Discussion:
time-nuts Digest, Vol 168, Issue 21
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Dan Kemppainen
2018-07-23 17:41:24 UTC
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FYI,

I would tend to agree about the drop in traffic. Not sure if it's just a
quiet period, but there are many less messages than before in the digest.

Dan
Could it be that the reflector drops messages?
I did not get Ulrich's? NO-Message and also not that from msimon6808.
It has become quite silent since the move to the new server.
Subjectively -6 dB.
Cheers, Gerhard
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Bob kb8tq
2018-07-23 17:44:05 UTC
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Hi

I think it’s just the normal summertime drop-off. Everybody is on vacation…..

Bob
FYI,
I would tend to agree about the drop in traffic. Not sure if it's just a quiet period, but there are many less messages than before in the digest.
Dan
Could it be that the reflector drops messages?
I did not get Ulrich's? NO-Message and also not that from msimon6808.
It has become quite silent since the move to the new server.
Subjectively -6 dB.
Cheers, Gerhard
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Andre
2018-07-23 19:41:23 UTC
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Hi, has anyone ever combined an SDR and Anthorn atomic clock receiver module so that they can be synchronized?
the idea here is as a variant of a thinned array similar to WebSDR but using lots of smaller units to get around the thinned aray curse (tm)
and combining signals with suitable offset delays so that signal increases at the expense of noise.

-Andre


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Tim Shoppa
2018-07-24 00:56:04 UTC
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I thought the thinned array curse was specifically referring to
transmitting coherent arrays?

In terms of synthetic aperture receiving arrays, I'm not sure the absolute
timestamping accuracy you can extract from say, the on-off encoded minute
marker on the 60kHz reference signal will be good for much of any synthetic
array used at HF or above. I'm guessing their transmit antenna has a Q of
100 or so, meaning that the on-off encoding will have a rise or fall time
in the millisecond ballpark.

Wikpedia tells me NPL used to have a "fast code" 100 bit per second signal
which would imply that their antenna Q is not more than 1000.

Tim N3QE
Post by Andre
Hi, has anyone ever combined an SDR and Anthorn atomic clock receiver
module so that they can be synchronized?
the idea here is as a variant of a thinned array similar to WebSDR but
using lots of smaller units to get around the thinned aray curse (tm)
and combining signals with suitable offset delays so that signal increases
at the expense of noise.
-Andre
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Sent: 23 July 2018 17:00
Subject: time-nuts Digest, Vol 168, Issue 21
Send time-nuts mailing list submissions to
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jimlux
2018-07-24 04:10:38 UTC
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Post by Tim Shoppa
I thought the thinned array curse was specifically referring to
transmitting coherent arrays?
In terms of synthetic aperture receiving arrays, I'm not sure the absolute
timestamping accuracy you can extract from say, the on-off encoded minute
marker on the 60kHz reference signal will be good for much of any synthetic
array used at HF or above. I'm guessing their transmit antenna has a Q of
100 or so, meaning that the on-off encoding will have a rise or fall time
in the millisecond ballpark.
Wikpedia tells me NPL used to have a "fast code" 100 bit per second signal
which would imply that their antenna Q is not more than 1000.
Why not use GPS to get your sync?
Using some sort of VLF, LF or HF signal seems subject to propagation
uncertainty which would be greater than the few ns you could get with
disciplining a decent oscillator with GPS.

In practice, what you probably want to do is have a pilot tone you can
distribute across your array.
Post by Tim Shoppa
Tim N3QE
Post by Andre
Hi, has anyone ever combined an SDR and Anthorn atomic clock receiver
module so that they can be synchronized?
the idea here is as a variant of a thinned array similar to WebSDR but
using lots of smaller units to get around the thinned aray curse (tm)
and combining signals with suitable offset delays so that signal increases
at the expense of noise.
-Andre
________________________________________
Sent: 23 July 2018 17:00
Subject: time-nuts Digest, Vol 168, Issue 21
Send time-nuts mailing list submissions to
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Graham / KE9H
2018-07-24 00:58:39 UTC
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It would be a lot easier if you used a GPSDO to synchronize your SDRs,
since both the time and frequency derived are several orders of magnitude
better than what you can get out of an LF receiver. Any systems like this,
that I am aware of, use GPS, not LF.

--- Graham

==
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jimlux
2018-07-24 18:07:16 UTC
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Post by Graham / KE9H
It would be a lot easier if you used a GPSDO to synchronize your SDRs,
since both the time and frequency derived are several orders of magnitude
better than what you can get out of an LF receiver. Any systems like this,
that I am aware of, use GPS, not LF.
This is essentially what we are doing for SunRISE - we're going to be
flying 6 small satellites in a cluster about 10-15km across to image the
sun from 0.2 to 22 MHz (or thereabouts).


http://www.tauceti.caltech.edu/science-at-low-frequencies-2016/slides/A/12_Lazio.pdf

We have a SDR which digitizes the RF, and a GPS receiver, and we log
both the GPS observables and the filtered/downconverted RF. Then, on
the ground, we reconstruct the time and position when the measurements
were made to do interferometry.

On Earth surface, this would be somewhat easier - I've been fooling with
using 4 RTL-SDR receivers, 4 beaglebone green wireless, and 4 GPS
receivers to do something similar.

As it happens, the challenge is in the non-determinate nature of the
data path from RTL-SDR digitizer via USB (on the space version, we can
arrange to use the same OCXO to drive the GPS sampler and the HF
sampler, etc.), so I'm working it by generating a pilot tone that's
"in-band".

Some years ago, we build a breadboard to demonstrate that this kind of
thing is possible for a large array at L-band.
http://luxfamily.com/jimlux/rtd.htm






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