[time-nuts] Small time server for mobile use.

Hal Murray hmurray at megapathdsl.net
Wed May 13 00:53:25 EDT 2015


mark at alignedsolutions.com said:
> Has anyone had practical experience with small commercially available time
> servers / ntp servers suitable for mobile  use in a vehicle.   

>  The use case is I am in need of an accurate (ie.  within 100 ms) time
> source for several pc's in moving vehicle.    Being able to run directly off
> a 13.8 or 28 VDC  source would be a major plus but AC power is also
> available. 

> Hold over if there are gaps in GPS coverage is also a major plus.

I don't have any experience with commercial units.  (except looking at a few 
price tags)

My straw man would be a Raspberry Pi.  Adafruit sells a GPS card.  (and all 
the other Raspberry Pi stuff you would need)
  http://www.adafruit.com/product/2324
Total parts cost would be under $200.

The GPS card comes with a patch antenna which works inside my house.  If 
that's not good enough, it has a connector so you can attach an external 
antenna.

The Raspberry Pi needs 5V.  The power connector is a micro-USB.  If you have 
PCs, you can probably steal power from one of them.

It's not ready-to-go.  You would need somebody familiar with Linux and ntp, 
or willing to learn.  That's both to set it up and for maintenance.  If David 
Taylor's web pages aren't good enough, I'll be glad to help.

----------

100 ms is pretty easy for ntp.

How long do you want it to run?  Is this for a ship which will be out for 
weeks, or a truck that goes out collecting data for an afternoon?

For a short trip, it might make sense to run in holdover for the whole trip.  
An important thing that ntpd does is correct for the "drift" of the local 
clock.  At 1 PPM error, it takes a whole day to drift 100 ms.  It's easy for 
ntpd to get better than 1 PPM, but the target moves if the temperature 
changes.


mark at alignedsolutions.com said:
> Yes I am aware I could feed a 1 pps signal into a laptop and use that as a
> time server and I may end up going that route.

Is there any serial data to go with the PPS?

> There is a small Ethernet LAN in the vehicle.  The pc's currently get their
> time via a wireless connection to various NTP servers.   I need to be able
> to ensure accurate time on the PC's if there is no wireless coverage. 

Why not run ntpd on one of the PCs and set the others up to get time from it?

Do you need time to agree with UTC or the PCs to agree with each other?


-- 
These are my opinions.  I hate spam.





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