Since starting this page there have been two developments which make the process somewhat easier - a program has been developed which allows the use of an unmodified operating system by working in user-mode rather than kernel-mode PPS, and a module is now available which plugs directly onto the 26-pin GPIO header of the Raspberry Pi, so no soldering is involved.My thanks to Folkert van Heusden and Anthony Stirk for these developments.Note that the Adafruit GPS Hat uses GPIO 4, physical pin 7, so you would need to change the commands given in this document.
As an experiment, I purchased one of the low-cost credit-card-size Raspberry Pi computers, and have configured it to run NTP (Network Time Protocol).
I have also used this board with a GPS receiver with pulse per second (PPS) output to make a stratum-1 NTP server, but as I know little of Linux, it has taken some time to achieve this aim!
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.
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.