Building a Stratum-1 GPS Based NTP Server with a Soekris net4501

Hardware & Software Used:

At the heart of this home-built NTP server is a Soekris net4501. The net4501 is an extremely popular and affordable embedded / single board computer (SBC) based on the AMD ElanSC520 CPU. While only a 486 class chip running at 133 MHz, it provides plenty of power to function as a NTP server (for my project), and could even double-duty as a firewall, router, VPN, or whatever common network needs one may have. I don't recall the actual maximum network throughput (which will vary based on application and configuration) but I believe it was somewhere in the neighborhood of 15 Mb/s. Power consumption for this little machine is probably around 10W max (5W average I think I read).

While looking at the little box above one may think... why on earth would he choose that? Reliability is one huge factor, but I had several reasons for choosing this particular piece of hardware. Common points of failure in PCs are the components that must move / rotate, which are fans (that keep the system cool) and hard drives (obviously storing your OS and other data). The net4501 requires neither, the CPU runs cool enough without any active or passive cooling, and the operating system is stored on a compact flash drive. While flash memory does have a limited amount of write cycles (even though that number is always growing as technology improves), the read cycles for all practical purposes is limitless. Once the OS is compiled and written to the CF card, there is little need to write to it again (but it is possible if necessary).

Another great thing about the net4501 is that the Elan SC520 CPU has internal time registers with a resolution of about ~125 nanoseconds (according to others) and can be accessed with a few simple hardware modifications and proper kernel settings in FreeBSD. However, to fully benefit from this level of precision would require replacing the stock clock crystal with a signal from a more stable source. That is something I did not do (yet), however John Ackerman and Poul-Henning Kamp have if you want to learn more. My goal for this little box was a little different from theirs, for my NTP server I was wanting something completely self-contained and portable.

To feed time to the net4501 I chose an old Motorola (now i-Lotus develops all new models) Oncore UT+ GPS Timing receiver. In the world of GPS receivers, models are typically tailored around "navigation" or "timing". A "timing" receiver is meant to be used in a fixed location and outputs a very precise "pulse" every second (PPS) aligned to UTC (Coordinated Universal Time). Even the cheapest receivers that output a PPS will spec the accuracy to within 1µs (one microsecond) of UTC, while more expensive models will be in the low ns (nanosecond) range! The pulse is typically fed to a PC's serial or parallel port, and software (like NTP) can more accurately set the computer's time via this direct method than if it was trying to sync over an Ethernet network.

I used NanoBSD (a modified minimal build of FreeBSD designed to be run from a Compact Flash card) for the operating system. Building a NanoBSD image requires running a box with FreeBSD installed and configuring a lot of custom options that are not well documented (Google was my friend on this one). Through a lot of trial and error (and many days) I finally managed to build an image that I was happy with.

Typical GPS based NTP server appliances sell for hundreds, even thousands of dollars. Most have lots of features that justify that cost, such as an ovenized crystal oscillator (or even a rubidium standard), additional frequency outputs, custom web interface, commercial support, etc. Building your own basic NTP server (like in this article) can be done on the cheap for well under $200, and for all practical purposes gives just as good time over a network.