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