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ESA contract for the study of the geodetic precession and frame dragging using the Galileo, GPS and GLONASS satellites

09-08-2021

UPWr has signed another contract for research on the impact of relativistic effects on the orbits of GNSS satellites. This time, two Keplerian parameters will be examined: the inclination angle, which describes the inclination of the orbit plane with respect to the Earth equator, and the right ascension of ascending node, which describes the orientation of the orbit intersection with the equatorial plane relative to the vernal equinox.

Geodetic precession is a relativistic effect. Geodetic precession should not be confused with the geometric precession of the figure of the Earth, which results from the gravitational interaction of the Sun and the Moon and causes, inter alia, slow shifting of the zodiac constellations on the ecliptic (e.g. the Aries point is currently in the constellation of Pisces and all constellations are shifted by one cycle relative to the position in antiquity). Geodetic precession, first described by De Sitter in 1916, results from general relativity, which predicts a change in the position of the ascending node due to the curvature of space-time by the Sun.

In an article recently published by UPWr scientists, it was found that geodetic precession also causes other effects, incl. change of the value of the satellite inclination in time depending on the mutual position of the satellite, the Earth and the Sun. So far, no one has experimentally confirmed this effect. Therefore, ESA decided to fund research in this area.

The second effect that will be investigated in the project is the dragging effect of the system, i.e. the formation of space-time vortices due to spinning celestial bodies. The effect was first described by Lens and Thirring in 1917. So far, the most accurate confirmation of the effect was based on a pair of LAGEOS-1 and LAGEOS-2 geodetic satellites, and a little later also on the observations of the LARES satellite (launched in 2012). However, the LAGEOS and LARES satellites orbit so low (1,400 - 5,900 km) that they are susceptible to temporal changes in the gravitational potential. Laser distance measurements are performed on these satellites, which are effective in determining radial distances but provide less accurate data on the angular parameters of the orbits of the satellites.

In the framework of the ESA project, we will, for the first time, use 80 GPS, GLONASS and Galileo satellites (instead of 3 LAGEOS and LARES satellites) to determine the effect of changing ascending nodes as a result of the frame dragging effect. GNSS satellites are located at different heights and in circular and eccentric orbits (a pair of Galileo FOC satellites), and are less susceptible to temporal changes in the Earth gravitational potential, which will increase the accuracy of confirming the frame dragging effect. Additionally, the GNSS technique provides the most accurate angular parameters describing the motion of the Earth (pole coordinates and length of day variability) due to the large number of ground tracking stations and active satellites. Thus, the research can provide much more accurate results than those based on laser distance measurements to the LAGEOS and LARES satellites.