[time-nuts] GPSDO Time Server

[Pluess, Tobias]

Hi all,
it has been quite a while ago since I worked on my GPSDO.
I plan now to extend it a bit and I wanted to connect it to my PC to serve
as precision time source, not only precision frequency source.
I figured out that ntpd on Linux can accept 1PPS input signals as well as
NMEA sentences that tell it the current time, so I would like to try this.
However the actual software implementation is not so easy as it sounds at
first.

I thought I will add a function that is called once per second and that
increments the seconds, minutes, days and so on. Since my GPSDO does not
contain a realtime clock, I need to start with some arbitrary date.
As soon as the GPS module knows the time and date, I can adjust the
"internal" time and date. However, how should I deal with leap seconds and
leap years, and how shall I proceed when the GPS module loses the fix (e.g.
antenna disconnected or something)?

is it better to have internally something like an unix time stamp that
increments each second and is then converted to a date and time, or is it
better to maintain the actual time and date in the software?

As far as I understand, the communication with a PC running ntpd works by
sending out the NMEA sentence GPRMC which contains the date and time, which
is valid when the next 1PPS pulse arrives. Is that correct?

In the long run, I wanted to add a Raspberry Pi compute module to my GPSDO,
which handles the ethernet and NTP stuff, while my microcontroller controls
the OCXO and so on. The microcontroller could be interfaced to the
raspberry such that it tells NTP the correct date and time. This would make
a nice NTP time server, plus the raspberry could collect other useful data
from the GPSDO and display it on a small web server.

I know the raspberry pi can be interfaced to a GPS module, but I have a
slightly different situation: my GPS module is connected to the
microcontroller, which in turn generates the precise 1PPS pulses which are
derived from the OCXO. Therefore, I thought I could use the microcontroller
to "fake" a GPS module, outputting the required NMEA strings, and use the
more precise 1PPS pulses derived from the OCXO, instead of simply routing
the GPS module to the raspberry.

Any hints?
best
Tobias