[ntp:questions] Indirect GPS time source options

Olivier Drouin ol.drouin at gmail.com
Thu Mar 13 17:52:51 UTC 2014

Thank you Greg, Jochen, William,

                               Great answers.

                               Diversity is really what I'm looking for and
I dont really need microsecond accuracy.
                               CDMA seemed to be the most 'convenient' way
to add diversity without a need for a working WAN link and/or line of sight
view to the sky.
                               Also, I already know that I get good cell
signal from inside the server room.
                               The thing with CDMA is that it looks like
it'll not be around for many years and I haven't seen any equipement for
4g, hspa, LTE, etc...
                               For your information, I think I can get away
with a 10K budget but I need to be absolutely sure It'll work beforehand.

                               I'll talk with the facility owner and do
some tests with handheld GPS so I can verify what kind of signal I can get
(and where). I'm a bit worried because I dont have roof access whenver I
want and I dont know how the antenna will behave with the snow we get
around here (quebec city).

                               Ill also verify if I can synthonize the CHU
radio frequency inside the server room if I can get my hand on a scanner.

                               Olivier Drouin.

On Thu, Mar 13, 2014 at 1:26 PM, Dowd, Greg <Greg.Dowd at microsemi.com> wrote:

> You are not incorrect but I think that "indirect GPS time" may be more of
> a marketing term.  Basically, GPS is the predominant method of time
> dissemination.  When you think of a commercial ntp server, really you just
> moved the GPS antenna from your PC (getting rid of the receiver and coax
> cable) over to the ntp server, and then distributed the timing information
> over a different cable (your ethernet cable).  Since NTP is layer 3, and
> the bus isn't dedicated, you introduce error but it's still GPS time.
> My term for the concept I understand you to be discussing is timing
> diversity.  I prefer to have a number of different methods of accessing
> time data.  GPS is the baseline but access to the L1 GPS signal is limited
> in some locations, particularly indoors.  Keep in mind the analogy that the
> GPS broadcast is like a light bulb shining from 22000 km away.  CDMA is
> nice as it's a powerful signal (compared to GPS) but it is limited in
> geographic deployment.  Most 3G/LTE cellular systems these days are tightly
> syntonized (frequency) but not synchronized (time).  So, no, you won't get
> microsecond level timing from GSM.  Next gen LTE has a profile for LTE-TDD
> which will require about the same level of synchronization as CDMA.
> If you are looking to add diversity to an installation, your first best
> bet is to move to a GNSS receiver.  Like your smartphone, a GNSS receiver
> typically works with both GPS and the Russian Glonass satellite systems.
>  There's some diversity.
> For Canada, you could also look at eLoran.  That operates on a completely
> different frequency plan to satellite timing.  I think Canada has CHU radio
> timing as well (3330kHz).  There are a number of signals of opportunity
> that require custom hardware to access either time or frequency.  You
> mentioned CDMA.  Digital television is another example.
> As someone mentioned earlier, to get a more precise answer you have to
> provide a more detailed analysis of your requirements and budget.  Anything
> is possible.  There are neutron stars out there dieing to provide you with
> a frequency reference :-)
> -----Original Message-----
> From: questions-bounces+gdowd=symmetricom.com at lists.ntp.org [mailto:
> questions-bounces+gdowd=symmetricom.com at lists.ntp.org] On Behalf Of
> Olivier Drouin
> Sent: Thursday, March 13, 2014 7:29 AM
> To: William Unruh
> Cc: questions at lists.ntp.org
> Subject: Re: [ntp:questions] Indirect GPS time source options
> On Wed, Mar 12, 2014 at 3:59 PM, William Unruh <unruh at invalid.ca> wrote:
> > On 2014-03-12, Olivier Drouin <ol.drouin at gmail.com> wrote:
> > > Hello,
> > >       I'm located in Canada and I'm trying to find indirect GPS time
> > sources.  I like the idea of using CDMA as a time source but it seems
> > that this technology will not be around for a lot longer.
> >
> > What is an "indirect gps timing source"? How does that differ from
> > using ntp to go to a level 1 server which gets its time from gps.
> > While cdma uses accurate time, it is not really designed to deliver
> accurate time.
> > GPS is.
> >
> Well, from what I understand the cell networks are closely synchronized
> against the GPS constellation. So, synchronizing against the cellular
> network is called "indirect gps", at least according to the endrun website
> (I haven't seen this nomenclature anywhere else).
> It is different of GPS because if your WAN links are down, which can
> happen once every few years,  then, hopefully, you can still reach the
> cellular network in your neighborhood or else its the end of the world or
> something...
> Also, from what I understand and please correct me if I'm wrong but
> cellular networks are indeed designed to be accurately timed because it's
> needed for the normal operations of the cell network.
> Thank you for your reply, best regards,
> Olivier Drouin
> >
> > >       For exemple, Endrun Technologies sells the Sonoma N12 which
> > > seems
> > to allow time synchronization against CDMA signal without the need for
> > a carrier subscriptions.
> > >
> >
> > And a gps receiver with PPS output is probably 1/100 the price.
> >
> > >
> > >        Is the CDMA method still available with these other cellulare
> > network technologies like HSPA or LTE? If not, do I have other options
> > beside a GPS antenna with a sky view?
> >
> > CDMA also needs an antenna with cell tower view. What's the difference?
> > (Note my gps on an east facing window through a large cedar tree works
> > just fine.)
> >
> >
> As Terje is mentioning, the cell antenna doesnt need a line of sight to a
> cell antenna, so It can be installed inside the server room and you dont
> have to deal with the maintenance problems (lots of snow around here) of a
> GPS antenna on the roof...
> > >
> > >
> > >
> > >        Thank you and best regards,
> > >        Olivier Drouin.
> >
> > _______________________________________________
> > questions mailing list
> > questions at lists.ntp.org
> > http://lists.ntp.org/listinfo/questions
> >
> Thank you and best regards,
> Olivier Drouin.
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