No. Besides I do not think a RPi has an RTC.

Also, you can use the NMEA on the gps to give the seconds. PPS and NMEA are
two separate reference clocks. NMEA gives the labeling of the seconds, but is
a horrible time source (good to about 100ms is you are careful). PPS is a
wonderful time source (good to a few nanoseconds or microseconds on most PCs)
but has no idea of the labeling of the seconds. Thus if you run both NMEA and
PPS, chrony can get the seconds from NMEA and the accuracy from PPS. (I
thought that was what you were doing anyway).

But NMEA is not good for .01 and even 1 second can be dicey (GPS receivers
issue NMEA sentences when all of the rest of the work is done) and if you ask
the receiver to send more than one NMEA sentence that makes the accuracy even
worse. But NMEA is usually good enough to deliver the time to within about .2
sec, and that is good enough for chrony to use to label the seconds from PPS.

If you use gpsd, it will label the seconds for you, but it adds an additional
layer between the GPS transmitter and chrony, which can degrade the accuracy
of the PPS.



William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
UBC, Vancouver,BC _|_ Program in Cosmology |____ un...@physics.ubc.ca
Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/

On Tue, 9 May 2017, Deven Hickingbotham wrote:

On 5/9/2017 12:52 PM, Bill Unruh wrote:
Note that the main problem with pps is that it tells chrony exactly when
the
transition at the top of the second takes place, it does not tell it which
second that was.

So for PPS to work, there has to be a second time source to identify the second. What happens if you start up the system without an internet connection (a very real possibility in my case)? Would chrony use the RTC to figure out the second?

Deven

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