开云体育

Having recently set up a RPi/GPS for the purpose of making a time server I learned that getting really accurate results requires more than just marrying a GPS to the RPi. For the record I wasn't after super accurate time, I only wanted to go through the exercise of seeing what I could end up with. I learned along the way that ...
1. the majority of USB GPS's are not capable of accurate time because they don't provide PPS. There are few exceptions but most convert RS232 to USB in the GPS and don't provide PPS
2. PPS is an absolute requirement for accurate time
3. Modern Linux kernels have "kernel pps" to minimise latency. Older kernels could be patched.
4. Navigation GPS units are not necessarily providers of really accurate time, timing grade GPS's are made for that purpose

My time server uses a uBlox LEA4 GPS which is not a timing standard unit, the uBlox MAX8 is MUCH better in that regard. My server is located in a steel clad shed with poor thermal stability and ntpstat currently reports 5mS accuracy, better than I need for my LAN. The RPi (model 2) is fitted with a DS2321 RTC, set up as per instructions on this site


There are numerous places on the web to gather information about accurate time, hardware requirements, and instructional howtos. Many utilise the RPi.






The third link above contains a link to suitable (and some unsuitable) GPS units. I followed the instructions on that site when setting up my RPi. To me, it seemed the most complete instructions ... and it worked.

Those links will answer questions you have asked, and questions you probably haven't thought about (;->)

HTH
Ray vk2tv