[ntp:questions] Discipline performance comparison of SHM(1) and Atom(PPS)

f.hiesinger f.hiesinger at studium.fau.de
Thu Jun 11 12:18:09 UTC 2015

 Hi Folks,
 I wonder, if a SHM(1) disciplined NTPd can reach the same accuracy than 
 one with the ATOM(PPS) Driver.
 [1] says, that SHM(1) is used for "PPS from GPS". SHM(0) is sourced 
 from NMEA-Messages.

 I'm currently parsing very precise timestamps via SHM(1) to NTP 
 (Operation mode=0). Every timestamp in SHM(1) is derived from a 
 pps-source and tweaked individually. Up to here everything works fine, 
 including Nanosecond resolution. See ntpshmmon output:

 ntpshmmon version 1
        Name   Seen@                Clock                Real            
   L Prec
 sample NTP0 1434023966.919852901 1434023966.105478000 
 1434023966.000000000 0  -1
 sample NTP1 1434023967.094973227 1434023966.999997298 
 1434023967.000000000 0 -20
 sample NTP0 1434023967.920931333 1434023967.099366000 
 1434023967.000000000 0  -1
 sample NTP1 1434023968.096867651 1434023967.999997890 
 1434023968.000000000 0 -20

 Unclear is, whether NTP accounts SHM(1)-timestamps with the same weight 
 as ATOM(PPS)-timestamps. Any experiences with this?

 BR, fran

 [1] www.support.ntp.org/bin/view/Support/ConfiguringSHMRefclocks

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