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?
In: 3751
It takes 12.5 minutes to download the sky chart and other info needed to search for GPS signals. The data rate of GPS data download is 20 bps, really, only 20 bits per second. The first satellite was launched in 1978.
Your phone can download all the same info for its search from the internet, 1 to 10 million times faster. It can also get an idea of time and frequency and coarse location from it cell connection.
It then uses all this information to search for a signal that is smaller than the noise, like listening for a mouse in a rock concert.
Old GPS units had to find the mouse at a rock concert with a couple of guys looking , cell phones get to find the mouse lit up by a spotlight with some light elevator music and 10000 guys looking for it.
Everything is 100x better 30 + years later so 7 seconds for a location instead of 700 seconds.
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