why did old (like 90s/2000s) GPS take up to 15 minutes to get a lock, in giant bulky units, but my running watch gets it in 15 seconds?
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One thing I don’t think anyone has mentioned is that modern chips in smartphones, watches etc will often have support for the other navigation systems too. This greatly speeds up getting a lock and also increases accuracy.
For example my Pixel 6 Pro supports GPS, GLONASS, BDS, GALILEO and QZSS.
If you install this app: https://play.google.com/store/apps/details?id=com.android.gpstest
Then you can see the different systems and satellites your phone is picking up.
GPS from the 90s ? Wasn’t it opened to the public in 2000 ?
Nowadays your devices download the GPS almanac (using GPS-A functionality), which usually requires an Internet connection.
Rather than having to “remember” where you were (and hoping you haven’t moved since you last had GPS turned on), it has to find you all over again from scratch and that initial lock can take a while to find.
GPS does transmit its own almanac but it’s slow to do so in order to be compatible.
With an up-to-date GPS almanac, which includes all the latest course corrections and orbit calculations for all the GPS units, it’s quicker to get a first-lock.
So if you have Internet, and a modern device, you can get first-lock faster.
Also, a tiny GPS chip now interacts with American GPS, Chinese Beidou, Russian GLONASS, European Galileo etc. GPS constellations all over the planet, so it’s far quicker to find 3 sats (basic 2D fix) or 4 sats (3D fix) much quicker.
The real ELI5 here is, GPS receivers and satellites have improved significantly in the past 20 years.
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