The Celestial Frame and the Weighting of the Celestial Pole Offsets in the Computation of VLBI-Based Corrections for the Main Lunisolar Nutation Terms

Very long baseline interferometry (VLBI) is the only technique in space geodesy that can determine directly the celestial pole offsets (CPO). In this paper, we make use of the CPO derived from global VLBI solutions to estimate empirical corrections to the main lunisolar nutation terms included in the IAU 2006/2000A precession–nutation model. In particular, we pay attention to two factors that affect the estimation of such corrections: the celestial reference frame used in the production of the global VLBI solutions and the stochastic model employed in the least-squares adjustment of the corrections. In both cases, we have found that the choice of these aspects has an effect of a few μas in the estimated corrections.

Linking the optically bright Gaia frame to the third International Celestial Reference Frame

<p>The new data release of the Gaia satellite operated by the European Space Agency recently published its 3rd data release (Early Data Release 3, EDR3). The dataset contains astrometric data of about 1.8 billion objects detected at optical frequencies and therefore it outperforms any catalog of astrometric information up to date. The reference frame defined by Gaia EDR3 is aligned to the International Celestial Reference System by referring to counterparts in its realization, the third International Celestial Reference Frame (ICRF3), which is calculated from very long baseline interferometry (VLBI) observations of extragalactic objects at radio frequencies. <br>The Gaia dataset is known to be magnitude-dependent in terms of astrometric calibration. As the objects in ICRF3, although bright at radio frequencies, are mostly faint at optical frequencies, the optically bright Gaia frame has to be linked to ICRF3 by additional counterparts besides objects in ICRF3. The non-rotation of the optically bright Gaia frame is especially important as optically bright objects can, besides astrophysical studies, be used for navigation in space, where other geodetic systems like global navigation satellite systems are out of reach. Suitable additional counterparts are radio stars which are observed by VLBI relative to extragalactic objects in ICRF3. We discuss the orientation and spin differences between the optically bright Gaia EDR3 and VLBI data of radio stars and their impact on the Gaia data usage.</p>

Достижения современной астрометрии, "Земля и Вселенная"

Астрометрия – самая древняя часть астрономии – основа астрономических наблюдений и измерений координат и времени. Главной задачей астрометрии является реализация системы отсчета – той самой инерциальной системы отсчета, о которой говорится в первом законе Ньютона. Лучшей на сегодняшний день реализацией системы отсчета на практике являются звездные каталоги, которые астрономы создавали для этой цели еще со времен античности. Работа над уточнением данных в каталогах привела к открытию прецессии и нутации земной оси, собственных движений и параллаксов звезд, орбитального движения двойных звезд. Наблюдения в радио- и оптическом диапазоне очень далеких объектов – квазаров – привели к созданию самой точной современной системы отсчета International Celestial Reference Frame (ICRF). Именно к ним привязывается система GPS или ГЛОНАСС в навигаторах. XXI век с его вычислительными возможностями привел к созданию звездных каталогов невиданной мощности, содержащих свыше миллиарда объектов. Но основной прорыв, даже революцию, в астрометрии совершили космические наблюдения. Уже два космических аппарата создали звездные каталоги, точность которых фантастична и позволяет прикоснуться к решению таких задач, сама постановка которых была ранее немыслима. Обзору успехов астрометрии за последние два тысячелетия, массовым звездным каталогам и космическим астрометрическим проектам посвящена эта статья.

The third realization of the International Celestial Reference Frame by very long baseline interferometry

