[time-nuts] A couple of answers to recent questions

Tom Clark w3iwi at toad.net
Tue Aug 9 16:33:12 EDT 2005

     Relative timing like that should certainly be doable using the
     right equipment
     and right setup. People that do this uses Geodesic receivers such
     Ashtech Z12 and a good antenna.
     With the relative proximity of only a few km away, the common view
     aspect will
     correlate quite well.
     Using terms like "common view" and "Ashtech Z12" you will Google
     yourself to
     alot of usefull information. You should harvest the archives of
     NIST, PTTI etc.
     for good articles on the topic.

   I didn't suggest the use of high quality, dual frequency, expensive
   receivers (like the Z12) because Shaun made the statement that his
   area of interest was only a few km in scale. Over that size network,
   nearly all of the errors due to the ionosphere (which forces the need
   for two-frequencies), troposphere & satellite orbit errors cancel out
   as a common mode source (i.e. the stations see identical errors). Only
   differential multipath, cable stability and mechanical integrity of
   the antenna are of much concern. Note that I also made the presumption
   that what he really needs is RELATIVE timing. BTW, I know of several
   other examples of the use of GPS to achieve "network" timing at <10
   nsec over scales of tens of km.

Hi All, 
The otherday I asked about the Jupoiter T receivers and Brooke mentioned the re
sults of his experience with it, I was curious what he meant when he spoke of t
he "clock zero-beat times (TvB calls them "hanging bridges")" .Brooke, can you 
or Tom direct me to somewhere where I might get a better understanding of this?
Thanks; Rich


   Take a look at figures 9 & 10 in
   [1]ftp://ftp.cnssys.com/pub/ION-GPS2000/ion-time.pdf and you will see
   examples of the receiver clock going thru zero-beat.
   For the ONCORE & M12+ receivers, the explanation for the zero-beat,
   hanging bridge phenomena (as well as the underlying sawtooth error) is
   seen in [2]ftp://ftp.cnssys.com/pub/ivs-tow-2005/tow-time2005.ppt in
   slides 25-27.
   For the Jupiter-T, the effect is the same but the reason is slightly
   different. The Collins/Rockwell/Conexxant chipset generates its 1PPS
   signal from a programmable numerically controlled oscillator (NCO)
   programmed with the differential frequency error for the next pulse
   (instead of the long counter for the ONCORE where the integral time
   error is controlled). The manufacturer does not keep track of the
   differential "seed" (or equivalently the constant of integration) for
   the NCO. The result is that the Jupiter-T does not have a serial data
   output with the pulse error as does the ONCORE. Aside from that
   difference, the Jupiter-T is as good a timing receiver as any of the
   Motorola products.
   Tom Clark


   1. ftp://ftp.cnssys.com/pub/ION-GPS2000/ion-time.pdf
   2. ftp://ftp.cnssys.com/pub/ivs-tow-2005/tow-time2005.ppt

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