss when they are on a flight to Bermuda with their mistress?

In that case they may need contact with their lawyer... ;)

A simple demo of the interference effect of smart phones is quite easy to do. Leave a gsm phone next to a speaker (computer speakers will do) and text or phone. You will hear pulses in the speaker.
 

Tom dinning

Registrant*
In that case they may need contact with their lawyer... ;)

A simple demo of the interference effect of smart phones is quite easy to do. Leave a gsm phone next to a speaker (computer speakers will do) and text or phone. You will hear pulses in the speaker.

Yeah, but they don't run a cockpit with computer speakers, do they? At least I hope not.
"This is the pilot speaking. Would you turn of your smart phones please. They interfere with me listening to James Taylor."
You might have a point Jerome but your argument has holes in it.
 

Jerome Marot

Well-known member
Yeah, but they don't run a cockpit with computer speakers, do they?

I am just trying to give simple examples of radio interference. For example: if the interference can be picked up by speakers, it may be picked up by headphones. Pilots wear those to listen to flight control.

The radio transmitter in a cell phone just outputs a rather strong electrical signal, strong enough for it to interfere with speakers, which are not radio receivers. In practice, it is unlikely to interfere with the pilot radio and headphones unless you are in the cockpit or the airplane is very small. But there are plenty of things in a large airplane it can interfere with, fire detectors for example. There are some in the overhead bins.
 

Tom dinning

Registrant*
I am just trying to give simple examples of radio interference. For example: if the interference can be picked up by speakers, it may be picked up by headphones. Pilots wear those to listen to flight control.

The radio transmitter in a cell phone just outputs a rather strong electrical signal, strong enough for it to interfere with speakers, which are not radio receivers. In practice, it is unlikely to interfere with the pilot radio and headphones unless you are in the cockpit or the airplane is very small. But there are plenty of things in a large airplane it can interfere with, fire detectors for example. There are some in the overhead bins.

It's just that simple examples of secondary information isn't always taken as simple by the simple minded. It's taken as gospel. Extrapolating to the nth degree is a favorite pastime of the tabloids and politicians. If we stick to factual relevance we have a better chance of arriving at a satisfactory outcome. Qantas has just made an announcement that they will allow electronic devices to be used between gates except for phone conversations. I don't want to know all the details. I trust them they know what they are talking about. After all they have kept us in the air so far.
Perfect safety isn't attainable. Affordable safety is. I can live (or die) with that.
Cheers
Tom
 

Andrew Stannard

pro member
A friend of mine used to work doing EMC testing of military jet aircraft. Once whilst testing they managed to raise the throttles all the way from idle to afterburner!

This was at a very specific frequency, and at a high power, but interesting to see the extent to which systems can be affected.

Cheers,
ANdrew.
 
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