[time-nuts] Cable length calibration
brooke at pacific.net
Fri Jul 1 12:04:23 EDT 2016
For quite a while I was heavily into "chirp" transmissions. These are HF ionosphere radio transmissions that sweep from
2 to 30 MHz at 100 kHz/sec.
In order to "tune" the radio to a specific station (you can not tune by frequency) you need to know the start time
schedule for that specific station (time nuts content).
When GPS became popular the transmitters switched to GPS.
You can use a pulse of RF to calibrate the time delay through your HF receiver to get a more accurate time of reception
value. That helps because with a GPS synchronized transmitter you can determine it's great circle distance from you.
Under some conditions you can see a transmission going around the Earth 2 or 3 times.
In a similar way if you used a pair non amplified versions of a GPS antenna back to back you could determine the time
delay of the pair and then divide by two.
The lesser of evils is still evil.
-------- Original Message --------
>> 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.
>> 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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