[time-nuts] The name "Totally Accurate Clock"
Tom Clark, K3IO
tom.k3io at gmail.com
Tue Aug 5 17:10:15 EDT 2008
Rob noted:
Good point. How was the position of the so called "Totally Accurate Clock"
obtained. Strange name though - no such thing as a totally accurate clock...
:-)
Rob
Here is the story -- and some of the details are in my contributions on
[1]http://gpstime.com:
For many, many moons, I headed up NASA's program that developed Very
Long Baseline Interferometry (VLBI) as a tool to make various
geophysical measurements. The measurements yielded up
* Accurate vector determination of the motions of the earth's
tectonic plates ("continental drift", now known to ~20 microns/yr).
* Very accurate measures of the rotation of the earth with respect to
the celestial reference frame, aka UT1 (in effect "sundial time"),
now observed at levels of a few tens of usec on a daily basis. VLBI
is the "official" BIPM and USNO source of UT1, including the info
on which we have a leap-second pending.
* Accurate positions of the earth's rotation pole (polar motion,
nutation & precession) and of a few hundred extragalactic radio
sources at the sub-milliarcsecond level.
* And some nifty astronomy too!
Each VLBI station around the world has a Hydrogen Maser as its local
clock (Jim Palfreyman's note concerned the VLBI station near Hobart,
Tasmania) serving as both the stable short-term phase reference for
microwave observations, as the timing clocks for data bits written onto
tape (now replaced with disk arrays), and as the still longer term
reference to tie between UT1 and UTC. If you look at my VLBI timing
tutorials on [2]http://gpstime.com you will see info on VLBI's timing
requirements.
In the good old days (thru the mid-80s) we used some combination of
Loran and traveling clocks to keep the network tied together. In the
mid-80s we started using GPS for the task. The original GPS receivers
were large, clunky devices. In the mid-90s I decided to come up with
low-cost GPS timing receiver for the VLBI stations and I needed a short
catchy name. At that time, Heathkit was selling a WWV receiver which
could produce timing at levels of a few msec (on a good day) & Heath
called it "The Most Accurate Clock". Also around that time,
Hammacher-Schlemer was selling a WWVB wall clock that they also called
"The Most Accurate Clock".
My simple timing receiver was already producing sub-usec results using
Motorola's PVT-6 "Six-Pack" 6-channel receiver, even in the face of
Selective Availability (SA). Since I was achieving several
orders-of-magnitude better results than the commercial "Most" receivers
and since we could trace its ACCURACY all the way back to the USNO
master clock, I needed a word much stronger than "Most" and thus was
born the "Totally Accurate Clock" name; I modestly note the coincidence
that TAC happen to be my initials! I began a campaign of replacing the
timing systems at various stations with the first batch of 25 TACs
using the PVT-6 receivers; I think that the unit Jim used for his GPSDO
was an original 6-channel unit. You can judge the performance back then
by the 6-week span of Maser/TAC data from Onsala Sweden plotted in my
tutorials. Joe Taylor (Nobel Prize for his Pulsar work) verified that
the original TAC was able to transfer time to Arecibo at levels of
accuracy comparable to NIST's common view time transfer system (the
service and equipment rental cost ~$5k/yr).
Motorola then came out with their ONCORE series with 6 and then 8
channel receivers and I updated the design to the TAC-2. Because there
was a lot of interest outside the VLBI community, I worked with TAPR to
make a kit-form TAC-2 available to anyone. My friend Rick Hambly
([3]http://www.cnssys.com) saw a need for this receiver in a turn-key
form, so he put the TAC-2 into his CNS Clock. He also transformed my
crude "SHOWTIME" TAC support software into a full Windoze-based
application which he called TAC32. He rolled into TAC32 the ability to
automatically correct the timing results for the annoying 100 nsec
sawtooth dither by providing a way to read a low-cost HP 53131 (or 132)
counter, correcting its reading, and generating corrected logs. All
told, between TAPR and CNS, something like 1000 of my TAC-2s found
their way into the world.
Rick has also adapted the newer Motorola (now iLotus) M12+ 12 channel
receivers and has now rolled out is CNS Clock 2 which includes hardware
sawtooth removal, IRIG time codes, a GPSDO and a bunch of other
widgets. See his web site for more details.
In the meantime, I retired from NASA in 2001 but I still stay involved
with the VLBI and timing communities as the resident curmudgeon and
referee of mis-spoken factoids.
So that's where the name came from and how it evolved.
Tom Clark
References
1. http://gpstime.com/
2. http://gpstime.com/
3. http://www.cnssys.com/
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