Download - It’s About Time !!!!!
15 Sep 2006 IVS VLBI2010 Meeting @ Haystack 1
It’s About Time !!!!!
15 Sep 2006 IVS VLBI2010 Meeting @ Haystack 2
Clock & Calibration for Clock & Calibration for VLBI2010VLBI2010
Portions have been adapted from “Timing for VLBI ” presented at IVS TOW Meeting
Haystack – May 9-12, 2005
Tom ClarkNASA/GSFC & NVI
mailto: [email protected]
VLBI2010 Working Group
Haystack – Sept 15, 2006
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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
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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°1010Hz103sec)] 2.810-
15 @ 1000 sec
In Geodetic applications, the station clocks are modeled at relative levels ~30 psec over a day [3010-12/86400 sec] 3.510-16 @ 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 [5010-9/ 106 sec] 510-14 @ 106 sec
Since 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
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The Allan Variance – A graphical look at clock performance
B
A
C
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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.
• If VLBI2010 is to produce accurateaccurate UT1 as a major data
product, then “absolute” clocks need to be a fundamental design consideration.
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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 . . . . . .
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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.
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The Real Signal Path
VLBI Analysis
assumes
the inter
sectio
n of
axes as t
he “fundamen
tal” refer
ence
point.
VLBI’s “REAL” Clocks (#1): Fundamental reference
point, Geometry & Cables
Remember – the lengths of the red - - - cables contribute to Clock (#1)
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VLBI’s “REAL” Clocks (#2): The Microwave & IF Signal Path
& Phase CalibratorH-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)
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VLBI’s “REAL” Clocks (#3): Converting IF Signals
into BitsH-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)
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VLBI’s “REAL” Clocks (#4): Synchronizing the bits with GPS to establish [ UTCVLBI minus UTCUSNO ]
H-Maser
GPS TIMING CLOCK (like my TAC)
Formatter
Clock
5 MHz
GPS Antenna
Counter #1 Counter #2
1 PPS
1 PPS
Initial Sync
Remember – the length of every red cable & the properties of every red box contributes to Clock (#4)
GPS Constellation
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For the VLBI2010 Era
IMHO, We mustmust 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.
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”.
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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