[time-nuts] "Selective availability" af accurate time for Apple users?

Paul tic-toc at bodosom.net
Tue Oct 20 15:09:25 EDT 2015

On Tue, Oct 20, 2015 at 4:02 AM, Brian Garrett <garrettbrian1960 at gmail.com>
> ...  As measured by NTP apps such as Emerald Time and Watchville,
millisecond accuracy comparable to the soon-to-be-released Apple Watch was
now commonplace, with typical offsets of 5 ms or less, rather than the
several seconds of offset typical under the previous release.

Network connected iOS devices not paired with a watch use (s)ntp at large
intervals rather than the mobile network or GPS.  The offsets appear to be
somewhat random -- both positive and negative, large (s) and small (ms).  I
assume it's related to the environment and the last time the offset was

> Now, with the latest version, iOS 9, the Apple Watch level accuracy
previously available to iPhone users has gone away.  Once again, your
iPhone will be as much as several seconds slow as measured by NTP, unless
you force an update to the phone’s clock by opening a GPS app.

I checked on an iPad (but presumably that shouldn't matter).  My current
offset is -1.6 s (fast, and slowly increasing) as measured by Time
independent of GPS which I woke with Observatory which uses both NTP (for
time) and the GPS (for location).  Restarting the iPad causes a step. This
compares with +1 to -3 ms on my phone.

I'll admit the above is slightly speculative because I haven't checked
network traffic yet but I will shortly.

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