Letters of Intent received in 2016

LoI 2018-1900
GA Symposium: Reference Frames

Topics

ICRF3 and celestial reference frames and systems, Gaia and the link between optical-based and radio-based celestial reference frames and systems, terrestrial reference frames and systems, Earth rotation, fundamental standards

Rationale

Reference frames are fundamental to positioning and navigating objects in space and on the Earth. Celestial reference frames (CRFs) are fundamental to space observations. The positions of celestial bodies are given within some celestial reference frame, whether it be optical-based or radio-based, and the accuracy with which celestial bodies can be positioned ultimately depends on the accuracy of the CRF. In 1997, the International Celestial Reference Frame (ICRF), based on the positions of 608 extragalactic radio sources derived from VLBI observations at S/X bands, was adopted by the IAU as the fundamental celestial reference frame, replacing the FK5 optical frame. This first ICRF was replaced in 2009 by ICRF2, also based on S/X observations, and is the current IAU standard celestial reference frame. ICRF2 is a great improvement over the first ICRF in both the number of sources included in the frame and in the positional accuracy of the sources. However, ICRF2 is aging and it has become evident that further improvements are needed. So a new generation celestial reference frame, ICRF3, is currently under development that makes use of both new VLBI observations and new developments in data analysis. ICRF3 is expected to be released by 2018.

Gaia is an ESA space mission with the goal of charting a three-dimensional map of our Galaxy, the Milky Way. Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population. The first data from the Gaia mission was released on September 14, 2016 and the results already provide interesting insights into the link between the radio and optical CRFs and give a taste for the future improvements in the optical materialization of the International Celestial Reference System (ICRS).

Just as celestial reference frames are fundamental to space observations, terrestrial reference frames (TRFs) are fundamental to Earth observations. The TRF is the foundation for virtually all space-based, airborne and ground-based Earth observations. Positions of objects are determined within an underlying TRF and the accuracy with which objects can be positioned ultimately depends on the accuracy of the TRF. The TRF is the indispensable foundation for all geo-referenced data used by science and society. It plays a key role in modeling and estimating the motion of the Earth in space, in measuring change and deformation of all components of the Earth system, and in providing the ability to connect measurements made at the same place at different times, a critical requirement for understanding global, regional and local change. Through its tie to the celestial reference frame by time-dependent Earth orientation parameters (EOPs), the TRF is also fundamentally important to interplanetary spacecraft tracking and navigation. The most accurate global terrestrial reference frame available today is ITRF2014 which was released in 2016.

Earth orientation parameters are used to transform the positions of objects in the TRF to their positions in the CRF. But the theory of the Earth’s rotation that is currently used to do this, based upon an ellipsoidal Earth model, is no longer adequate. The current theory was developed in the late 1970s and early 1980s when Earth rotation observations were much less accurate than they are today. For example, in the late 1970s observations of polar motion were accurate to a few milliarcseconds. Today they are accurate to better than 30 microarcseconds, an improvement in accuracy of two orders of magnitude. This great improvement in the accuracy of the observations allows smaller signals to be studied today than could be studied when the theory was developed, such as effects due to the triaxiality of the Earth. The need to improve the theory of the Earth’s rotation led the IAU and the International Association of Geodesy (IAG) to establish a Joint Working Group on the Theory of Earth Rotation and Validation. As a result, a theory for the rotation of a triaxial Earth is now available.

Standards are required to ensure that reference frames and Earth orientation parameters are consistently determined. There are continuing requirements within both the astronomical community and other scientific and engineering communities for standard data and models involving the concepts of fundamental astronomy. These standards are incorporated into products and services (e.g., publications, data services, software, websites; hereafter, "services"), many of which are provided under the auspices of governments, national/international agencies, and institutes. The services want to be able to ensure their broad user community that the standards they use are produced with the authority of the relevant "professional body", the IAU.

Given the fundamental importance of reference frames, the determination of a new radio-based celestial reference frame (ICRF3), the recent release of the first Gaia data set, the recent release of a new terrestrial reference frame (ITRF2014), the progress being made in extending the theory of the Earth’s rotation, and the need for improved standards to support improved reference frame and Earth rotation determination, the IAU General Assembly in 2018 is a timely opportunity to hold a Symposium on Reference Frames. The topics to be included in the Symposium are, astrometry, terrestrial and celestial reference frames, the link between the radio and optical materializations of the ICRS, the Earth Orientation Parameters that connect the terrestrial and celestial reference frames to each other, and the fundamental standards that are used to determine reference frames and EOPs.