One of the (minor) drawbacks of the Raspberry Pi is the lack of a hardware clock. Normally, you'd work around this by configuring a good pool of NTP servers to connect to. What do you do though, if you can't guarantee there will be an Internet connection available when needed?
The solution is obvious, so obvious that many have already done it - use the time provided by a cheap GPS dongle. The gpsd daemon helpfully pushes the time to Shared Memory Segments (SHM) so it's a simple adjustment to the NTP configuration file to have NTPD pull the time from the dongle.
Except, it seems on Raspbian, it isn't quite so simple. You've followed all the instructions (simple as they are) but are still seeing an entry like this
# ntpq -p
remote refid st t when poll reach delay offset jitter
SHM(0) .GPS. 0 l - 16 0 0.000 0.000 0.001
No matter what you try, reach stays at 0.
Frustrating, and there's very little to give you a guide. This post will tell you what the issue is, as well as how to go about finding it should it re-occur