15 Sep 2006 IVS VLBI2010 Haystack0 It’s About Time !!!!!

15 Sep 2006 IVS VLBI2010 Haystack1 Clock & Calibration for VLBI2010 Clock & Calibration for VLBI2010 Portions have been adapted from “Timing for VLBI ” presented at IVS TOW Meeting Haystack – May 9-12, 2005 zTom Clark NASA/GSFC & NVI mailto: VLBI2010 Working Group Haystack – Sept 15, 2006

15 Sep 2006 IVS VLBI2010 Haystack2 Long-Term seconds - years Events that occur with a defined nsec -- minutes There is a difference between Frequency and Time: The Oscillator Pendulum Escapement Wheel Crystal Oscillator Oscillator Locked to Atomic Transition Rubidium 6.8 GHz) Cesium 9.1 GHz) Hydrogen Masers 1.4 GHz) The Integrator & Display = “Clocks” Gears Electronic Counters Time transferred from “outside” (GPS) The Rotating Earth (i.e. UT1 & sundials) Oscillators and Clocks

15 Sep 2006 IVS VLBI2010 Haystack3 zThe VLBI community (Radio Astronomy and Geodesy) uses Hydrogen Masers at remote sites all around the world. To achieve ~10° signal coherence for ~1000 seconds at 10 GHz we need the two oscillators at the ends of the interferometer to maintain relative stability of  [10°/(360°  Hz  10 3 sec)]  2.8  sec zIn Geodetic applications, the station clocks are modeled at relative levels ~30 psec over a day  [30  /86400 sec]  3.5  10 1 day zTo correlate data acquired at 16Mb/s, station timing at relative levels ~50 nsec or better is needed. After a few days of inactivity, this requires  [50  / 10 6 sec]  5  sec zSince VLBI defines [ UT1-UTC ], we need to control the accuracy of our knowledge of [UTC (USNO) - UTC (VLBI) ] to ~100 nsec or better. What Timing Performance Does VLBI Need? A B C

15 Sep 2006 IVS VLBI2010 Haystack4 The Allan Variance – A graphical look at clock performance B A C

15 Sep 2006 IVS VLBI2010 Haystack5 Why do we need to worry about “Absolute Time” (i.e. Accuracy) in VLBI? To get the correlators to line up for efficient processing, the relative time between stations should be known to ~ 100 nsec. In the past, geodetic and astronomical VLBI data processing has been done by fitting data with “station clock polynomials” over a day of observing, and then discarding these results as “nuisance parameters” or “instrumental constants” that are not needed for determining baseline lengths, source structure, etc. The uncalibrated and unknown offsets now range from 1-10  sec at many VLBI stations. accurate If VLBI2010 is to produce accurate UT1 as a major data product, then “ absolute ” clocks need to be a fundamental design consideration.

15 Sep 2006 IVS VLBI2010 Haystack6 Why do we need to worry about “Absolute Time” (i.e. Accuracy) in VLBI? The MAIN reason for worrying about “absolute time” is to relate the position of the earth to the position of the sun, planets & stars: Generating Sidereal Time to point antennas (especially big arrays, including VLBI!). Measuring UT1(i.e. “Sundial Time”), Nutation & Precession to observe changes due to redistribution of mass in/on the earth over long periods of time. Knowing the position of the earth with respect to the moon & planets to support interplanetary navigation. To improve the accuracy of GPS/GALILEO/GLONASS navigation etc......

15 Sep 2006 IVS VLBI2010 Haystack7 Why do we need to worry about “Absolute Time” (i.e. Accuracy) in VLBI? At the stations this means that we will need to pay more attention to timing elements like: Frequency Standard and Station Timing, including changes within a one-day experiment. The lengths of all cables in the signal & timing paths. The geometry of the feed/receiver to the antenna, including deformation with pointing & temperature. Calibration of instrumental delays inside the receiver and backend. The development of new instrumentation is needed. The care with which system changes are reported to the correlators and the data analysts.

15 Sep 2006 IVS VLBI2010 Haystack8 The Real Signal Path VLBI Analysis assumes the intersection of axes as the “fundamental” reference point. VLBI’s “REAL” Clocks (#1): Fundamental reference point, Geometry & Cables Remember – the lengths of the red cables contribute to Clock (#1)

15 Sep 2006 IVS VLBI2010 Haystack9 VLBI’s “REAL” Clocks (#2): The Microwave & IF Signal Path & Phase Calibrator H-Maser Phase Cal Ground Unit: Monitors Cable Length Changes -- UP + Down Phase Cal Counter Cable Length Transponder Divide by 5 5 MHz Microwave Receiver 1 MHz 1 Pulse/  sec DOWN UP This is the Phase Cal “data clock” that is used to analyze VLBI data. Note: The 1/  sec pulse has a 200 nsec ambiguity because of  5 stage. Quasar Pulse Generator ON ANTENNA CONTROL ROOM IF Signals to Control Room 5 MHz Remember – the length of every red cable and the properties of every red box contributes to Clock (#2)

15 Sep 2006 IVS VLBI2010 Haystack10 VLBI’s “REAL” Clocks (#3): Converting IF Signals into Bits H-Maser IF From Microwave Receiver IF Distributor Video Converter 5 MHz Formatter Clock 5 MHz Clipper/ Sampler Recorder This is the “clock” that the correlator uses to make fringes Remember – the length of every red cable and the properties of every red box contributes to Clock (#3)

15 Sep 2006 IVS VLBI2010 Haystack11 VLBI’s “REAL” Clocks (#4): Synchronizing the bits with GPS to establish [ UTC VLBI minus UTC USNO ] H-Maser GPS TIMING CLOCK (like my TAC) Formatter Clock 5 MHz GPS Antenna Counter #1Counter #2 1 PPS Initial Sync Remember – the length of every red cable & the properties of every red box contributes to Clock (#4) GPS Constellation

15 Sep 2006 IVS VLBI2010 Haystack12 For the VLBI2010 Era must z IMHO, We must insure that all four of these different types of clocks used by VLBI are calibrated throughout the data acquisition, correlation and data processing chain at every station. These clocks need to be “harmonized” at the ~100 nsec level at each station. zThis will allow VLBI2010 to be a reliable source of UT1 at the (hopefully) sub-  sec level, free from biases and long-term drifts with no network-to-network & day-to-day mismatch “seams”.

15 Sep 2006 IVS VLBI2010 Haystack13 One Possible Solution: Calibrate small antennas very accurately & then use them to transfer calibration to the more “difficult” stations: This is taken from the latest Japanese IVS NICT-TDC NEWS