[deceptionatd]

> More importantly, satellite positioning receivers need a cached "ephemeris" file that helps them predict which satellites will be overhead at a given time to operate efficiently, accurately, and quickly.

Interesting, I've never heard of that particular optimization for GNSS before. I know GPS transmits ephemeris information in each frame, since that's the input data for the positioning calculations. I've got a number of Garmin watches, and they've always been able to get a position fix even after being disconnected for weeks.

I find GNSS implementations very interesting, which manufacturer is making their watches like this?

[saltcured]

Garmin watches do! Go into the System -> About menu and there is a page showing ephemeris status.

Their documentation states that this may expire after approximately 30 days or if you travel more than 200 miles, while it will update during syncing.

I've seen it expire in less than two weeks with daily use of GPS but phone syncing disabled. It will still get position, but it can be the difference between an almost immediate fix after opening an activity menu or a delay for tens of seconds to minutes. Distance and pace measurements also seem to be lower quality when operating without a current ephemeris file.