Not urgent, more of a curiousity…
I am wondering whether Arc will continue to know my location even though there’s no phone signal out in the desert.
Yes, Arc will continue logging location without signal because GPS is still active. Then, when you’re back online, it will fill in the data. I recommend coming out of Airplane mode and opening Arc at least once a day, as well as any time you restart the phone.
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Yep, everything @earthsaver’s said!
The recording itself doesn’t use the internet or cell network at all - location/motion data is recorded in real time and saved to the database without any network involvement. Only the later timeline processing might want an internet connection.
Caveats / Matt’s signature way-too-long technical answer:
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If it’s an area you haven’t been to before, then iOS / the phone itself will prefer to have a cell network connection and internet connection, so that it can do a) do cell tower triangulation and wifi hotspot triangulation to improve its location data accuracy. This doesn’t stop Arc recording, it just means the raw location data might be more rough than desired.
Without cell tower and wifi hotspot triangulation the phone has to rely solely on GPS, which suffers from line-of-sight limitations. If it’s an outdoor area with no tall buildings around, that’ll be fine - the phone will see enough GPS satellites to do a good job of it. But if it’s a built up city area the phone might struggle, and would definitely benefit from cell tower and wifi hotspot triangulation.
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When you open the app to view the timeline, timeline processing will kick in to clean up the raw data. At that point Arc might want an internet connection before it can do the job properly, if it’s an area you haven’t been to before.
In order classify recorded samples into activity types (walking, running, etc) Arc needs model data. Those models either come from your own data (ie it’s an area you’ve been to before and have confirmed/corrected data in), or from the shared models which come from the server (and are then cached on the phone for later reuse). So if it’s somewhere you’ve never been before, Arc will be like “I can’t classify these samples until I get a shared model from the server”, and it will leave the timeline data in a partially processed state, until an internet connection is available. No harm done, just a bit of delay in the timeline cleanup.
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Thank you @earthsaver and @matt for your answers. I don’t mind answers that are a bit longer and give me more info. I wouldn’t be tracking my activities if I wasn’t that sort of person. 
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