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Comment by kuroguro | Hacker News Reader

I wonder why phones don't use GPS to sync clocks? Or do they? Hmm...

At least in 3GPP networks (ie. GSM/UMTS/LTE, all of which depend on precise 27MHz clock in the handset) the PLL lock to network's time reference gives more precise time reference that you can reliably get from GPS and essentially for free. The MS (ie. your phone) needs to maintain that PLL lock in order to stay associated to the network (which itself has traceable time/frequency calibration to some kind of national frequency standard and GPS clock).

AFAIK in 3GPP2 (ie. US-style CDMA) the handsets actually have GPS receiver that gets used in the initial association phase to speed up the timing PLL lock (which for the initial cell acquisition has to be significantly more precise than in 3GPP systems)

Phones usually just use network time, because the GPS is an expensive component to spin up.

Can you explain 'spin up'?

Do you mean that its Power/battery intense?

Yup. The GPS drains battery.

But when GPS is going for another purpose phones could grab the time. Don't know if they do.

Initial GPS signal acquisition is power intensive, but if it is already tracking, then there should be no incremental cost to read the time information.

GPS time tends to be very precise as well. In the NTP world, (simplifying greatly) GPS can serve as a reliable “stratum 0” time source which feeds stratum 1 servers, which go on to feed the rest of the NTP network.

Building accurate NTP stratum 0 GPS time source is somewhat nontrivial endeavour. Which explains why most of stratum 1 NTP timeservers (typically GPS based) give wrong time (with error on the order of 500ms).

But what value would that add? How often would you need to sync your phone clock but not be connected to network (which will be providing time sync anyways)?

If you're off the grid somewhere. Or just a cool bragging right.

I own a watch that uses GPS as a time source and it is not without its limitations. It only works outdoors, and it takes several seconds to get as sync - over a minute if you also need to sync your position (to adjust the time zone).

Towers do use GPS to sync clocks and typically they also have rubidium clocks so that they can last a considerable time without GPS. Why feed and wait for your hungry hippo GPS chip when you can do almost as good (in an absolute sense) or better (in a relative sense) by just observing tower timestamps?

To be more complete and precise, they use GPS to condition a rubidium clock. For several reasons it isn't a good idea to take timing directly from GPS without a highly stable local oscillator if you are using the output over long or continuous periods.

And to be even more precise in the pre-CAE days the main source for disciplining was SDH/SONET clock of the network interfaces. It is higher stratum clock source than GPS, but as the frequency is higher it ends in more accurate PLL lock.

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