JSG T.39

Terms of Reference

Gravitation belongs to the four known fundamental physical interactions and represents a crucial quantity reflecting the state of attracting masses. Its knowledge stands at the core of important applications, e.g., 1) establishing planetary reference systems for positioning and predicting orbits of artificial satellites in geodesy; 2) studying inner structures, (sub-)surface processes, and thermal evolutions of planetary bodies in geophysics and planetary sciences; 3) detecting mass transport for understanding climate change and mechanisms of natural hazards in environmental sciences; or 4) navigating terrestrial or space vehicles in transport, military and space exploration. In general, gravitation is indispensable for advancing science, industry, and research; and for addressing a broad range of societal issues, such as sustainable energy, environmental aspects, or infrastructure development.

The science of determining gravitational fields at macroscales is called physical geodesy. This intriguing subject has been an inherent component of the International Association of Geodesy (IAG) and is officially considered one of main pillars of the modern geodetic research. The current status of physical geodesy may even be understated as countless scientists take numerous key products, i.e., static and time-variable gravitational fields, for granted and access them freely from IAG international services, e.g., the International Centre for Global Earth Models (ICGEM), International Gravimetric Bureau, International Gravity Field Service and International Service for the Geoid, as well as from ESA’s Planetary Science Archive or NASA’s Planetary Data System Geoscience Node.

The international services and their products originate from an intricate modelling. This non-trivial process essentially combines these three key components:
• Experimental data are geometric, gravitational, and auxiliary measurements collected by various sensors. For the Earth, these data originate from an ultimate infrastructure of the IAG called the Global Geodetic Observing System.
• Methodology is the underlying mathematical apparatus. Physical geodesy employs potential theory by studying and advancing boundary-value problems (BVPs), integral transformations and equations, and forward modelling of potential fields. Alternatively, statistical methods, e.g., the least-squares collocation, have been developed.
• Computational tools are elements of discrete mathematics and computer science, e.g., numerical methods and algorithms, software, and hardware.

The standard conceptual framework for the gravitational field determination by potential theory often relies on spherical approximation. Nevertheless, as proved by the expeditions of the French Academy of Sciences to South America and Lapland already in the 18th century, Earth’s shape is much closer to a rotational ellipsoid flattened at the poles (flattening ≈ 1/298). Contemporary investigations of solar system planetary bodies have revealed that many resemble prolate or oblate ellipsoids with many of them flattened more significantly than the Earth. Four such spheroidal bodies have recently been of an immense research interest: 1) Mars (flattening ≈ 1/170) being intensely explored by satellite and lander missions as it represents a potential target for future colonisation, 2) the asteroid Bennu (flattening ≈ 1/8.5) explored by the samplereturn satellite mission OSIRIS-REx, 3) the dwarf planet Ceres (flattening ≈ 1/13.4), and 4) the asteroid Vesta (flattening ≈ 1/5.7), both explored by the satellite mission Dawn. In addition, several comets and asteroids with spheroidal (ellipsoidal) shapes have been subject to an intense small body research. Thus, there is an urgent need for formulating a modern conceptual framework for the gravitational field determination.

Objectives

• To complete the class of spheroidal integral transformations relating various types of gravitational data.
• To derive the mathematical theory for the spheroidal forward modelling in the spatial and in the spectral domain.
• To propose a rigorous method for estimating surface mass variations for flattened planetary bodies.
• To develop efficient and accurate software tools for the spheroidal gravitational field modelling.

Program of activities

• To cooperate with related IAG entities (Commission 2, ICGEM).
• To present research findings at geodetic and geophysical conferences.
• To monitor research activities of JSG members and of other scientists, whose research interests are related to the scopes of JSG.
• To organise a session at Hotine-Marussi Symposium 2026.

Members

Michal Šprlák (Czech Republic); Chair
Blažej Bucha (Slovakia)
Sten Claessens (Australia)
Mehdi Eshagh (Sweden)
Khosro Ghobadi-Far (USA)
Elmas Sinem Ince van der Wal (Germany)
Martina Idžanović (Norway)
Pavel Novák (Czech Republic)
Vegard Ophaug (Norway)
Georgios Panou (Greece)
Martin Pitoňák (Czech Republic)
Mahdiyeh Razeghi (Australia)
Natthachet Tangdamrongsub (Thailand)