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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GPS needs to know the[ position of satellites](https://en.wikipedia.org/wiki/Ephemeris) and the [condition](https://en.wikipedia.org/wiki/GPS_signals#Almanac) of each of these satellites even though they are in a very predictable high earth orbit about 12,000 miles above Earth; this information is known as almanac and ephemeris data. The US Space Force maintains the GPS constellation and tracks all these satellites; they upload this information to the GPS satellites for rebroadcast to every GPS receiver. Your phone, or any internet connected device, will download this data automatically over the internet. Old GPS receivers had to rely on the GPS satellites rebroadcasting ephemeris and almanac data to get this information; the broadcast takes about 12.5 minutes to send all the information. Once it knows where each satellite is, it needs to figure out which satellites it can actually see to get a rough idea of what part of the world you’re in. Your phone doesn’t have to do this as it can roughly figure out it’s location using your IP address and [the wifi networks around you](https://slate.com/technology/2018/06/how-google-uses-wi-fi-networks-to-figure-out-your-exact-location.html); once it knows roughly what area of the world it’s in, it will look for specific GPS satellites it knows will be in view.
edit: you got downvoted for asking a simple question, but the answer is a lot more complicated and interesting than most people think.
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