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I've been looking at radio controlled watches online. I noticed that some radio controlled Casio watches have an 'airplane mode', which disables GPS and terrestrial time signal reception:

Casio watch airplane mode

Now, why would signal reception be restricted? The watch in question does not have a Bluetooth module or similar.

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    \$\begingroup\$ I see you have never flown international and had to argue about the legality of toothpaste. Or prove to a flight attendant that your smart watch isn't doing anything bad while switched off \$\endgroup\$
    – PlasmaHH
    Jul 24, 2016 at 17:41
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    \$\begingroup\$ Please enter an actual model number of a watch that has no bluetooth but has an airplane mode. When I searched, I found a variety of watches that have airplane mode, but they also have bluetooth. \$\endgroup\$
    – user57037
    Jul 24, 2016 at 17:54
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    \$\begingroup\$ @mkeith MRG-G1000B-1ADR \$\endgroup\$
    – Geier
    Jul 24, 2016 at 17:55

4 Answers 4

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Most useful radio receiver designs utilize some form of the superheterodyne architecture, where one or more radio-frequency local oscillator signals are used to shift the frequency of the received signal to an intermediate frequency where it is more readily processed.

Typically, the local oscillator signal will radiate from a receiver to some degree - how much depends on the specific design, but it is not uncommonly detectable in near proximity.

Restrictions on radio frequency sources on passenger airlines have historically tended to err on the side of caution, restricting anything that could theoretically emit moreso than being specifically tailored to demonstrated concerns. Since a local oscillator theoretically radiates, the argument was that you should not be operating one, which effectively meant that you should not be operating a radio receiver.

Of course digital processing circuitry typically radiates clocks and their harmonics, too... But then, the shift in the last several of years towards services like in-flight WiFi also indicates a shift in thinking away from vague theoretical concerns.

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  • \$\begingroup\$ This is the correct answer. For an example of how useful local oscillator emissions can be, this was the method used by Spycatcher Peter Wright and his team of MI5 boffins to search for KGB illegals listening in to Moscow in Cold War London. It is also, reputedly, the basis of the notorious TV detector vans in the UK. \$\endgroup\$ Jul 25, 2016 at 9:08
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    \$\begingroup\$ Note that interference from receivers had actually contributed to the deadliest airplane crash in history by masking the stand by for takeoff command with heterodyne whistle. \$\endgroup\$ Jul 25, 2016 at 11:47
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    \$\begingroup\$ @DmitryGrigoryev: According to the article, that crash was caused by interference from a simultaneous transmitter, not from a receiver. \$\endgroup\$
    – David
    Jul 25, 2016 at 16:16
  • \$\begingroup\$ @David It seems to blame "three-second-long whistling sound (or heterodyne)". Are heterodynes used in transmitters as well? \$\endgroup\$ Jul 25, 2016 at 16:18
  • \$\begingroup\$ It's called "heterodyne interference" because the effect is the same: when two transmitters are simultaneously operating on the same channel, their carrier frequencies usually don't match precisely. So the difference between the two carriers turns into a loud audible tone that drowns out the intended signal. \$\endgroup\$
    – David
    Jul 25, 2016 at 16:22
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I think that there is a very simple answer to this question that has nothing to do with electronics. A number of airlines prohibit the use of GPS receivers in flight. Their rationale does not actually matter. In order to comply with their rules, the watch must be able to turn off the GPS receiver somehow or it would be prohibited from being on the plane entirely.

Here is a list of airlines which do not allow GPS receivers: http://gpsinformation.net/airgps/airgps.htm

Does GPS actually cause interference? Doubtful. http://gpsinformation.net/airgps/gpsrfi.htm

If you wish to keep GPS enabled during flight, check with the pilot.

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Mobile phones, GPS receivers, laptops or indeed smart watches have no effect whatsoever on avionic equipment. Most kit is designed and tested for restricted emissions and EMI tolerance in reverse.

The reason this equipment is banned has nothing to do with safety, but financial expedience. There's a lot of electronic equipment in the world. To rigorously prove my assertion in a way that would absolve an airline in a damages court case would take £100Ms. No one wants to pay that, especially if it would have to be passed onto customers. Better just ban the stuff. People will still keep flying as they're a captive market. You'll fly naked sat in a transparent plane if necessary.

Try leaving your phone fully on the next time you fly a long way across Europe. If you survive to land at the other end, you'll find perhaps 10 -15 messages on it welcoming you to various mobile networks that you over flew. No problem.

Edit following comments:

There are approximately 1 million people flying in the air at any one time across the world. If 1 in 1000 people randomly forget to switch off their e devices in flight (plausible), this theory is tested 1000 times/day across all randomly selected airliners in the world. That's 365,000 times a year on every conceivable aircraft type. That's a million hours /year of testing based on a 3hr mean flight time (plausible). That's a pretty extensive systematic experiment. Nothing dangerous happens.

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  • \$\begingroup\$ That irresponsible to advocate leaving said devices on. The issue is there are a lot of planes in circulation that were designed way before wifi, personal gps etc.. really existed. I am thinking the 747 specifically here. They were designed to consider the DO160 at that time. Now chuck within the main structure another source of RF and well... There are clearly logged incidence where a radio transmitting device interfered with comms (didn't bring the plane down for sure but...). Newer planes have better shielding, routing, susceptibility limits. \$\endgroup\$
    – user16222
    Jul 24, 2016 at 22:20
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    \$\begingroup\$ @JonRB Which original bit of the 747 from 1970 do you think is still in service? The multi functional computerised instruments or the GPS navigation system? \$\endgroup\$
    – Paul Uszak
    Jul 24, 2016 at 23:03
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    \$\begingroup\$ My "phone fully on" example occurs probably thousands of times a day with no significant effects - people forget to turn them off and there's a million people in the air at any one time. If a phone had any tactical capability they would be immediately banned from all flights in the present climate. They would be confiscated at security and you'd be arrested and waterboarded. \$\endgroup\$
    – Paul Uszak
    Jul 24, 2016 at 23:07
  • \$\begingroup\$ publicapps.caa.co.uk/docs/33/CAP756.PDF "transmission levels produced by a portable telephone used near the flight deck or avionics equipment bay exceeded demonstrated RF susceptibility levels for equipment qualified to standards published prior to July 1984, and as such, as equipment qualified to these standards is installed both in earlier certificated aircraft and in newly built aircraft, the current policy for restricting the use of portable telephones on all aircraft will need to remain in force; and". I would trust the CAA over some random individual on the internet \$\endgroup\$
    – user16222
    Jul 25, 2016 at 8:05
  • \$\begingroup\$ New planes are a lot better but that doesn't dismiss the existence of older planes still in usage. At the end of the day the CAA/FAA/RTCA/EASA/ANAC/.../Airlines set the rules not the passengers. As long as they say "turn off during takeoff and landing" you run the risk of being removed if you do not adhere. \$\endgroup\$
    – user16222
    Jul 25, 2016 at 8:07
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All radio receivers have circuitry that could emit RFI if not properly shielded. Better to be safe then try and determine the possible harm

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  • \$\begingroup\$ Welcome to EE.SE. Can you expand your answer with some citations and references? One-line answers aren't very helpful. \$\endgroup\$ Jul 24, 2016 at 20:15
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    \$\begingroup\$ This is true for all electronic devices, even if the receiver is disabled, or even if they don't even have a receiver. \$\endgroup\$
    – pipe
    Jul 25, 2016 at 4:04

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