Division A - Abstracts

Gaia Data Release 2

Brown, Anthony

An overview of Gaia DR2 will be presented focusing on the photometry, radial velocities, astrophysical parameters, and variable stars.

The International Terrestrial Reference Frame: ITRF2014 and future plans

Altamimi, Zuheir

We review the progress and continuous improvements being made since more than 30 years in the determination and development of the International Terrestrial Reference Frame (ITRF). We evaluate the precision and accuracy of the main geodetic and geophysical products of the latest ITRF release, namely the ITRF2014, using some key performance indicators. These indicators include the evaluation of the performance of the annual and semi-annual signals and Post-Seismic Deformation (PSD) models: the two main innovations introduced in the ITRF2014 elaboration. A special emphasis will be given to discuss the Earth Orientation Parameters (EOPs) resulting from the ITRF2014 combination. We evaluate in particular the impact of estimating periodic signals on the EOPs, versus applying a non-tidal atmospheric loading model. We discuss the level of agreement between techniques in terms of Earth Orientation Parameters, but also of the frame physical parameters (origin and scale). A short discussion on the transformation between the ITRF and the ICRF will be made in the light of the ITRF2014 EOP results. Main conclusions will be drawn to guide and improve our analysis and combination strategy for future ITRF releases.

Gaia mission overview and status

Prusti, Timo

Gaia has been operating in its nominal phase since mid-2014 for four years. The operations are smooth and routine. The presentation summarises the scientific performance parameters, the status of the spacecraft and a look into the future.

The Gaia Celestial Reference Frame: Properties and relations to ICRF3

Mignard, Francois

The second release of the Gaia data includes a materialisation of a Celestial Reference Frame (called the Gaia-CRF2) agreeing with the ICRS prescriptions. It is based on a set of 556,869 extragalactic distant sources regularly distributed on the sky and ranging from magnitude 16 to 21 in the G-band. The median position accuracy is 0.12 mas for G<18 and 0.5 mas at G = 20. Large-scale systematics are believed to be in the range 20 to 30 muas. New versions with improved astrometry will appear with the next Gaia releases.  I will report on the overall properties of this frame and show a comparison to ICRF3 based on the 2820 sources in common. Then I will take on the issues raised by the  simultaneous availability of several realisations of the ICRS at different wavelengths and how this should be handled at the IAU level in terms of terminology, alignement, comparison procedures and communication to the astronomical community.

The Third Realization of the International Celestial Reference Frame

Charlot, Patrick

This talk will review the new realization of the International Celestial Reference Frame, ICRF3, which is presented for adoption by the IAU General Assembly through a resolution. ICRF3 is aimed at replacing the second realization of the International Celestial Reference Frame, ICRF2, in use since 2010. ICRF3 has been generated by a Working Group of the IAU appointed in 2012. It is based on state-of-the-art astronomical and geophysical modeling and takes advantage of the wealth of VLBI data acquired on various observing networks (IVS, VLBA, DSN,…) since 2009 when ICRF2 was built. Compared to ICRF2, ICRF3 represents a significant improvement in terms of source characterization, position accuracy and total number of sources. About twice as many observations have entered ICRF3 compared to ICRF2. ICRF3 comprise source positions at three radio frequencies, 8.4 GHz, 22 GHz and 32 GHz. It is meant to be the reference for aligning the Gaia optical frame onto the International Celestial Reference System with the highest accuracy.

The consistency of TRF, CRF and EOP - a VLBI perspective

Heinkelmann, Robert

Currently three realizations of the ITRS, the DTRF2014, ITRF2014, and JTRF2014, are available. Although these frames are based on identical input data, the applied mathematical models and thus some results significantly differ.ICRF3, probably the next realization of the ICRS, is very close to be presented to the IAU General Assembly by the corresponding IAU WG. In the sense of the IUGG Resolution No. 3 (2011), which advocates the consistent determination of CRF, TRF, and EOP (Earth Orientation Parameters), one of the main tasks will be to make sure that the two reference frames, TRF and CRF, and with that the EOP are utmost consistent. VLBI is the space geodetic technique that connects the TRF with the CRF. It is uniquely capable of providing the complete orientational difference, i.e. all five EOP, at once in a consistent manner.In the presentation, we will quantify the orientational differences between the CRF and the TRF through the analysis of data of the IVS archives. The obtained frames and EOP will in turn be compared to the current conventional products: ICRF2, IERS 14 C04, ITRF2014 what allows the assessment of the reference frame consistency. The second aspect of consistency addressed will be the application of observation and correction models. In this respect, we will test state-of-the-art models in VLBI analysis as well as the analysis configuration that is specified through IERS Conventions 2010 and the corresponding IERS Convention updates. The systematic effects found will be in the focus of this study, for example the choice of the mean terrestrial pole for the pole tide loading model. As a third aspect of consistency, we discuss the parameterization of the data analysis. In particular, we will look at the different approaches applied to 24 h and Intensive VLBI sessions. The science-driven quality criteria defined by IAG and its GGOS will be used to judge whether the size of the identified inconsistencies is significant or can be neglected.

Gaia DR2: astrometry of small Solar System bodies

Tanga, Paolo

A review of the properties of Gaia observations of asteroids in DR2 and the best approach to exploit them are presented. The results from orbital fits reach the sub-mas accuracy. The combination of additional ground-based astrometry, with the difficulty to receduce zonal errors of past astrometric catalogs, is discussed. The expected progress in different domains, including the prediction of stellar occultations and the detection of Yarkovsky drift for a larger sample of objects, are now at the reach of the community. 

Gaia Second Data Release Early Science

vallenari, antonella

In this presentation we review the main scientific results based on Gaia Second Data Release

Gaia DR2 astrometry

Lindegren, Lennart

The main astrometric properties of Gaia DR2 will be described: random errors, systematic errors (to the extent that they are known), spatial correlations.