A Brief History of the NOAA Very Long Baseline Interferometry Program
Starting in the mid 1970s, researchers from NOAA's National Geodetic Survey played a leading role in developing Very Long Baseline Interferometry stations and collecting observations that resulted in more accurate celestial and terrestrial reference frames. This pioneering work played an important role in increasing the fundamental understanding of our planet.
- Historical Background
- The Introduction of VLBI
- POLARIS VLBI Network
- Applying VLBI
- Works Consulted
Photograph of the VLBI station at Fortaleza, Brazil, jointly developed and operated by NOAA and the Brazilian Space Agency. Click image for larger view.
Did you know that major weather systems like El Niño can actually cause the Earth's rotation to speed up or slow down? In fact, one of the effects of the 1997 El Niño weather system was to lengthen our day by 0.6 milliseconds! Or did you know that the continents are in constant motion? North America and Europe are drifting apart at a rate of about 1 centimeter per year. How were these discoveries made? The answer might surprise you…
Much of what we know about our own planet has been gained by looking to space. Astronomers use time differences in the arrival of microwave signals from radio sources outside of our own galaxy (extragalactic) to study the distant cosmos. This same technique, called Very Long Baseline Interferometry or just "VLBI," can be used to study our own planet and its place in the universe and to monitor the changes in both. VLBI produces very precise distance measurements on the Earth's surface, allowing us to learn about the Earth's size, its shape, variations in its spin rate, changes in the orientation of its polar axis—all by observing quasi-stellar objects (quasars) and other natural radio sources.
For two decades, starting in the mid 1970s, researchers from NOAA's National Geodetic Survey (NGS) played a leading role in developing VLBI stations and collecting observations that resulted in more accurate celestial and terrestrial reference frames. These reference frames were used to make the first accurate measurements of the motions of Earth's major tectonic plates and to monitor changes in the Earth's orientation and length of day with greater resolution and accuracy.
NOAA no longer operates VLBI observatories nor participates in international VLBI observing programs, but stations developed in Brazil, South Africa, and Australia with NOAA support continue to operate, and VLBI continues to be the single most important technique used by the International Earth Rotation Service.
When the mathematician and physicist Leonhard Euler first reported that his research on rotating bodies suggested that the Earth's axis of figure might wobble slightly with respect to its axis of rotation, Pierre LaPlace responded that "all of astronomy…depends…upon the invariability of the Earth's axis of rotation…and upon the uniformity of this rotation." If Euler's research was correct, it would mean that lines of latitude would vary by about 9 meters north and 9 meters south of their mean position over a period of 10 months.
Despite LaPlace's skepticism, many of the leading astronomers of the next century devoted substantial time and resources seeking to (unsuccessfully) detect the variation of latitude.
In 1891, Seth Carlo Chandler, Jr., an insurance actuary, amateur astronomer, and a former employee of the U.S. Coast Survey, stunned the international scientific community by announcing his detection of a variation of latitude at a period of 14 months. After further analysis, Chandler determined that the "wobble" of the Earth's axis is actually more complex, comprising at least annual and 14-month oscillations and a long-term drift, and perhaps other variations as well.
The Gaithersburg Latitude Observatory located in Gaithersburg, Maryland, circa 1910. Click image for larger view and image credit.
In 1878, the name of the U.S. Coast Survey was changed to the U.S. Coast and Geodetic Survey (C&GS), and the variation of latitude was certainly of interest to this agency charged with developing a national geodetic control network for the United States. To verify the variation of latitude, C&GS participated in an important international observing program, sending a team to Japan in 1891 to perform simultaneous observations with an International Geodetic Association team. And, in 1899, C&GS joined in establishing and operating the International Latitude Service (ILS). The ILS regularly monitored polar motion, a more appropriate name for the "wobble," by observing stars with "zenith telescopes." All observatories observed the exact same stars, so that errors in the coordinates of the stars tended to cancel out.
For more than 80 years, C&GS (and later NGS) continued to participate in the ILS. Observatories in Gaithersburg, Maryland, and Ukiah, California, (and for a short while in Cincinnati, Ohio) regularly submitted nightly astronomic latitude observations to the central ILS Bureau in Mizusawa, Japan.
