From jimr Tue Aug 25 11:27:29 EDT 1998
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From: Jim Ray (USNO 202-762-1444)
Message-Id: <199808251527.LAA20676@maia.usno.navy.mil>
Subject: Sigma Tau adjustment ?
To: jdw@erl.noaa.gov (J.D. Williams (NOAA))
Date: Tue, 25 Aug 1998 11:27:28 EDT
Cc: jom@CygX3.usno.navy.mil (Jim Martin (USNO)), jimr@maia.usno.navy.mil
X-Mailer: Elm [revision: 212.4]
Status: RO
Hi J.D.,
I recall that a few years ago you took some pains to set the Fortaleza Sigma
Tau maser synthesizer to get a very small rate difference compared with GPS
Time. You did a very good job because that site has consistently had one of
the smallest clock rates of any station in the VLBI or GPS networks. Recently,
the IGS started a campaign to study improved time transfer using GPS (see
the web site at http://maia.usno.navy.mil/gpst.html for details). As
part of that effort, I have started to collect and post the clock differences
for IGS stations used in our GPS data analysis (wrt the USNO Master Clock).
Looking at the FORT - USNO clock differences for last week (16-22 August), I
find a clock rate difference of:
(FORT - USNO) = -.196837E-08 seconds/day
= -2.278206-14 seconds/second
where FORT is the GPS receiver at Fortaleza (connected to the Sigma Tau
maser) and USNO is the GPS receiver here connected to Master Clock 3.
You can see postscript plots at
http://maia.usno.navy.mil/gpsclocks/0971/FORT.971.ps raw
http://maia.usno.navy.mil/gpsclocks/0971/FORT.971.res.ps detrended
I would like to suggest that perhaps you could ask the Fortaleza crew to
adjust the Sigma Tau maser synthesizer to bring FORT even closer to zero
(wrt USNO). To do so, the synthesizer setting would need to INCREASE by
(2.278206E-14 s/s) / (4.659E-17 / digit)
= 489 digits
If you decide to ask them to make this change (preferably not during a VLBI
session, please), I would appreciate it if you could find out when they make
the change and let me know. I'll keep you posted on the outcome.
Thanks very much for your help,
--Jim
From jdw@windom.erl.noaa.gov Wed Aug 26 12:23:04 EDT 1998
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Wed, 26 Aug 1998 10:21:43 -0600 (MDT)
Date: Wed, 26 Aug 1998 10:21:43 -0600 (MDT)
From: JD Williams
To: Jim Ray
cc: "Jim Martin (USNO)"
Subject: Re: Sigma Tau adjustment ?
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Status: RO
Hello Jim,
I will ask them to change the synthesizer.
J.D.
From jdw@windom.erl.noaa.gov Thu Sep 10 12:45:32 EDT 1998
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Date: Thu, 10 Sep 1998 10:45:04 -0600 (MDT)
From: JD Williams
To: "Jim Ray (USNO)"
cc: JD Williams
Subject: Fortaleza Sigma Tau
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Status: RO
Jim,
The Fortaleza Sigma Tau synthesizer was changed from 309 644 52 to
309 649 41 on 10 Sep 98 at 16:02:45 UT.
J.D.