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?

Question

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

Answer 1

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).