[time-nuts] Cable length calibration

Scott McGrath scmcgrath at gmail.com
Thu Jun 30 04:19:43 EDT 2016

If the nominal velocity of propagation is known and length is known delay is easily determined mathematically 

Time Domain Reflectrometry is the usual technique for finding cable length but even there the cables NVP is an essential parameter if you want to compute length but not essential in time nuts application because we are interested in delay which a TDR reads directly When using a TDR its best if cable is unterminated as the discontinuity at the end is helpful as a marker.   Also most TDRs like the Tek 1502 can put 100v or more on the cable which will blow most GPS antennas

> On Jun 29, 2016, at 4:08 PM, Brooke Clarke <brooke at pacific.net> wrote:
> Hi Hal:
> I think the cal process is essentially a time domain reflection measure of cable length.  The GPS receiver and the cable cal hardware would be in the antenna unit.
> The 1 PPS signal would be aligned at the output of the cable.
> -- 
> Have Fun,
> Brooke Clarke
> http://www.PRC68.com
> http://www.end2partygovernment.com/2012Issues.html
> The lesser of evils is still evil.
> -------- Original Message --------
>> brooke at pacific.net said:
>>> 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?
>> The obvious way is to compare the time you get with a known-good time, but if
>> you had that, why would you want this new GPS with an unknown cable length.
>> You might be able to do it by measuring the DC drop.  Getting enough accuracy
>> seems tough.
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