[time-nuts] GPS module recommendation for Pi timing

[jimlux]

On 3/13/20 8:50 AM, Tim Shoppa wrote:
> If your product depends on monotonic times, there are a lot of good reasons
> for using NTPD. It will slew the clock rather than jump it which can be
> hugely important for many applications if there is some need for data to be
> accumulated with monotonic timestamps. It will also apply good drift
> correction for times when sky and satellites might not be visible (which in
> terms of real world products is very common.
> 
> You prominently mention both milliseconds and TOD RTC. Note that all common
> RTC chips (e.g. DS1307) only directly read out time to the previous second.
> They do not read out directly for milliseconds although you can try to
> interpolate based on seconds edge.
> 
> Tim N3QE
> 
> 
> On Fri, Mar 13, 2020 at 10:07 AM Brian Lloyd <brian at lloyd.aero> wrote:
> 
>> I have an application where I need to synchronize the internal TOD RTC in a
>> raspberry pi and need to pick a GPS module. We are building our own
>> hardware but still using the Pi so interconnection will be via GPIO/serial.
>> We won't try to use USB.
>>
>> This is not an NTP application. These units will be in the field and will
>> most likely not have Internet access. I need their clocks to be pretty
>> close. I am shooting for 1ms ... if possible.
>>

I've been working on a similar project - using a battery powered 
beaglebone and a GPS with an RTL-SDR to build an interferometer.

I was able to get a NEO-7 hooked up, chrony installed and running, etc. 
on 4 nodes.   Attached are screen shots running pdsh to see all 4 nodes

what I have not been able to do is figure out how to sync the RTL-SDR 
(or, at least, get a 1pps hack into the data stream) - I might just go 
to a fancier RF interface like a Lime mini.




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