The Introduction of VLBI
With the onset of the space age, it became clear that a more accurate monitoring service was needed to support space navigation and modern geodesy. Technologies developed for the exploration of space could now be used to better monitor variations in the orientation of the Earth in space, including not only polar motion, but also Universal Time, precession, and nutation (a small periodic "nodding" of the axis of rotation in space).
A team of National Aeronautics and Space Administration (NASA) and Massachusetts Institute of Technology (MIT) researchers began exploring the use of VLBI to measure motions of the Earth's tectonic plates and Earth orientation. In VLBI, two or more radio telescopes track natural sources, generally quasi-stellar objects (called "quasars") located great distances from Earth. The radiation received from most quasars was emitted long before our solar system was formed. Because the quasars are so far from Earth, they appear to not be moving, thus forming a nearly inertial, or fixed, reference frame, making them the obvious choice to use as a celestial reference frame.
By using two or more radio telescopes to observe and record the signals received from the same quasar at exactly the same time, scientists can determine the time difference between the arrival of the signal at each radio telescope. These differences can then be used to calculate very precise distances and directions between the telescopes. VLBI can determine distances between radio telescopes to within a millimeter across an entire continent! Image courtesy of NASA.
However, also because the quasars are so distant, their signals are extremely weak when they arrive at Earth, requiring the use of large aperture radio telescopes (with collecting surfaces typically tens of meters in diameter) with cryogenically cooled sensors. Even with these powerful telescopes, the signals are so weak that they are buried in noise. The noise and signals are recorded on wideband digital tape recorders, with respect to highly precise time tags provided by a hydrogen maser frequency standard located at each observatory. At the completion of an observing session, typically 24 hours in length, the tapes are transported to a special correlator center for processing. The noise is different on the tapes recorded at each observatory, but the signal is the same, making it possible for the correlator to determine the difference in time of the arrival of signals between pairs of stations.
The delays in arrival times change as the Earth rotates. If times are determined at several time periods for several quasars, scientists can estimate very precisely the coordinates of the sources, the differences in the clocks at the different stations, and the baseline vectors (both magnitude and directions) of the lines between observing stations.
With the third generation Mark III VLBI system developed by the NASA-MIT team, the baseline lengths could be determined to a few millimeters for stations separated by thousands of kilometers and the orientation of the baseline could be determined to a fraction of a millisecond of arc.
The POLARIS VLBI Network and Project MERIT
In 1977, NGS launched an initiative to establish an improved Earth orientation monitoring system using VLBI. Project POLar-motion Analysis by Radio Interferometric Surveying (POLARIS) involved the development of three VLBI observatories, in a continental-scale open triangle, with stations near Ft. Davis, Texas; Richmond, Florida; and Westford, Massachusetts.
One year later, at a working meeting held in Spain, the international community decided to launch a project to explore the potential benefits of establishing a new international Earth rotation service using a mix of space techniques, most importantly, Very Long Baseline Interferometry, Lunar Laser Ranging, and Satellite Laser Ranging. With Lunar Laser Ranging and Satellite Laser Ranging, retroreflectors were placed on the moon (by astronauts) and on artificial satellites. Telescopes fire pulses of laser light to the reflectors, and the round-trip travel time of the light is recorded. The one-way travel time multiplied by the speed of light gives the distance. Measurements from different stations can be used to determine the orbits of the satellites and moon, along with the locations of the ranging stations on Earth.
Map showing the motions of tectonic plates derived from NOAA VLBI measurements. Click image for larger view.
Folded together, these space techniques were important components of the new project, which was named Monitor Earth Rotation and Intercompare Techniques (MERIT).
The POLARIS VLBI network was important in the deliberations leading to the launch of project MERIT, because it assured that a regular series of Earth orientation parameters would be available from VLBI. W. E. Carter, NGS, was asked to serve as the VLBI technique coordinator for project MERIT. NOAA, NASA, and the U.S. Naval Observatory (USNO) signed an interagency agreement to collaborate on the application of VLBI to Earth orientation. The collaboration between these three agencies was known as the National Earth Orientation Service (NEOS) and resulted in the building of a next-generation VLBI correlator center at USNO.
The success of project MERIT led to the establishment of the International Earth Rotation Service (IERS). Today, the IERS regularly provides accurate Earth Orientation Parameters (EOP) to the international scientific community, using Lunar Laser Ranging, Satellite Laser Ranging, and Very Long Baseline Interferometry, as well as other techniques, including use of the Global Positioning System (GPS).
