[time-nuts] Handy iPhone app

Oz-in-DFW lists at ozindfw.net
Fri Jul 16 08:06:30 EDT 2010

On 7/16/2010 12:00 AM, Peter Monta wrote:
> Here in the Bay Area, AT&T/iPhone time has gotten noticeably worse
> recently.  The error used to be around 4 seconds; now it's 49 seconds (!).
> Emerald Time is fine for interactive use, but what I find very impolite
> is that AT&T's bad timestamps are written into the EXIF headers on photos.
> Sometimes I take pictures of sundials, for example, and a 49-second
> error is not negligible for a carefully made dial.
> It would be amusing to arrange for a long-term record of the offset of
> one's phone (which can of course change across multiple providers during
The problem is more challenging than this.  It can depend on:

   1. What air interface your phone is using (2G,3G, 0.0005g)
   2. Where in the system you are.  The time messages provided to a
      phone is usually referenced to some element of the billing system
      - not over the air operation. 
   3. The phone make, model, and internal settings.  While the standards
      usually specify how this is to be done,not all manufacturers worry
      about this much.  Apple is one of the worst, but not by much. 
      It's further complicated by multiple standards which can have
      inconsistent requirements. Most phones don't do step corrections
      and instead use some sort of correction filter to gradually
      correct to the reference.  For various values of gradual.

So you'll need to record a lot more information to get anything really
useful, though just average error would be interesting.

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