[time-nuts] Cable length calibration
kb8tq at n1k.org
Fri Jul 1 07:34:58 EDT 2016
> On Jul 1, 2016, at 3:40 AM, Mike Cook <michael.cook at sfr.fr> wrote:
>> Le 29 juin 2016 à 22:18, Poul-Henning Kamp <phk at phk.freebsd.dk> a écrit :
>> In message <20160629192850.19C2940605C at ip-64-139-1-69.sjc.megapath.net>, Hal Mu
>> rray writes:
>>>> At one point they were looking into making a GPS time receiver where the
>>>> cable length calibration would be built-in.
>>> How would you do that?
>> TDR ?
>> If it wasn't behind a choke, the inrush current to the antenna
>> preamp power filtering capacitor could be measured, but the choke
>> ruins that.
>> The trouble is how to do it without frying the antenna preamp...
>> GPS antennas and receivers are cheap, I would just use two GPS antennas
>> with a known difference in cable-length.
> Sounds simple, but even after a days reflection I don’t see how you find the complete path delay. You would get the cable delay (OPs concern) provided they were the same antenna/cable type combinations, but not delay induced by the antenna electronics. From another post that delay seems to be non-negligable. I find it curious that antenna manafacturers don’t seem to give this parameter. I looked at some datasheets on Trimble and Leica sites but they don’t have it.
> As for me, I measure cable delay by injecting a 1PPS into it through a T and for the RG174 attached to the patch antennas, just sacrificed one by cutting the head off and measuring that.
If you dig into the papers on calibrating the delay of the whole antenna, there are a bunch of things they run into. I suspect
the biggest one is quite simple: nobody but TimeNuts care. The survey guys are happy once they know the phase center. The
NIST category guys are going to calibrate it anyway. The cell phone outfits never did any real GPS time calibration, they did
it all on the other end of the system. Way to many cables in a cell site to measure them all ….
>> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
>> phk at FreeBSD.ORG | TCP/IP since RFC 956
>> FreeBSD committer | BSD since 4.3-tahoe
>> Never attribute to malice what can adequately be explained by incompetence.
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