[time-nuts] GPS for ntp

[Simon Marsh]

Running the BBB as an NTP server is a breeze and has a couple of 
advantages over the Pi. Specifically, on the BBB, the kernel module is 
pre-built and configuring the PPS driver is done at runtime using the 
device tree. No kernel re-compilation is required to get up and running, 
just plug and go on any of the available GPIO. The ethernet interface on 
the BBB is also a SoC peripheral so gives much better latency than the 
USB ethernet on the Pi. This gives a big improvement if you are using it 
to serve time for other NTP clients.

I have one of the Reyax u-blox 8 modules and it works as described, 
_but_ be aware that they do not have an external antenna connector. I 
found the reception indoors was poor to the point of making it 
unuseable, so depending on your circumstances, this could be a major 
problem.

E-bay is riddled with GPS modules and pretty much any of them will be 
fine for NTP use. For a more off the wall suggestion, I currently have a 
BBB hooked up to one of these:

http://www.ebay.com/itm/Mini-U-blox-B39-PCI-5S-1-500-PCI-E-Express-Wireless-Card-GPS-Module-for-MID-Lapt-/201172829176?pt=Radio_Control_Parts_Accessories&hash=item2ed6d5c3f8

A little bit of soldering is required (see here: 
http://emerythacks.blogspot.co.uk/2013/01/u-blox-pci-5s-cheap-gps-module-for-your.html), 
but what you get is a dirt cheap u-blox 5 module powered at 3.3v 
directly by the BBB. It's not cutting edge, and I'm not going to suggest 
using one of these to discipline your frequency standard, but for simple 
NTP use it works a treat.

No claims are any good without being backed by data, so attached are a 
couple of plots showing my NTP performance from the past 4 hours. 
'Sheliak' is the BBB with the PCI-5S and 'Albali' is a Pi with an 
adafruit ultimate GPS breakout (MTK3339). It's easy to see the heating 
coming on at ~6:20, as both servers are out in the open and it's 
interesting to see how they both respond to the temperature change. The 
scale is in milliseconds, so 5m is showing 5us.

Cheers



Simon

> What does everyone think of this GPS module for ntp use? According to the
> specsheet, it uses a Ublox Neo-7N.
>
> http://www.ebay.com/itm/RY725AI-10Hz-UART-USB-interface-GPS-Glonass-QZSS-antenna-module-flash-memory-/181562403752 
>
>
> I'm thinking about using it for a Beaglebone ntp server. I know there was
> some discussion about Beaglebones a while back. Has anyone gotten ntpd 
> with
> PPS and time stamping running on a Beaglebone yet (like on the Soekris
> Net4501)?
> []
> I have one Beaglebone White that I got cheap, so I have something to get
> started with. Later, as I figure out things, I'll buy a few Beaglebone
> Blacks.
>
> Joe Gray
> W5JG
> ============================================
>
> Joe,
>
> You might also consider the Raspberry Pi as an NTP server using PPS:
>
>  http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
>
> You can get the ublox 7Q in a ready-to-go no=-soldering board here:
>
>  http://ava.upuaut.net/store/index.php?route=product/product&path=59_60&product_id=95 
>
>
> I would reckon on sub-100 microsecond timing accuracy with the 
> Raspberry Pi and kernel-mode PPS, but I've not done any comparison 
> with the BB.
>
> Cheers,
> David

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