Next, I note a couple of problems I had with the first GPS receiver I tried, and how I cured those with a different GPS receiver to produce a stratum-1 NTP server consuming about 4 watts.The easiest approach with good performance is described here.
Note that the Adafruit GPS Hat uses GPIO 4, physical pin 7, so you would need to change the commands given in this document.
Later additions have included remote monitoring of the NTP server performance, and more general monitoring of the Raspberry Pi using the standard SNMP functions, with an additional CPU temperature monitoring add-on. Note that good performance is dependant on the GPS unit having a clear view of the sky, particularly the southern part of the sky if you are in the northern hemisphere.
There are some helpful Linux commands scattered throughout this page.
These notes are almost as much for my own records for the next time I need to visit this project, but I hope they may be helpful to others.
If you want to get started quickly, with the best results for minimum fuss, please see my Raspberry Pi NTP quick-starter page.
Please also see that page for issues with the Jessie release of Linux, and with the newer Raspberry Pi model 3.