[time-nuts] GPSDO+PC as a NTP server

[Adam Kumiszcza]

Hi!

I just bought my first GPSDO (BG7TBL dated 2019-08-03 with Trimble 73090
and u-blox 7 series GPS chip – more info here:
https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/msg2749310/#msg2749310).
I intend to connect it to a PC and use it as NTP and maybe PTP server, and
additionally share the 1PPS signal directly to additional computers in the
same room (max 2 more). I would like to know about best practices of doing
this project.

1. The GPS chip onboard can be set via RS232C, for instance by u-center
program. I currently set stationary mode, disabled SBAS, 3D fix mode only,
minimum elevation 20 degrees, position and time accuracy masks of 40
meters, C/No threshold 20 dbHz.  All of this filters out some satellites
and increases DOP, but my understanding is it also removes some errors and
makes the GPS more suitable for timing? Unfortunately, this chip can only
use only 1 constellation of satellites at once. The question here is then:
is it better to have less precise, high DOP fix or more precise fix that
can be absent from time to time? I guess the precision of OCXO disciplined
by GPS is an overkill anyway for my intended use.

2. I would like to get both NMEA and PPS signal from it on the NTP server.
Currently PPS is on pin 8 (CTS) but according to
http://doc.ntp.org/4.1.1/driver22.htm and
https://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html it should
rather be on pin 1 (DCD). Is it possible to use it anyway on Linux NTP
servers, or is it better to split it to 2 different COM ports – one for
NMEA, second for PPS? Or maybe is it better to use PPS from BNCs or get
some additional device and use 10MHz signal?

3. I'm thinking of using Linux as the NTP server with recompiled kernel
options for some better precision. But maybe there is a Linux/FreeBSD/other
system distribution prepared especially for NTP servers? I will probably
install additional Intel i210 network card for future PTP usage.
Do you have any other suggestions for the hardware/software of NTP server?

Thank you all in advance,
Adam Kumiszcza