Chopo Ma

The IAU 2000 Resolutions for Astrometry, Celestial Mechanics, and Metrology in the Relativistic Framework: Explanatory Supplement

We discuss the IAU resolutions B1.3, B1.4, B1.5, and B1.9 that were adopted during the 24th General Assembly in Manchester, 2000, and provides details on and explanations for these resolutions. It is explained why they present significant progress over the corresponding IAU 1991 resolutions and why they are necessary in the light of present accuracies in astrometry, celestial mechanics, and metrology. In fact, most of these resolutions are consistent with astronomical models and software already in use. The metric tensors and gravitational potentials of both the Barycentric Celestial Reference System and the Geocentric Celestial Reference System are defined and discussed. The necessity and relevance of the two celestial reference systems are explained. The transformations of coordinates and gravitational potentials are discussed. Potential coefficients parameterizing the post-Newtonian gravitational potentials are expounded. Simplified versions of the time transformations suitable for modern clock accuracies are elucidated. Various approximations used in the resolutions are explicated and justified. Some models (e.g., for higher spin moments) that serve the purpose of estimating orders of magnitude have actually never been published before.

The Celestial Reference Frame at 24 and 43?GHz. I. Astrometry

We present astrometric results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24 hr observing sessions made over a five-year period. These observations were motivated by the need to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies to enable improved deep space navigation after 2016 and to improve state-of-the-art astrometry. Source coordinates for 268 sources were estimated at 24 GHz and for 131 sources at 43 GHz. The median formal uncertainties of right ascension and declination at 24 GHz are 0.08 and 0.15 mas, respectively. Median formal uncertainties at 43 GHz are 0.20 and 0.35 mas, respectively. Weighted root-mean-square differences between the 24 and 43 GHz positions and astrometric positions based on simultaneous 2.3 and 8.4 GHz Very Long Baseline Interferometry observations, such as the ICRF, are less than about 0.3 mas in both coordinates. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted systematic errors limit the overall accuracy of the catalogs.

THE CELESTIAL REFERENCE FRAME AT 24 AND 43 GHz. II. IMAGING

We have measured the sub-milli-arcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high frequency celestial reference frame (CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of

Site displacement due to variation in Earth rotation

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Second Epoch VLBA Calibrator Survey Observations - VCS-II

5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.

Effects of ICRF2 on the TRF, CRF, and EOP

We study site displacement caused by changes in the rate of rotation of the Earth and in the orientation of the Earths pole. We include small effects which have been previously neglected, most notably those caused by polar motion induced ocean loading. Using very long baseline interferometry (VLBI) data we estimate the real and imaginary Love numbers associated with vertical and horizontal deformations caused by polar motion. The estimates of the real part of the Love numbers are consistent with theoretical predictions, although the imaginary parts are too large. Including the loading effect improves the agreement of the real part.

Commission 19: Rotation of the Earth

Six very successful VLBA calibrator survey campaigns were run between 1994 and 2007 to build up a large list of compact radio sources with positions precise enough for use as VLBI phase reference calibrators. We report on the results of a second epoch VLBA Calibrator Survey campaign (VCS-II) in which 2400 VCS sources were re-observed at X and S bands in order to improve the upcoming third realization of the International Celestial Reference Frame (ICRF3) as well as to improve their usefulness as VLBI phase reference calibrators. In this survey, some 2062 previously detected sources and 324 previously undetected sources were detected and revised positions are presented. Average position uncertainties for the re-observed sources were reduced from 1.14 and 1.98 mas to 0.24 and 0.41 mas in RA and Declination, respectively, or by nearly a factor of 5. Minimum detected flux values were approximately 15 and 28 mJy in X and S bands, respectively, and median total fluxes are approximately 230 and 280 mJy. The vast majority of these sources are flat-spectrum sources, with ~82% having spectral indices greater than -0.5.

The ICRF-3: Status, plans, and progress on the next generation International Celestial Reference Frame

The ICRF2 became official on Jan 1, 2010. It includes positions of 3414 compact radio astronomical sources observed with VLBI, a fivefold increase over the first ICRF. ICRF2 was aligned with the ICRS using 138 stable sources common to both ICRF2 and ICRF-Ext2. Maintenance of ICRF2 is to be made using 295 defining sources chosen for their historical positional stability, minimal source structure, and sky distribution. The switchover to ICRF2 has had some small effects on the terrestrial reference frame (TRF), celestial reference frame (CRF) and Earth orientation parameter (EOP) solutions from VLBI. A CRF based on ICRF2 shows a relative rotation of ~40μas with respect to ICRF, mostly about the Y-axis. Small shifts are also seen in the EOP, the largest being ~11μas in Xpole. Some small but insignificant differences are also seen in the TRF.

A Proposed Astrometric Observing Program for Densifying the ICRF in the Northern Hemisphere

The activities in scientific research related to Commission 19 are mostly developed in the different institutions that have sent their reports here enclosed, in the different meetings that have been organized in related themes, and in the WGs of the Division 1. An important additional activity has been developed in the frame of precession and nutation. This research has been initiated by the Descartes Prize received by the Nutation Consortium in 2003. 1. Terms of Reference of Commission 19 The OC has prepared the following charter: Commission 19 of the International Astronomical Union (IAU) is a part of the IAU Division I. The Commission is created to fulfill a specific scientific goals related to the Earth rotation, Earth orientation, and related reference frames. The objectives of Commission 19 are: (a) Encourage and develop cooperation and collaboration in observation and theoret- ical studies of Earth orientation (the motions of the pole in the terrestrial and celestial reference systems and the rotation about the pole). (b) Serve the astronomical community by linking it to the officialorganizations pro- viding the International Terrestrial and Celestial Reference Systems/Frames (ITRS and ITRF) and Earth orientation parameters: International Association of Geodesy (IAG), International Earth Rotation and Reference System Service (IERS), International VLBI Service for Geodesy and Astrometry (IVS), International GPS Service (IGS), Interna- tional Laser Ranging Service (ILRS), International DORIS Service (IDS). (c) Develop methods for improving the accuracy and understanding of Earth orienta- tion and related reference systems/frames. (d) Ensure agreement and continuity of the reference frames used for Earth orientation with other astronomical reference frames and their densification. (e) Provide means of comparing observational and analysis methods and results to ensure accuracy of data and models. The organization of Commission 19 is the following. The Commission consists of its members and consultants, and is chaired by the President. To coordinate its activity, the Commission forms the Organizing Committee (OC) (see above for its composition). The Organizing Committee includes ex-officiomembers (the past Commission President and representatives from the IAG, IERS, IVS, IGS, ILRS, and IDS) and members at large. Each IAU member who is interested in the participation in the Commission activity may be a member of the Commission. No election procedure for a membership is established; only recommendation from the Commission 19 OC is needed.