Plot of polar motion determined from VLBI observations. Click image for larger view and complete caption.
VLBI soon proved to be the most powerful technique for maintaining celestial and terrestrial reference frames and for providing the highest accuracy and most complete suite of Earth orientation parameters.
To expand the international VLBI network, NGS made agreements with agencies in Brazil, South Africa, and Australia. NGS provided VLBI recording terminals, and in the case of Brazil, a radio telescope. In turn, the host nations agreed to operate stations near Fortaleza, Brazil; Hartebeesthoek, South Africa; and Hobart, Tasmania, Australia. These stations participated in observing sessions with the POLARIS stations and also in observing sessions including stations developed by other nations, most importantly Germany and Norway. Other VLBI observatories were developed by NASA and USNO in Hawaii and Alaska, and other nations built observatories, including Italy, Japan, Russia, and China.
Plot of variations on the length-of-day determined from VLBI (black line) and the change in atmospheric angular momentum (red line). Click image for larger view and complete caption.
In the early 1980s, for a period of a few years, NGS also operated two mobile VLBI units to monitor crustal motions in North America and Europe. That work was eventually taken over by GPS, which was less expensive and provided continuous 24-hour time series.
The U. S. Naval Observatory 20-meter radio telescope at Green Bank, West Virginia, monitors Earth rotation and orientation. Click image for larger view and image credit.
An unusual combination of a nearly century-old historical mission, the need and means to improve the accuracy of the determination of polar motion and other Earth orientation parameters by at least two orders of magnitude, enlightened management, Congressional support, and collaboration with NASA and USNO, all led to the extraordinary success of the NOAA VLBI program. In fewer than 20 years, VLBI measurements verified plate tectonic theory, created celestial and terrestrial reference frames never before thought possible, compiled time series of Earth orientation parameters of unprecedented accuracy and temporal resolution, and confirmed the deflection of electromagnetic radiation in a gravitational field predicted by Einstein's theory of relativity to a new order of magnitude.
Budget constraints ultimately resulted in the decision to terminate the NOAA VLBI program in favor of GPS, which more directly addresses the highest-priority operational responsibilities of NOAA. However, the VLBI correlator built at the USNO continues to process data collected by a global network of stations, many of which were in part inspired by project POLARIS. Ironically, it is only because of the VLBI Earth orientation observations pioneered by NOAA and still collected by other organizations around the world today that NOAA is able to rely on GPS.
Contributed by William E. Carter, formerly with NOAA's National Ocean Service, National Geodetic Survey, now at the University of Florida
Carter, W.E. & Carter, M.S. (2006). Simon Newcomb, America's Unofficial Astronomer Royal. St. Augustine, Florida: Mantanzas Publishing.
Carter, W.E. & Carter, M.S. (2002). Latitude, How American Astronomers Solved the Mystery of Variation. Annapolis, Maryland: Naval Institute Press.
Carter, W.E., Robertson, D.S., Nothnagel, A., Nicolson, G.D., Schuh, H., & Campbell, J. (1988). IRIS – Extending Geodetic VLBI Observations to the Southern Hemisphere. Journal Geophysical Research, 93: 14947-14953.
Carter, W.E. & Robertson, D.S. (1986). Studying the Earth by Very long Baseline Interferometry. Scientific American, 255 (5): 44-52.
Carter, W.E., Robertson, D.S., Petty, J.E., Tapley, B.D., Schutz, B.E., Eanes, R.J., & Lufeng, M. (1984). Variations in the Rotation of the Earth. Science, 224: 957-961.
Robertson, D.S., Carter, W.E., & Dillinger, W.H. (1991). A New Measurement of the Solar Gravitational Deflection of Radio Signals Using VLBI. Nature, 349: 768-770.
Robertson, D.S., Fallon, F.W., & Carter, W.E. (1986). Celestial Reference Coordinate Systems: Sub-millisecond of Arc Repeatability Demonstrated with VLBI Observations. Astronomical Journal, 91: 1456-1462.
Robertson, D.S., Carter, W.E., Tapley, B.D., Schutz, B.E., Eanes, R.J. (1985). Polar Motion Measurements: Sub-Decimeter Accuracy Verified by Intercomparison. Science, 229: 1259-1261.