U.S. Naval Observatory Earth Orientation Department

The Washington Correlator

Very Long Baseline Interferometry (VLBI) observations are used by the Earth Orientation Department to determine and predict the variable orientation of the Earth in three-dimensional space.

In order to determine the Earth's orientation, you must compare it to something that does not move, or at least whose motion is so small it can be neglected. The ideal objects to use as points of reference are quasars, those myserious sources of tremendous energy that are so far away they appear point-like and show almost no motion. Even though quasars are intrinsically very bright, they are so far away that the signals we receive are very weak. Most are visible only in radio frequencies so we use large radio telescopes to detect them. But because the radio waves used are 100,000 time longer than light waves, we would need to build 100-km telescopes to detect the small angular changes that tell us how the Earth's rotation is changing.

A more clever way of achieving this resolution is by using a technique called Very Long Baseline Interferometry (VLBI) that virtually creates a radio telescope the size of the entire Earth. In a VLBI observation radio telescopes at various locations on the globe all look at the same distant quasar simultaneously. The radio noise from the quasar sweeps across the Earth striking the telescopes at slightly different times. If we can measure these time differences very precisely, we can apply geometry and find the precise location of the telescopes and the orientation of the Earth with respect to the stationary quasars.

[USNO Mark IIIA Correlator]
The Washington Mark IIIA Correlator

The difference in the arrival times at widely separated radio telescopes is found by making high-quality instrumentation tape recordings of the quasar signal at each telescope. The signal is time-tagged by having a high-stability clock (a hydrogen maser) at each site. This enables the tapes to be compared, or correlated, and their signals shifted until they align at the given time offset. At USNO we have a special purpose supercomputer, called the Washington Correlator, designed especially to play back VLBI tapes and find the delay times and the rates of change of delay between two VLBI stations.

The Washington Correlator can play back data from six radio telescope stations at once, forming 15 separate pairs of observations, or baselines. The six tape drives of the Washington Correlator (blue with black fronts) can be seen on the left in the above photograph. Each telescope records data in 14 different frequency bands at a rate of 4 to 8 million bits per second on each frequency. Thus, when the six stations are being played back the Correlator is inputting data at a rate of more than a billion bits per second per radio telescope, and the computer is executing more than 100 million instructions per second. The radio signals are recorded at each telescope on special, very thin tapes so that 14-inch diameter reels hold more than 3.5 miles of tape, enough for 24 hours of data recording. The tape playback units shuttle the tape past the read heads at more than 10 feet per second while keeping all the tapes synchronized to a few millimeters. As a result, after correlating several minutes of VLBI data, the quasar signal delay between a pair of telescopes separated by inter-continental distances can be found with an accuracy of a few picoseconds (1/1,000,000,000,000 of a second).

Once the delays and delay rates of a series of VLBI observations are determined, usually for a 24-hour period, the Earth Orientation Department uses these values to solve for Earth orientation parameter values by comparison with theoretical models by the method of least-squares.

VLBI observations to study the secular motion of the pole, continental drift, Earth tides and corrections to the fundamental astronomical constants are made and correlated in order to improve observations of Earth Orientation Parameters and radio source positions. VLBI data are currently the most accurate data available to estimate polar motion, UT1, precession and nutation. All other data types are "calibrated" with respect to the VLBI determinations.

The Mark IV Correlator now at USNO was designed and constructed by MIT Haystack Observatory and is operated in cooperation with NASA. It currently operates 24 hours a day, 5 days per week, and for 16 hours on the weekends. (For more information on the Mark IV Correlator processor, see this link.)

The Washington Correlator is a joint project of the National Geodetic Survey, the National Aeronautics and Space Administration, and the U.S. Naval Observatory and is responsible for the processing of a variety of VLBI experiments.

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Last modified: 11 May 2001 Approved by EO Dept. Head, USNO