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.
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.
Something wasn’t right with the hardware you used in 90s/2000s and/or the hardware was operated too infrequently. GPS may need 15 minutes to get a lock only if it failed to receive a GPS almanac (a file describing long-term orbital parameters of the satellites) during the last 3-6 months. Normally a GPS receiver tries to save it in the background every time the receiver is on. If you operate a GPS receiver for 15-30 minutes continuously just once in 3-6 months it should update almanac and never take 15 minutes to get a lock. Even if it fails to receive an update, once updated it’s good for 3-6 months so a 15 minute long lock should not happen more than 2-4 times a year.
Besides the almanac a GPS receiver needs to get a GPS ephemeris (a smaller file describing short-term high precision deviations of satellite orbits). It is valid for 2-4 hours. It takes 30 seconds to receive it. A standalone GPS receiver that was off for 2-4 hours needs 30-60 seconds to get ephemeris before getting a lock.
To avoid the delay pretty much all smartphones download GPS almanac and ephemeris over wi-fi or cellular network either on demand or periodically. The file is only a few kilobytes so it takes less than a second to download it from an Internet server. That’s what your watch is doing to avoid 30-60 seconds delay to get ephemeris. It most likely downloads almanac and ephemeris via your phone. Unpair your watch, put it where it can’t get GPS signal, and you will see 45-95 seconds delay to get the first lock after 4 hours.
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Part is that they cheat. Turn your phone off and fly to another country and turn it on and it will take a noticeably long time. But phones being always on means it can store and update your location and remember where you were. So it gets to start with the hypothesis you are still around where you were last time it checked.
Some missing details that I haven’t seen, so I’ll add to them:
First, in addition to using satellite positioning, phones (and watches if they also connect to your phone) can also use cell towers to triangulate position. This was an upgrade made to cell towers in the early 2010s as a response to slow emergency response times (in the US at least). The US government subsidized a large portion of the upgrades. Source: I worked in the cell industry when it was going on.
Second, your phone and watch have accelerometers so they can “cheat” by adding known distance traveled to last good position.
GPS signals are transmitted in such a way that to receive them, the GPS receiver needs to first accurately know the parameters of the signal.
These parameters are:
1. satellite id, which determines the pseudorandom sequence emitted by the satellite
2. the Doppler shift of the radio signal
3. the correct phase of the pseudo-random code transmitted by the satellite.
There are roughly 32x40x1024 possible combinations of these parameters, which the receiver has to search through. Without the correct parameters it simply does not see the signal from the satellite. Once the correct parameters are found, then the receiver can start getting the data from the satellite.
New digital chips are not only faster, but they are also built to try many combinations in parallel. This allows them to find satellite signals much faster than the old receivers could.
As had already been mentioned in other comments, if the receiver can get the data on the orbits of the satellites and the exact time and its own approximate location from other sources, this greatly simplifies the problem. In this case the correct signal parameters can be calculated with quite small error, and only a small amount of fine tuning is required, allowing the receiver to lock onto the signal almost instantaneously.
Here is a more detailed explanation from StackExchange: [Why do GPS receivers need so many correlators](https://electronics.stackexchange.com/a/11900).
How does the gps system lock out opponent clients, by that I mean, us gps isn’t going to ‘aid’ or be available to, Russian cruise missiles to assist them in their mission accuracy? Do they just make the data or clock signal slightly inaccurate so it’s to be functionally worthless? I know that military GPS is far more accurate than consumer receivers, is that inaccuracy good enough to discourage use?
One reason (there’s a few more) is improvements in computing and signal processing algorithms. Old GPS receivers only had a few “channels”, let’s say 12. The GPS satellites transmit a signal that looks random, unless you know it. But even if you do, it took many minutes to just find the signal (this process is called acquisition) from one satellite, and you need to track four for about 30 seconds to learn about where it is (50 bits per second, pretty slow). That’s why it took forever back then. Today, you often have a network connection of some sort giving you the information about where the satellites are in a fraction of a second. And time and rough position. Even if it doesn’t, frequency domain acquisition algorithms searches through the full search space in seconds instead of many minutes.
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