[time-nuts] GPS module recommendation for Pi timing

[Bob kb8tq]

Hi

> On Mar 14, 2020, at 1:36 PM, Brian Lloyd <brian at lloyd.aero> wrote:
> 
> 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.

Only you can evaluate how important this or that aspect of the design is.
If *any* timing drift past 1 ms becomes an issue, some sort of TCXO based
GPSDO may be the real answer. 

With a bare crystal, you could easily have a 10’s of ppm sort of error. If 
you do, then any outage over about 100 seconds will hit your limit. 


> 
> 
> 
>>> 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.

The output will have jitter at the same level as the time pulse. If it’s a 20 ns
time pulse pre-sawtooth, then the 10 MHz will also hop by 20 ns.


> 
> 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."

There are maybe another two dozen modules out there that might work and
cost the same or less. Buying a few of this and that / trying them out is about 
the only way to know if they are “good enough” for all the details of your application. 

The bigger issue is - how much can you invest to dig through a pile of modules?
The simple answer is indeed to just pick one and accept the cost impact. 

Bob


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