GPS: How NTP time injection works

Question

I\'ve recently known about a gps.conf file in the /system/etc/ directory. Seems that tweaking NTP_SERVER values to NTP servers nearer to the usual loca

Answer 1

Well scouting a bit of wikipedia and some other sources, let me have a few guessses.

1. Yes, you can infer GPS time from UTC time. You just have to know the offset, which is transmitted every 15 seconds and changes once in approximately 18 months. Source: Wikipedia

2. NTP does not give you exact time. It measures the time message gets from client to server and the time the response gets from server to client. These times are then used to calculate the delay of the connection. Which is then applied as an offset to received time. This works for symmetrical routes. If the routes are assymetrical, there is an error. So closer the server, lower the the chance and level of assymetry, thus lower the error. Source: Wikipedia again

3. NTP signal is not directly used to obtain the GPS fix. But for an accurate fix you need very accurate clocks. We're talking nanoseconds here. GPS satellites do transmit current GPS time, but even as it travels at speed of light, there is some delay. GPS receiver has no way to know what the delay is, so it has to approximate from several received signals. With every transmission received the clock get more precise. So the better time you have at the beginning, the less time signals you have to receive to have an accurate clock. Source: Wikipedia

4. Well pretty much explained in 3. - the lower clock error the less signals needed to approximate the correct time.

5. I'm little guessing here, but having approximate location can help you better approximate the distance from satellite and thus the delay. (Not sure if that is really used.)

I hope it makes at least a little sense ;-)