[time-nuts] Low cost synchronization

[Magnus Danielson]

From: "Tom Van Baak" <tvb at leapsecond.com>
Subject: Re: [time-nuts] Low cost synchronization
Date: Sat, 20 Aug 2005 13:35:05 -0700
Message-ID: <001701c5a5c6$a68ea100$8633f304 at computer>

Tom,

> > Just as with WWVB receivers, he does not have to have the GPS powered up
> very long for and
> > then only once a week or so to keep the oscillator tuned up. Once a GPS
> solution has been
> > found, the local time and the GPS solution time give a time-difference and
> by remembering
> > the GPS solution time from the last time you have the /|t you need to
> calculate the
> > frequency error. So, a GPS solution could be possible.
> 
> I'm curious what the power requirements are.
> 
> My Casio WWVB wrist watch works on one
> battery for two years while my Casio GPS
> wristwatch is lucky to run for more than a
> two days, even when in intermittent mode.

It all depends on what you aim for. If you really wish to, you can run an
ordinary 32k oscillator, have a GPS receiver turned on for say 15 min every
week or so. With the GPS constellation in memory and the local time to within
a few seconds it should be possible to keep the hot-time of the GPS receiver
down to a minimum. Doing linear or exponential backoff on failure to grab
signal should make the failure handling less power-insensitive.

> > Depending on which standard you have, the phones only may have a sense of
> "real" time.
> > In GSM for instance, the phones traces network time only in a relative
> aspect, but there
> > is no real way to get an accurate UTC. The phones is being synchronised to
> the base
> 
> This sounds odd to me given that cell phones
> I've seen can display the date & time and they
> appear to be accurate to a second.

As I said, this all depends on the particular technology (there is a so many
that I don't recall them all) and in some cases how the operator have chosen to
operate their network. All I am saying is that it may not be as simple and
straightforward as one first may think. When time synchronisation is there,
the relative effects are more important than absolute time. This is true for
CDMA too, infact it is imperative or else the network fails miserably as I have
understood it. In GSM it fails more gracefully, so much infact that
synchronising base stations is not always done.

> All we need are some counter-examples. Does
> anyone on this list have a cell phone that displays
> the time of day with an error greater than a few
> seconds? (if yours has a HH:MM-only display
> compare the instant when MM changes). If so,
> then Mike can scratch cell phones from his list
> of accurate time sources.

I have. My GSM phone is not updated over the GSM network, since my particular
operator have chosen not to transmitt the necessary signals. The reason I know
all this is since I wondered why I could not have the service, and I was
enlightend on this particular issue. Remember, if you are a time-nut, you do
expect things to be synchronised just to learn that people doesn't care as much
as you do. On the other hand, I can correct time of my GSM phone when I feel it
is out of time. I usually only care about hours and rought minutes.

So, I think this option should be taken out of the list since it may not be
universally applicable.

> By the way, Mike, have you considered if your
> battery-operated, fob-sized, world-wide, low-cost,
> synchronization device will be allowed through US
> airports?

What the hitch?

Cheers,
Magnus