[time-nuts] Relationship of relative stability between distant locations using GPS and environmental factors

Tom Clark, K3IO K3IO at verizon.net
Sun Aug 13 20:17:38 UTC 2006


   Way back on June 30th, Stephan asked:

A number of recent entries to this list have mentioned topics relating to
GPS timing and environmental corrupting factors (e.g. Ionosphere, Temp.,
Humidity, etc.). Personally, I am very interested in setting up a very
precise relative time between locations (maybe 100s of meters to 10s of
kilometres apart) on time scales of (maybe 100s of seconds to 10s of
minutes). I noted some members referred to dual frequency receivers for
overcoming these effects. Can anyone point me to some literature, articles
or links to overcome these environmental factors?

   Sorry to be slow in responding. I'm just getting to the July pile of
   Email!
   You might find some tests that Rick Hambly (see [1]http://cnssys.com)
   and I did on a 22 km baseline across Washington, DC. See the paper
   "Critical Evaluation of the Motorola M12+ GPS Timing Receiver vs. the
   Master Clock at the United States Naval Observatory, Washington DC."
   at [2]http://www.gpstime.com/. You will see (in Fig.7) that we
   obtained ~2 nsec RMS syntonization for 6+ days despite a ~50 nsec
   ionospheric disturbance that happened during a major storm & aurora
   event.
   In the paper you will also see that, at least for the 4 receivers
   tested at USNO, the different receivers showed ~5 nsec "absolute"
   agreement without calibration. Of course, I would recommend that you
   run a zero-baseline calibration verification on the different
   receivers if the "absolute" offset is important.
   You also asked about the use of simple, single frequency receivers
   instead of professional, dual-frequency surveying instruments. The
   basic answer is cost and simplicity. The board-level single frequency
   receivers cost a few hundred dollars, while the professional
   instruments are in the $5-10,000 range. The single frequency
   instruments produce 1PPS pulse outputs that can be used "now" while
   the professional instruments require post-processing of the data {the
   1PPS "now" outputs from the professional instruments are AFAIK derived
   from the single frequency L1 data}.
   You might find the other papers [especially "Low-cost, High Accuracy
   GPS Timing" and "Timing for VLBI (2005)"] to be of interest to see how
   we have used the board-level (especially Motorola) receivers. Recently
   Motorola has dropped the production of their ONCORE receivers; Rick
   and I are in the process of examining the successor iLotus M12M Timing
   receiver (see
   [3]http://www.synergy-gps.com/images/stories/pdf/m12m%20timing%20preli
   m%20v11.pdf) and plan to report on the tests at this December's PTTI
   meeting.
   73, Tom

References

   1. http://cnssys.com/
   2. http://www.gpstime.com/
   3. http://www.synergy-gps.com/images/stories/pdf/m12m%20timing%20prelim%20v11.pdf



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