[time-nuts] Newbie to Time Nuts; Seeking wisdom, re Hydrogen MASER applications
apolloeme at gmail.com
Sun Nov 12 18:34:04 EST 2017
Hello fellow Time nuts,
I am a newbie here, just joined, first post. But back in college dabbled in
"precise time" recording in the field, for
Geodetic Surveying field measurements, Star Shots. transiting the
meridian. Very crude by today standards, but effective for
field measurements that had to be time stamped to sub second accuracy, I
got to about 3-4ms UTC absolute, and prior to my senior project most
geodetic surveyors used a Wooden boxed, marine chronometer, to get sub
second UT1 time, or back then, GMT. I have a project involving use of a
Hydrogen Maser as a frequency reference, a STEM activity coming up next
year that may be of interest, and for which I need some technical
assistance. Involves precisely measuring received frequency of a lunar
orbiting satellite, in the 70 cm band. over a period of an hour or so, and
from widespread locations (multiple MASERS) around the Earth.
But that project will be revealed in my next post.
Mean time, for your enjoyment, I copy here a summary of what/how the
Arecibo Radio Telescope capabilities were, when I visited there for a
moonbounce activity I set up, called "Echoes of Apollo", in 2012. Google
and search Youtube videos, KP4AO for commentary.
I was interested after returning home from Echoes Of Apollo, to learn more
about frequency standards they had and used at
the Arecibo Observatory. So I wrote and got an interesting reply from one
of their awesomely knowledgeable staff. Here is his reply on Arecibo Time:
"Any radio observatory worth its salt has time information at hand to the
sub-microsecond level of accuracy. Having experienced only the AO,
I can speak of how we do it, but can't speak for other observatories.
Basically we start with a highly stable reference oscillator, an active
hydrogen maser whose frequency we generally maintain within about
1 part in 10^14. It has an internal synthesizer which provides a 10-MHz
output. Note that this level of frequency accuracy is needed primarily for
our planetary radar and pulsar timing work, so not all observatories
would be as well equipped.
The 10-MHz signal is distributed about the place for locking LOs and
other synthesizers, but it also "regulates" our master station clock.
This clock produces time-of-day (distributed in one of the IRIG formats)
and a 1 pulse per second digital output (1PPS). These are also
distributed about the observatory.
We compare our station clock against UTC via a "common view GPS"
measurement scheme, in which we locally record time differences between
our master clock and GPS signals received by a special receiver. This
receiver, among other things, pays attention solely to GPS signals which
come from satellites known to be simultaneously receivable both here and
at NIST in Boulder at the time of each comparison measurement. Then
we send our locally-recorded difference data to NIST via internet, and they
apply relevant corrections and can tell us our time error with respect to
UTC. We receive this information as a monthly report, whose major
component is a graph of our daily average difference from UTC. We
try to keep this difference within ~200 ns. It does drift around, however,
because the maser itself does drift around a wee bit and even very tiny
frequency errors integrate up to substantial time errors if left alone.
>From time to time we make small (a few parts in 10^14) adjustments to
our maser's frequency if the time error is getting too large, in order to
start our clock drifting back towards correct alignment with UTC. The
end result is that an observer has immediate real time access to time
within a few hundred ns of correct. Further, he can do much better if he
is willing to wait for the monthly report and apply the corrections- then he
can get down to ~10 ns error."
More, re MASER questions, later...
Best, 73, Pat Barthelow AA6EG
apol <apolloeme at gmail.com>loeme at gmail.com
*"The most exciting phrase to hear in Science, the one that heraldsnew
discoveries, is not "Eureka, I have found it!" but:*
"That's funny..." ----Isaac Asimov
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