How do we measure Earth Orientation?
In order to observe Earth orientation, observations must be made from the Earth of objects located in space. Objects that are used include stars, artificial satellites, the Moon, and distant radio sources called quasars. These provide useful reference directions with which to measure the Earth's orientation. To determine the Earth's orientation very accurate observations of these objects must be made. Stars have been observed photographically for decades to determine the motion of the pole and the rotation of the Earth. Recently, more accurate methods have been devised including the use of lasers and radio telescopes. Laser bursts can be bounced off of artificial satellites or the Moon. This provides information on exactly where the satellite is at a particular time which, in turn, can be used to determine the Earth's orientation in space.
Radio telescopes can also be utilized in a technique called Very Long Baseline Interferometry (VLBI). By having several radio telescopes observing the same quasar at the same time and recording the information that is seen at each telescope, the Earth's orientation can be determined. USNO operates a VLBI network for this purpose, in close cooperation with other groups in the U.S. and abroad. The recorded information then needs to be processed further before the final results can be determined. The reduction procedure involves the use of a highly specialized computer called a correlator. Currently, there are three such correlators in the United States, one located at USNO, one in Boston (Haystack Observatory), and one located in Socorro, NM. Once the data from a VLBI observing session has been correlated, it can be processed further to produce information on Earth orientation and other useful quantities.
Astronomical observations are made routinely by a number of observatories located around the world for this purpose. The IERS is the international organization responsible for the coordination of observations of polar motion and nutation as well as astronomical time. The IERS organization consists of various product centers to provide specific services to users, such as rapid service/predictions of Earth orientation, data related to the motions of geophysical fluids (e.g., atmosphere and oceans), and the celestial and terrestrial reference frames. Observations are contributed to the IERS by individual observing techniques, which in turn receive results from numerous observatories, laboratories, and analysis centers around the world. The IERS Rapid Service/Prediction Center then combines these data into a series of x, y, UT1-UTC, and celestial pole offsets. This information is recomputed at least daily and disseminated by e-mail, anonymous ftp, and the World Wide Web. Additional related information can be obtained upon request.