[ntp:questions] Public NTP server as Falsetick while preferred clock is GPS time

Roland roland.heise at gmx.de
Tue Nov 25 08:40:08 UTC 2003

I am looking for some support to identify and resolve the issue of my
configuration. I built an NTP time server by using the GPS NMEA
protocol sentences from a Trimble SV-6 module and the 1PPS-signal of
the same box. Due to the transfer rate of 4,800 baud I stretched the
1PPS-signal from 1..5us to about 110ms by using a CMOS timer chip 555
to ensure that the signal can be converted to RS232 (using Maxim-IC
MAX232) and detected by the serial port (UART 16550A). The PC system
runs on a pentium III, 450 MHz with Linux kernel-2.4.21 and the
corresponding kernel patch of the PPSkit 2.1.2. The NTP version is
Beside the ref-clock of NMEA and PPS a third ref-clock is used from a
public NTP time server (ntp1.ptb.de). The relevant part of the
ntp.conf is as followed:
   server mode 2 prefer minpoll 4 maxpoll 4
   fudge stratum 0
   pps /dev/pps0 assert hardpps	# PPS device
   server minpoll 4 maxpoll 4
   fudge flag3 1	# enable PPS API
   server ntp1.ptb.de		# public NTP time server
 So far no issue with that. The query with ntptime shows no unexpected
results (at least for me):
linux-2.4.21-NANO:~# ntptime
ntp_gettime() returns code 0 (OK)
  time c36d87f4.332da110  Tue, Nov 25 2003  8:49:08.199, (.199915111),
  maximum error 802 us, estimated error 7 us, TAI offset -1073743976
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset -1.082 us, frequency 10.069 ppm, interval 256 s,
  maximum error 802 us, estimated error 7 us,
  time constant 4, precision 9.219 us, tolerance 496 ppm,
  pps frequency 10.069 ppm, stability 0.281 ppm, jitter 4.688 us,
  intervals 45, jitter exceeded 9, stability exceeded 2, errors 0.

The thing that worries me is the offset of about 117ms of the public
NTP time server I got through the ntpq -p command.

I would be happy if you could give me some advice what I need to do to
reduce the offset significantly. I would expect that the offset of the
public NTP time server is as low as the others.

I already changed in the ntp.conf the pps line by replacing ASSERT by
CLEAR and vice versa. However the effact to the offset time of the
public time server was close to zero.

linux-2.4.21-NANO:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset
+GPS_NMEA(0)     .GPS.            0 l    8   16  377    0.000   -0.016
oPPS(0)          .PPS.            0 l   14   16  377    0.000   -0.020
xntp1.ptb.de     .PTB.            1 u   59   64  377   77.193  117.255

linux-2.4.21-NANO:~# ntpq -c as
ind assID status  conf reach auth condition  last_event cnt
  1 53124  9414   yes   yes  none  candidat   reachable  1
  2 53125  9714   yes   yes  none  pps.peer   reachable  1
  3 53126  b114   yes   yes  none falsetick   reachable  1

linux-2.4.21-NANO:~# ntpq -c "rv 52126"
status=b114 reach, conf, sel_falsetick, 1 event, event_reach,
srcadr=ntp1.ptb.de, srcport=123, dstadr=, dstport=123,
leap=00, stratum=1, precision=-17, rootdelay=0.000,
rootdispersion=0.092, refid=PTB, reach=377, unreach=0, hmode=3,
hpoll=6, ppoll=6, flash=00 ok, keyid=0, ttl=0, offset=117.255,
delay=77.193, dispersion=2.343, jitter=0.470,
reftime=c36d8817.bc339000  Tue, Nov 25 2003  8:49:43.735,
org=c36d8818.6a45c000  Tue, Nov 25 2003  8:49:44.415,
rec=c36d8818.564a16a4  Tue, Nov 25 2003  8:49:44.337,
xmt=c36d8818.41d81f10  Tue, Nov 25 2003  8:49:44.257,
filtdelay=    79.70   82.05   77.19   78.37   80.68   79.74   80.03  
filtoffset=  117.91  117.76  117.26  117.53  118.06  117.56  117.03 
filtdisp=      0.02    0.98    1.95    2.90    3.86    4.85    5.84   

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