Perhaps it is time that I step in to point the newer members of Time-Nuts to some of the historical (hysterical?) info on using GPS to achieve sub-microsecond global timing sync. I refer you all to the website http://gpstime.com that Rick Hambly (http://cnssys.com) maintains to document some of our timing exploits over the past ~25 years starting with the early Motorola PVT-6 "SIXPACK" and proceeding thru the later ONCORE, M12 and UBLOX receivers.
By explanation, I was one of the principal researchers that developed the technique of Very Long Baseline Interferometry (VLBI) for high accuracy Geodesy and Astrometry. In VLBI we record raw noise from a wide RF bandwidth at a network of radio telescopes around the world. Most of the noise picked up at the stations is from the microwave receivers, the earth's atmosphere & the 2.7 degree remnant background noise from the big bang -- the system noise resulting from all these sources at each station is independent and uncorrelated with noise at all the other telescopes.
But a small fraction of the noise originated at quasars which are (nearly) point sources with an angular sizes measured in fractions of milliseconds of arc (1 milliarcsec is about the size of George's head on a quarter held over over San Francisco as seen from Boston); the noise from these compact sources is correlated at the stations separated thousands of km and serve as reference points for the celestial reference frame. The telescopes define fixed points on the earth which constitute the terrestrial reference frame.
To extract the correlated signal VLBI each station uses an independent Hydrogen Maser clock as a time and frequency reference. The independent clocks need to be synchronized at nsec levels and to hold this synchronization for time scales up to several days. I realized that, if each station used a GPS-based timing system operating 24/7/365, we could solve the time/frequency sync needed to measure the "real-time" motions of the earth as the continents drift and the rotation of the earth and the sub-milliarcsec astrometric position and structure of the quasars.
FYI -- After 3+ decades of such measurements, we now know the few cm/yr (about the same speed as your fingernails grow) of relative motions of the continents (a.k.a. "continental drift") to uncertainties of tens of microns/yr. The celestial reference frame as defined by the positions ~1000 extragalactic radio sources is known to ~10 microarcseconds. You can see get a feeling for these results and the network of stations that produced them by browsing http://ivscc.gsfc.nasa.gov .
As you browse the material on http://gpstime.com you will see snapshots of our efforts at delivering low-cost GPS timing system intended for the VLBI station operators ("telescope drivers") updated every couple of years. Also you will find presentations we gave at several Precise Time and Time Interval (PTTI) meetings.
To answer a couple of the recent questions in more detail, take a look at the 2006 "Low Cost" PTTI paper :
1. For all the ONCORE and UBLOX receivers: The "sawtooth correction" is in the binary data message for the NEXT second. The receivers have an counter register that is updated based on the navigation firmware (typically a Kalman filter) to the integer number of clock counts corresponding to the next 1 PPS epoch. The sawtooth correction is the fractional part of the epoch that is left over after the integer clock count is set into the hardware. 2. In Rick's CNSCLOCK, a programmable delay line is fed from the fractional counter error (the "sawtooth correction" plus a small constant "DC" to center the correction in the delay line). The "DC" bias is treated as if it were a cable length correction. 3. My email address has changed from verizon.net: Now it is mailto:tom.k...@gmail.com 73 de Tom K3IO --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
