It all started with an article in "Linux Format LX282" where there was a tutorial on how to setup a Stratum 0 Reference, "Setting atomic-clock precise time on your Pi". The article describes a way to get accurate time on a Raspberry Pi. Unfortunately the chosen solution, using PPS-client doesn't provide server capabilities. For this to work either a ntpd, ntpsec or chrony server is needed. But interestingly enough all these different solution uses the same GPS with PPS-capabilities, namely a "Adafruit Ultimate GPS HAT for Raspberry Pi".
There are many different problem popping up which are not always documented in the available descriptions on the net. Below is a list of articles where I have found useful information:
Also the manual for the different servers contain a lot of useful information.
For the GPS I have used the gpsd server.
The Raspberry Pi I am using is a RP4, mostly because that is what I had available. After a lot of testing I figured out that the easiest and best working SW for my configuration was the chrony server.