[time-nuts] Low cost synchronization
cfmd at bredband.net
Sat Aug 20 16:02:38 EDT 2005
From: "Tom Van Baak" <tvb at leapsecond.com>
Subject: Re: [time-nuts] Low cost synchronization
Date: Sat, 20 Aug 2005 11:59:45 -0700
Message-ID: <00a701c5a5b9$55fe7740$470ff204 at computer>
> > 2. WWVB Receiver
> These are exceedingly cheap now and should fit
> all your requirements. Contact Rod Mack who has
> probably done more WWVB R&D than anyone on
> the list (he did the Ultralink receivers using Temic
> chips). Email me offline for his contact info.
> WWVB reception quality is not an issue since
> it's only used to intermittently re-synchronize the
> internal XO. One decent reception every couple
> of days or even weeks will take care of your
> Note also that many WWVB chipsets are now
> "global", meaning they will also receive signals
> from LF time services in Europe and Japan
This is indeed a solution that should be looked at.
> > 3. GPS Receiver
> > 4. GPS Time Receiver
> As many cell phones now include GPS receivers
> sizes and prices are dropping. But I'm guessing
> you are not going to meet your fob-size nor power
> specs with GPS (or other satellite nav systems).
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.
BTW, recall that it is mostly US cell phones that has the GPS in them for the moment.
The big markets does not have them (yeat). Never the less, small GPS solutions is around
> > 5. Cellular
> What percent of your thousands to millions of
> users world-wide already have a cell phone? To
> me this is the obvious solution. I would guess
> all cell phones know the time to a millisecond
> internally and this means a billion people on the
> planet are already carrying just what you need.
> Battery life is not a problem because all users
> already know how to recharge theirs.
> Now if each brand of cell phone would just have
> a standardized 1PPS output connector you'd be
> all set.
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
stations such that the transmitted slot from the phone fits right into the timeslot at the
base-station, which is done by having the base station continously sending advance time
corrections which could be used backwards to know the time from the base-station. However,
this is only relative timing from the base station, but the base station is not even
guaranteed to be synchronised to UTC or even a PRC. Infact, some GSM vendors is proud of
this fact since synchronisation is such a hassle in their eyes. Sigh! Also, this
correction is only available as when you are transmitting.
Then, getting the time from the cellular network may not be as easy as you think either.
The operator of the network may choose to turn on the optional time announcement and last
time I checked this was not universally done.
Yes, these networks could potentially be capable of giving good time, but I think the
real life prooves a bit more disappointing than at least I first thought. It all depends
on which cellular standard and maybe also details about how a certain operator is actually
using the technology.
I would not look at cellular technology for a global solution since there is so many
standards in operation right now and will continue to be. Also, there is a shift towards
3G which is slowly progressing.
> > 6. TV Stations
> Two methods come to mind. The XDS timecode
> (used by PBS stations) is good in principle but
> perhaps not in practice. The other approach is
> to discipline a 32 kHz XO against the 3.58 MHz
> colorburst frequency. This seems dated, though.
Ah well, that will become toughter as the analogue TV stations is being shut down. DVB
and ASTC is taking over. There is a goal to have the signals traceable to TAI/UTC and work
is undergoing, such as the SMPTE EPOCH and other work.
> For any solutions that give you stable frequency
> only (XO, RF carriers, 60 Hz) you will need a way
> to set the initial time and to reset the time when
> the batteries fail.
For some countries will 60 Hz or 50 Hz no longer be maintained on 24 h basis, so it may be
a bad idea to depend on it.
> For any solutions that give you time only you will
> presumably need to convert from UTC to local
> time. Also, are you concerned with DST?
> At least with your requirements, you don't have
> to worry about leap seconds!
Leave that to us others! ;O)
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