It’s About Time !!!!!

1. It’s About Time !!!!! IVS VLBI2010 Meeting @ Haystack

2. Clock & Calibration for VLBI2010Portions have been adapted from “Timing for VLBI ” presented at IVS TOW MeetingHaystack – May 9-12, 2005 • Tom Clark NASA/GSFC & NVI mailto: K3IO@verizon.net VLBI2010 Working Group Haystack – Sept 15, 2006 IVS VLBI2010 Meeting @ Haystack

3. Oscillators and Clocks 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) Events that occur with a defined nsec -- minutes FREQUENCY • The Integrator & Display = “Clocks” • Gears • Electronic Counters • Time transferred from “outside” (GPS) • The Rotating Earth (i.e. UT1 & sundials) Long-Term seconds - years TIMING IVS VLBI2010 Meeting @ Haystack

4. What Timing Performance Does VLBI Need? • The VLBI community (Radio Astronomy and Geodesy) uses Hydrogen Masers at 40-50 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°1010Hz103sec)] 2.810-15 @ 1000 sec • In Geodetic applications, the station clocks are modeled at relative levels ~30 psec over a day  [3010-12/86400 sec] 3.510-16 @ 1 day • To 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10-9/ 106 sec] 510-14 @ 106 sec • Since VLBI defines [ UT1-UTC ], we need to control the accuracyof our knowledge of [UTC(USNO) - UTC(VLBI)] to ~100 nsec or better. A B C IVS VLBI2010 Meeting @ Haystack

5. The Allan Variance – A graphical look at clock performance FREQUENCY TIME C A B IVS VLBI2010 Meeting @ Haystack

6. 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 relativetime 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. • If VLBI2010 is to produce accurateUT1 as a major data product, then “absolute” clocks need to be a fundamental design consideration. IVS VLBI2010 Meeting @ Haystack

7. 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 . . . . . . IVS VLBI2010 Meeting @ Haystack

8. 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. IVS VLBI2010 Meeting @ Haystack

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

10. VLBI’s “REAL” Clocks (#2):The Microwave & IF Signal Path & Phase Calibrator CONTROL ROOM H-Maser ON ANTENNA UP Phase Cal Ground Unit: Monitors Cable Length Changes -- UP + Down Cable Length Transponder 5 MHz DOWN 5 MHz Divide by 5 Phase Cal Counter IF Signals to Control Room 1 MHz Quasar Pulse Generator 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. 1 Pulse/sec Microwave Receiver Remember – the length of every red cable and the properties of every red box contributes to Clock (#2) IVS VLBI2010 Meeting @ Haystack

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

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

13. For the VLBI2010 Era • IMHO, Wemustinsure 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. • This 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”. IVS VLBI2010 Meeting @ Haystack

14. 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 No.27@http://www.nict.go.jp/w/w114/stsi/ivstdc/news_27/pdf/tdcnews_27.pdf IVS VLBI2010 Meeting @ Haystack