[time-nuts] GPS module recommendation for Pi timing

[Bob kb8tq]

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. 

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. 

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. 

“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. 

> 
> 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.

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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