[time-nuts] Testing frequency using NTP Bruce GPS ps

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 4 06:34:41 UTC 2008

Mike Monett wrote:
>   Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>   >  Mike
>   > They actually  use an augmented form of GPS common view  for which
>   > the GPS PPS signal and its timing variations are largely common to
>   > both locations  and   thus   largely   cancel  when  comparing the
>   > frequencies at the customer site with the standards at NIST.
>   > With a  good timing receiver (and antenna  location)  the sawtooth
>   > corrected PPS signal timing noise can be as low as a few nanosec.
>   Is that  like the CNSC02-O1 High Performance PPS system?  From their
>   web site, they state:
>   "Provides for  dynamic hardware correction of the  1PPS quantization
>   ("sawtooth") error.  This reduces the noise on the 1PPS  pulses from
>   (typically) +/-27  nsec  (15 nsec 1-sigma) to +/-  11  nsec  (3 nsec
>   1-sigma). More importantly, eliminates periods of bias  error caused
>   by the quanization error going through a "zero beat" period that can
>   last one to two minutes about every 10 minutes or so."
>   http://www.cnssys.com/cnsclock/CNSClockII.html
>   That paragraph  is  very  interesting.   The  "zero  beat"  error is
>   particularly nasty.  I tried to get more information, but  it wasn't
>   really clear  how their correction method fixes this  problem. Could
>   you explain it a bit more?
The GPS receiver can only position the leading edge of the PPS pilse to 
the nearest transition of the receiver crystal oscillator or harmonic 
However it can estimate the resultant timing error with a resolution of 
1ns (for the M12 GPS timing receiver and its derivatives).
The resultant sawtooth error can either be corrected in hardware as in 
the CNSClockII or it can be corrected in software (if the PPS transition 
is time stamped with sufficient precision).
The NIST system has a time stamp resolution of 30ps or so software 
correction for sawtooth error is possible.
After sawtooth correction their is still some residual, largely random, 
noise of a few ns rms with a well sited antenna.
>   I am working on a new technique that might give one or two orders of
>   magnitude improvement  locking  to the 1PPS  signal.  It  might give
>   sub-nanosecond locking in a very low cost system. But the  zero beat
>   is a  serious  problem,  so  I am interested  to  learn  as  much as
>   possible how  other  solutions work. Maybe the simplest  is  to just
>   apply some jitter to the GPS crystal oscillator to keep it  off zero
>   beat.
>   > The "all in view" technique will reduce the noise  contribution to
>   > the comparison somewhat.
>   That term is a bit confusing. Does it mean using all satellites that
>   are in  view, or does it mean the NIST receivers see  the  same ones
>   that the user sees?
Common view means that the 2 GPS timing receivers use the same GPS SV 
for timing purposes.
Observing schedules have to be agree upon before hand.
This technique originated with early receivers that could only track one 
SV at a time.

All in view means that each GPS receiver use all SVs that it can see to 
derive its timing.
With modern timing receivers sufficient information can be logged so 
that the timing info from each individual SV can be extracted from the 
PPS time stamp sequence if desired.

>   > The timestamp  resolution of better than 30ps or  so  ensures that
>   > time stamp quantisation noise is negligible.
>   > It also  allows, in principle at least, standalone 3  cornered hat
>   > comparisons of  the  frequency   instabilities  of  the  3 sources
>   > connected to the customer instrument.
>   Another term I must study and learn how to do: "3 cornered  hat". It
>   sounds like  a very powerful technique, ideally suited for  a fairly
>   simple program and some data logging.
It is if and only if the  phase variations of 3 (or more ) oscillators 
are statistically independent.
It is then possible to derive the individual phase variances of each 
oscillator from the the 3 (or 0.5*N*(N-1) for N oscillators) sets of 
phase difference measurements between pairs of oscillators.
In practice all oscillators may share the same ambient thermal 
environment so that their finite oven gains will produce small cross 
variance terms.
Even humidity and atmospheric pressure variations may affect the 
frequency of an OCXO.
>   > For more detail see:
>   >  http://tf.nist.gov/timefreq/service/fms.htm
>   Thanks very  much  for the link. It is curious  they  don't  seem to
>   spend much  effort on correcting the user's  frequency  errors. They
>   just want to report how much they are off.
>   Why is that? You'd think they would perform a more  valuable service
>   by applying  advanced techniques to adjust the  user's  equipment to
>   minimize the  error, then report and certify the  actual  result. Is
>   there some  reason   they   want   to   leave  the  user's equipment
>   free-running?
>   > Only carrier  phase  GPS  techniques  are  potentially  capable of
>   > picosecond noise levels.
>   Now we are getting very interesting. How do you do that?
Most GPS receivers track the carrier phase variations internally.
Some make it available to the user (eg. Novatel Superstar GPS receivers, 
Rockwell/Navman Jupiter GPS receivers ).
However since these refer to the phase variations between the GPS 
carrier and the receiver LO, they are usually only useful if the 
receiver LO is phase locked to the OCXO output frequency.

>   > However there are a large number of effects that have to  be taken
>   > into account and data reduction and correction is very complex.
>   What if  we just want to stabilize an oscillator  frequency, without
>   caring much  what the exact phase offset is from the USNO?  The high
>   carrier frequency  should make it much easier to lock a  rubidium or
>   crystal oscillator, and it should give much lower phase noise.
There are commercial carrier phase disciplined oscillators available (eg 
from Quartzlock) if you can afford them.
One of these purports to have better short to medium term stability than 
a passive hydrogen maser.
>   Of course, auroras and other disturbances would be more significant,
>   but here  I'm mainly interested in getting a good  lab  reference to
>   measure and compare the performance of other commercial oscillators.
>   This is  a  separate  topic, but as long  as  we  are  talking about
>   precision signals,  do you happen to know what kind  of distribution
>   amplifiers are used at the USNO to distribute the signals from their
>   cesium and hydrogen standards?
>   www.usno.navy.mil/
No idea, as they don't say but the Spectradynamics amplifiers are used 
by national standards labs in several countries.
>   >  Bruce
>   Thanks very  much  for  your  help. It  might  seem  like  a  lot of
>   questions, but  I'm  pretty sure you will be very  pleased  with the
>   results. I think I can beat just about all the other  methods except
>   carrier phase techniques.
>   Best Regards,
>   Mike Monett

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