Another motion that must be considered in the coordinate transformation from ECI (Earth-Centered Inertial) to ECEF (Earth-Centered Earth-Fixed), besides precession and nutation, is polar motion. Many people often confuse polar motion with precession, even though they describe different phenomena related to Earth’s rotation.

Earth’s rotational pole position is described by using two parameters, x and y. x measures along the greenwich meridian while y is measured along 90 degree west meridian. these coordinates show how the instantenous rotation axis moves relative to Earth’s surface.

One of the main components of polar motion is the Chandler Wobble, a natural movement of Earth’s rotational pole with a period of about 433 days. Research by Richard Gross from JPL showed that this wobble is mainly caused by pressure changes at the bottom of the oceans, which are influenced by variations in temperature, salinity, and ocean circulation. His study found that about two-thirds of the Chandler Wobble comes from ocean-bottom pressure changes, while the remaining one-third is caused by changes in atmospheric pressure.

Polar motion is different from precession. Precession is the slow change in the direction of Earth’s rotation axis in space, caused mainly by the gravitational influence of the Sun and Moon. Polar motion, on the other hand, describes the small movement of the rotation axis relative to Earth’s crust.

Scientists observe polar motion using highly precise techniques such as:

• GNSS networks,
• VLBI (Very Long Baseline Interferometry),

Although polar motion is tiny, it plays a critical role in modern navigation and Earth observation systems. Without correcting for polar motion, positioning errors in GNSS and satellite orbit determination would significantly increase. Understanding Earth’s wobble is therefore essential for maintaining the precision of today’s global navigation systems.

References:

[1]https://www.nature.com/nature-index/topics/l4/polar-motion-excitation-and-earth-s-rotation-dynamics

[2]https://www.jpl.nasa.gov/news/a-mystery-of-earths-wobble-solved-its-the-ocean/

[3]https://www.aanda.org/articles/aa/full_html/2011/02/aa15894-10/aa15894-10.html

[4]https://maia.usno.navy.mil/information/what-is-eop