| Re: [chrony-users] Multi-PPS+RTC Setup Questions |
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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 |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/
On Tue, 29 Sep 2020, Hal Murray wrote:
So NMEA is one of the worst ways of getting accuracy (usually much worse than
internet ntp.) (PPS, network, NMEA, wristwatch is the order of accuracy.)
Some networks are really crappy - see bufferbloat. I have an old DSL line.
Round trip times are often over a second when I'm downloading something.
I would not call DSL "network" but yes, if the network becomes overloaded then
the packets cannot get through. Bufferbloat is in the microsecond range, not
millisecond.
Some NMEA is really crappy. SiRF is the one that comes to mine.
http://users.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
NMEA has the problem that it is transmitted by an extremely slow method and an
NMEA sentence has variable length.
There is nothing fundamental about timing on a serial port being crappy. If
the line is idle and the software gives it some data to send, I'd expect the
data to start within a bit time.
9600BPS = 100us per character = 2-5ms/sentence, and most of the NMEA readers
time it from the end of the sentence, not the beginning. NMEA is horrible
according to all the tests I did about 5-10 years ago. But PPS should be at
the 1us level for reaction to interrupt. There is no way that pps from any gps
chipset should be out by 30us and variable at that level.
Does anybody have data on any UBlox chips? NMEA may be different than their
binary mode.
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