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I noticed that watches with GPS signal receivers made by Seiko and Citizen have an airplane mode.

If it's a GPS receiver, it shouldn't cause interference, right?

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    $\begingroup$ Do they have Bluetooth too? $\endgroup$ – tomnexus Dec 6 '19 at 11:27
  • $\begingroup$ Only some of them (by Casio for example) $\endgroup$ – neuhaus Dec 13 '19 at 13:49
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Some years ago when electronics on planes were still a big deal, I remember GPS receivers, portable CD players and radio receivers being singled out as Not Allowed.

This could be because radio receivers can radiate some of their LO, and this could be at an unwanted frequency and interfere with the aircraft communications or navigation. GPS receivers are no exception.

Scratching around I found this page and this list of airlines which allow and do not allow GPS receivers on their planes.

Presumably if one were flying on Lufthansa in 2010, such a GPS-only watch would need to be in flight mode to comply with the airline regulations. In practice it would be forgotten or ignored, like so many mobile phones, but the watch manufacturer couldn't ignore the airlines' requests.

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  • $\begingroup$ What is "LO"?.. $\endgroup$ – neuhaus Dec 13 '19 at 13:49
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    $\begingroup$ Local Oscillator - a signal generated in most radio receivers, to mix with the incoming signal. $\endgroup$ – tomnexus Dec 14 '19 at 19:14
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If the watch has a GPS, it may also have bluetooth or wifi, and typically the function of the airplane mode is to turn these transmitters off.

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I have no authorative answer – but my guess would actually be "for power-saving reasons":

Especially during high-altitude flights, GPS might not work well anyways (for both technical and legal reasons). Successlessly trying to acquire a fix (which on top of that only has very limited validity, what with places often crossing time zone boundaries) consumes a lot of power.

But, even more likely: it's a user- or authority-demanded feature that has no technical reason. "It's a radio device, it needs to be switched off. Why would a 1000€ thing not have an off button?!"

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    $\begingroup$ I suppose the watch would already be good at not wasting power on its GPS when indoors, underground, etc. It might have a way of detecting that it's outdoors, like direct sunlight on the face, before it tries the GPS. $\endgroup$ – tomnexus Dec 6 '19 at 14:07
  • $\begingroup$ @tomnexus that might very well be a good idea. I presume it's also not continuously on receive, but merely tracking to use the least total energy (having to cold-start GPS reception is energetically very expensive; it's usually better to "stay a little bit on top of things" as long as possible; once you've had a good lock, you don't need as much SNR to work) $\endgroup$ – Marcus Müller Dec 6 '19 at 15:29

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