[time-nuts] GPS module recommendation for Pi timing

[Brian Lloyd]

On Sat, Mar 14, 2020 at 10:57 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
>
> > On Mar 14, 2020, at 10:54 AM, Brian Lloyd <brian at lloyd.aero> wrote:
> >
> > On Fri, Mar 13, 2020 at 8:49 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> >
> >> Hi
> >>
> >> If you always operate in fixed locations, your chances of seeing a slip
> >> are not that high. The original post question and reference here were to
> >> timing in a *mobile* situation.
> >>
> >
> > Thank you all for your inputs. Here is a little more data, prompted by
> > comments and questions. (Some of this I wasn't thinking about at first.)
> >
> > I realized that, while I expect the majority of units to be stationary
> with
> > the ability to properly position an antenna, there is a chance that one
> or
> > more of the units may be mobile. Regardless of mobility, the purpose will
> > be time and frequency reference, not position. Does that change my choice
> > of device?
>
> In order to get time, you first need location. Without location, there is
> no
> way to correct for the time of flight of the signals.
>

I understand. The question is, are the timing-oriented GPS modules going to
work in that application or should I opt for a position-oriented GPS and
accept lower accuracy of 1pps?

In a mobile situation, timing can be tough. At some point a ZED-F9P or a
> member of that family would do better if highly accurate mobile time is
> the
> goal. Since your needs are modest, it may not be worth the cost.
>

And that is the question.

There is still the issue of going under a bridge / into a “canyon” and
> having the GPS stop due to lack of signal. This sort of dropout can
> easily run into the “couple of minutes” range.
>

Let me put it this way, if I am going under a bridge or in a tunnel, the
point will be moot.

“Good antenna location” in this case is often not an easy thing to do.
> Ideally you want a full sky view down to 10 or 20 degrees above the
> horizon. Your typical “fast setup” location usually does not do this. With
> an “impaired” antenna location, you can get dropouts based on the
> limited amount of sky you can get at.
>

I understand completely. Still, I have to come up with a reasonably-priced
source of time and it does seem like GPS is the right answer. I can live
with units going off-line periodically due to insufficient timing accuracy,
but I cannot have them drifting away from correct time.



> > Some of the units may have access to the Internet and I could run NTP on
> > them but the majority will be using GPS as the single stand-alone source
> of
> > UTC. The goal is for all the units to be sync'd to UTC as they need to
> > perform repetitive functions concurrently.
> >
> > The starting point for me was going to be the Ublox LEA-M8T because it is
> > current generation and Ublox offers a common footprint so that, if
> > production runs into the future, there will be newer modules that can be
> > dropped onto the same board. I want the second output to use as a 10MHz
> > reference. I have need for a 3e-7 accuracy 10MHz frequency reference in
> > some cases and that looked like it would do what is needed without going
> > full-on GPSDOCXO.
> >
> > So is my first choice of the Ublox LEA-M8T a good one or should I be
> > looking at other units. While I don't want to spend $50(us) on a
> receiver,
> > if it does the best job, it will fit the budget and that extra timing
> > output I can use to generate 10MHz is quite attractive.
>
> That output is likely to be very “dirty” as far as spurs and noise are
> concerned.
> If you have any significant spectral purity needs … yikes.
>

The signal will be squared and used as a clock for something else.
Duty-cycle is flexible. As long as the rising edge is clean and there isn't
too much jitter, I can live with it. I have no erroneous notion that I am
getting a clean sine wave or a symmetrical square wave. Also, this is in
the "nice to have" category and I can do away with it and use a GPS module
that only has 1pps.

So I am still hoping that someone will say, "The U-Blox LEA-M8T is a pretty
good choice but for what you are talking about, you might want to look at
the XYZ module as well."

-- 



Brian Lloyd
706 Flightline
Spring Branch, TX 78070
brian at lloyd.aero
+1.210.802-8FLY (1.210.802-8359)