[time-nuts] Cable length calibration

Mike Cook michael.cook at sfr.fr
Fri Jul 1 03:40:28 EDT 2016

> 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...
> Seriously...
> 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. 

> -- 
> 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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