Time on our Hands

What is a NTP Server

Molly is destined to wander off-grid. Which means she is not often connected to the internet. Normally this is not a problem, but our Cradlepoint likes to know the time, and it typically gets this time from the Internet, and more specifically, from a NTP (Network Time Protocol) server.

Our Solution

It is possible to get a fairly accurate time signal from GPS satellites. All that is needed is a GPS receiver and the ability to decode the signal and extract the time. It is made a little more complex, as the time has to be quite accurate for the whole thing to work. Ideally accurate to better than a millsecond.

The Garmin 18x LVC is the GPS receiver we choose. This little guy is installed in the ceiling void space affording it an excellent view of the sky. The GPS sentences are output in NEMA format, allowing the computer to decode the messages. Most importantly the Garmin 18x has a PPS (Pulse Per Second) output. This is the key ingredient allowing the system to get an accurate time. This output will pulse each second, with the rising edge of the pulse being timed to the very start of the second.

The Software

There is some open source software call gpsd which support our Garmin GPS device. More to come on this later.

Garmin 18X LVC
Wiring Diagram, from the Garmin manual with an addition.