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/ _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