A new realization of the International Celestial Reference Frame (ICRF) is presented based on the work achieved by a working group of the International Astronomical Union (IAU) mandated for this purpose. This new realization follows the initial realization of the ICRF completed in 1997 and its successor, ICRF2, adopted as a replacement in 2009. The new frame, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry at the standard geodetic and astrometric radio frequencies (8.4 and 2.3 GHz), supplemented with data collected at higher radio frequencies (24 GHz and dual-frequency 32 and 8.4 GHz) over the past 15 years. State-of-the-art astronomical and geophysical modeling has been used to analyze these data and derive source positions. The modeling integrates, for the first time, the effect of the galactocentric acceleration of the solar system (directly estimated from the data) which, if not considered, induces significant deformation of the frame due to the data span. The new frame includes positions at 8.4 GHz for 4536 extragalactic sources. Of these, 303 sources, uniformly distributed on the sky, are identified as “defining sources” and as such serve to define the axes of the frame. Positions at 8.4 GHz are supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources. In all, ICRF3 comprises 4588 sources, with three-frequency positions available for 600 of these. Source positions have been determined independently at each of the frequencies in order to preserve the underlying astrophysical content behind such positions. They are reported for epoch 2015.0 and must be propagated for observations at other epochs for the most accurate needs, accounting for the acceleration toward the Galactic center, which results in a dipolar proper motion field of amplitude 0.0058 milliarcsecond yr−1 (mas yr−1). The frame is aligned onto the International Celestial Reference System to within the accuracy of ICRF2 and shows a median positional uncertainty of about 0.1 mas in right ascension and 0.2 mas in declination, with a noise floor of 0.03 mas in the individual source coordinates. A subset of 500 sources is found to have extremely accurate positions, in the range of 0.03–0.06 mas, at the traditional 8.4 GHz frequency. Comparing ICRF3 with the recently released Gaia Celestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames, in agreement with the ICRF3 noise level. Significant positional offsets between the three ICRF3 frequencies are detected for about 5% of the sources. Moreover, a notable fraction (22%) of the sources shows optical and radio positions that are significantly offset. There are indications that these positional offsets may be the manifestation of extended source structures. This third realization of the ICRF was adopted by the IAU at its 30th General Assembly in August 2018 and replaced the previous realization, ICRF2, on January 1, 2019.

Relativistic Effects in the Rotation of Jupiter’s Inner Satellites

The most significant relativistic effects (the geodetic precession and the geodetic nutation, which consist of the effect of the geodetic rotation) in the rotation of Jupiter’s inner satellites were investigated in this research. The calculations of the most essential secular and periodic terms of the geodetic rotation were carried out by the method for studying any bodies of the solar system with long-time ephemeris. As a result, for these Jupiter’s satellites, these terms of their geodetic rotation were first determined in the rotational elements with respect to the International Celestial Reference Frame (ICRF) equator and the equinox of the J2000.0 and in the Euler angles relative to their proper coordinate systems. The study shows that in the solar system there are objects with significant geodetic rotation, due primarily to their proximity to the central body, and not to its mass.

A multi-frequency Celestial Reference Frame at S/X, K and X/Ka-band

<p>We present a Celestial Reference Frame (CRF) based on the combination of independent, multi-frequency radio source position catalogs using nearly 40 years of Very Long Baseline Interferometry observations at the standard geodetic frequencies at S/X band and about 15 years of observations at higher frequencies (K and X/Ka). The final catalog contains 4617 sources.</p><p>The novelty in our approach is the combination of independent, multi-frequency radio source position catalogs through a rigorous combination by carrying over the full co-variance information of each catalog through the process of accumulation of normal equation systems instead of using only the positions themselves. Through the novel process of combination, a complete co-variance matrix of the entire set of sources across the three bands is provided. Special validation routines were used to characterize the random and systematic errors between the input reference frames and the combined one.</p><p>The resulting CRF contains precise positions of 4617 compact radio astronomical objects, 4536 measured at 8~Ghz, 824 sources being observed also at 24 GHz and 674 at 32 GHz. The frame is aligned with ICRF3 within ±3 μas and shows an average positional uncertainty of 0.1 mas in right ascension and declination. No significant deformations can be identified. Comparisons with Gaia-CRF remain inconclusive, nonetheless significant differences between all frames can be attested.</p>

Realization of a multifrequency celestial reference frame through a combination of normal equation systems

Context. We present a celestial reference frame (CRF) based on the combination of independent, multifrequency radio source position catalogs using nearly 40 years of very long baseline interferometry observations at the standard geodetic frequencies at SX band and about 15 years of observations at higher frequencies (K and XKa). The final catalog contains 4617 sources. Aims. We produce a multifrequency catalog of radio source positions with full variance–covariance information across all radio source positions of all input catalogs. Methods. We combined three catalogs, one observed at 8 GHz (X band), one at 24 GHz (K band) and one at 32 GHz (Ka band). Rather than only using the radio source positions, we developed a new, rigorous combination approach by carrying over the full covariance information through the process of adding normal equation systems. Special validation routines were used to characterize the random and systematic errors between the input reference frames and the combined catalog. Results. The resulting CRF contains precise positions of 4617 compact radio astronomical objects, 4536 measured at 8 GHz, 824 sources also observed at 24 GHz, and 674 at 32 GHz. The frame is aligned with ICRF3 within ±3 μas and shows an average positional uncertainty of 0.1 mas in right ascension and declination. No significant deformations can be identified. Comparisons with Gaia-CRF remain inconclusive, nonetheless significant differences between all frames can be attested.