It’s About Time !!!!!

15 Sep 2006 IVS VLBI2010 Meeting @ Haystack 1

It’s About Time !!!!!


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


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


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


The Allan Variance – A graphical look at clock performance

B

A

C


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.



•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 . . . . . .



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.


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)


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)


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)


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


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”.


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

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It’s About Time !!!!!