24 Hour Sessions

24 hr sessions are sessions that observe quasars over a 24-hr period by a network of radio telescopes all over the world. There are different types of 24 hr sessions that observe at different schedules. For instance, some sessions observe once a week, every week of the year. While others may only observe a few times during the course of a year.

Very Long Baseline Interferometry (VLBI) is the technique used by the 24-hr sessions to observe the radio emission from quasars. These observations are used to determine the five earth orientation parameters (EOP) - the celestial pole offsets (dX, dY) or precession and nutation, UT1-UTC and polar motion (x, y). Precession is the long-term motion of the Earth’s spin axis in space. While, nutation is the Earth’s periodic motion.


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UT1-UTC measures how much the Earth has rotated over a period of time. Daily 1-hr Intensive sessions are observed to determine how much the Earth has rotated in a day. More information can be found for the UT1-UTC sessions on the UT1-UTC Sessions page.


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Polar motion is the motion of the Earth’s rotational axis relative to the geographic north and south poles, i.e., the Earth’s crust. The x-axis is in the direction of the IERS Reference Meridian (the Greenwich Meridian). The y-axis is in the direction of 90 degrees West longitude.


VLBI EOP data from 24-hr sessions can be found under Data Products. They include SINEX Files, Analyzed Databases, Analysis Reports and the latest 24 Hour EOP series, EOPS and EOXY.

Final USNO EOP products which include additional processing can be found here.