Letters of Intent received in 2019

LoI 2021-2109
GA Symposium: Reference Systems and Rotation of the Earth and other Solar System Bodies

Date: 22 August 2021 to 26 August 2021
Location: Busan, Korea, Rep of
Contact: Florian Seitz (florian.seitz@tum.de)
Coordinating division: Division A Fundamental Astronomy
Other divisions: Division F Planetary Systems and Astrobiology
Co-Chairs of SOC: Seitz, Florian (Technical University of Munich)
Souchay, Jean (Observatoire de Paris)
Archinal, Brent (Archinal, U.S. Geological Survey)
Escapa, Alberto (University of León)
Chair of LOC: None (None)



- New developments, scientific challenges and the consistency of radio (VLBI) and optical (GAIA) Celestial Reference Frames
- Next generation Celestial Reference Frame: Towards a combined radio and optical multi-band CRF
- Practical aspects of defining and maintaining reference systems and frames for solar system bodies (planets, moons, asteroids)
- Combining celestial and planetary reference frames
- Rotation of the Earth and other solar system bodies: Theories, models, observations and analysis
- Internal structure modeling of planetary bodies
- New and emerging measurement systems



Celestial and planetary reference systems provide the fundamental framework and metrological basis for referencing astronomical and space-geodetic observations. Realizations of these systems (reference frames) at the highest level of accuracy require a precise determination of the rotation and orientation of solar system bodies, and they are of paramount importance for positioning and navigation on Earth and across the solar system as well as for the measurement of time. Furthermore, they are essential to study the dynamics of the solar system and of more distant objects, such as exoplanets. The knowledge of the rotation of solar system bodies also allows for studying their internal structure and geophysical phenomena.

The proposed GA Symposium "Reference Systems and Rotation of the Earth and other Solar System Bodies" involves:
(1) Commission A1 on "Astrometry"
(2) Commission A2 on "Rotation of the Earth"
(3) Inter-Division A-F WG Cartographic Coordinates & Rotational Elements
In addition, it is expected that other Divisions, Commissions, or Working Groups – partly in the process of creation – will also endorse the proposed Symposium.
Commission A2 will take the lead on organizing the Symposium. The proposed editors are identical with the Chairs of SOC.

Over the past years, great progress has been made in the improvement of the Celestial reference frames. In 2018, the IAU adopted the Third realization of the International Celestial Reference Frame (ICRF-3) determined from VLBI observations as the fundamental realization of the ICRS, and in 2013 ESA launched the Gaia space astrometry mission to determine a new highly-accurate celestial reference frame in the optical domain (Gaia Celestial Reference Frame, GCRF). The proposed Symposium will be a forum to discuss the new developments, scientific challenges and the consistency (link accuracy) of radio and optical frames with a particular focus on their mutual orientation and rotation. Furthermore, it shall be discussed how a next generation multi-band CRF combining both frames at micro-arc second level accuracy could be realized. It will in particular benefit from the expertise generated in the respective Division A WG Third Realization of the ICRF.

In addition, the quasars observed both by VLBI and by Gaia DR2 (and in the future DR3) are of particular interest. Their radio and optical data lead to challenging questions as: how can we explain their positional offset? Is there a correlation between radio flux and magnitude variability? What is the link between the positional uncertainty/instability in both domains? Some of these questions involve the astrophysical models of the quasars, and the ties between astrometric and astrophysical constitutes a new exciting field of interest.

For defining planetary coordinate systems, the IAU and the Working Group on Cartographic Coordinates and Rotation Elements (WGCCRE) established fundamental principles primarily in the 1970s. Related to these definitions, the Symposium shall address the following questions: Are these principles still adequate? Are changes needed? E.g. for bodies where a longitude definition has been previously defined based on ground-based observations, how should that (or should it) be maintained for spacecraft observations? What are the best methods for recording the fundamental parameters defining cartographic coordinates and rotational elements for well-studied bodies? What procedures need to be further recommended for establishing and updating these coordinate systems and frames? Does it make sense to switch from the existing widespread use of planetographic systems for planets and satellites to planetocentric coordinate systems?

The Celestial reference frame and the respective reference frame of a particular solar system body form an interconnected trio with its rotation, because the latter is defined as the link between the orientations of the two frames. One key topic of the Symposium will be the discussion of theoretical concepts for the realization of consistent frames and rotation parameters as well as the practical implications of this goal. In particular, for the Earth’s rotation the consistency between ICRF, ITRF (International Terrestrial Reference Frame) and the Earth Orientation Parameters is requested by two resolutions of IUGG and IAG and subject of a new joint IAG/IERS/IAU WG to be established in late 2019.

The Symposium will be a forum to discuss the theories, models, and the analysis of the rotation of the Earth and other solar system bodies. For the Earth, observations of its rotation are available since many decades with constantly increasing accuracy. However, the accuracies at the millimeter level on the surface of the Earth required by IAG’s Global Geodetic Observing System GGOS for the reliable determination of global change impacts can still not be met, as it has been recognized in the final reports of recent IAU/IAG WGs on the Theory of Earth Rotation (2013-2015, 2016-2019) hosted by Commission A2. This challenge requires further improvement of theories and models of Earth rotation as well as new and refined approaches for the combination of different space-geodetic observations. Concerning other planets, satellites, asteroids, and comets, observational data on their rotation are presently limited. In the future, we anticipate a dramatic improvement in our knowledge on the rotation states of Mars (from NASA’s InSight mission), the Galilean satellites (ESA’s JUICE and NASA’s Europa Clipper missions), and of several asteroids (e.g. from the Psyche and Lucy missions). It is peculiar that the various rotation models and underlying physics for planets, moons, and small bodies differ so greatly.

Another fundamental point concerns the theoretical framework for rotational models used since the WGCCRE was established about 40 years ago, when space exploration had just begun. It may be time to revise this framework and to adapt it to the new knowledge on the rotation states. To achieve this goal, much can be learned from the theoretical framework already in use for the Earth’s rotation.

Observations of the rotation of solar system bodies – encompassing the rate of rotation, the position of the rotation pole with respect to the planetary reference frame (polar motion), and with respect to the CRF (precession and nutation) – show changes on multiple observable timescales, reflecting a wide variety of exogenous and endogenous stimulating processes. The Symposium will provide a forum to discuss analyses of the observed changes for the Earth and other solar system bodies. With respect to the later, there is in particular great interest in research related to their internal structure, including the “Ocean Worlds” of Europa, Titan, Enceladus, and others. Related questions will be: What information on this structure has been derived or can be derived in the next years with current or future datasets, via analysis of body orientation parameters, e.g. for libration and tides? What types of new datasets need to be collected? What types of analysis techniques are useful for deriving such information? We aim at getting new insight on theses topics.

Finally, the Symposium seeks contributions discussing new or improved astrometric and geodetic observation techniques for the determination of reference frames and rotation series, including combinations of different observing techniques.

Notice: A colloquium “Journees 2019: Astrometry, Earth Rotation and Reference Systems in the Gaia era” will be held at Paris (2019, Oct. 7-9) gathering as many as 130 scientists from all over the world, for a large part members of Commissions A1 and A2. This large participation constitutes the proof that the topics presented above and which will be discussed during the colloquium are subject to exciting investigations. Thus it should be very adequate that this international meeting is followed in two years by an IAU Symposium including (but not exclusively) its common topics.