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We announce the detection of a new large jump in the phase of the free core nutation (FCN). This is only the second such large FCN phase jump in more than thirty years of FCN monitoring by means of a very long baseline interferometry (VLBI) technique. The new event was revealed and confirmed by analyzing two FCN models derived from a long-time series of VLBI observations. The jump started in 2021 and is expected to last until the late fall of 2022. The amplitude of the phase jump is expected to be approximately 3 rad, which is as much as 1.5 times larger than the first phase jump in 1999–2000. A connection of the new FCN phase jump with the recent geomagnetic jerk started in 2020 is suggested. Santiago Belda was partially supported by Generalitat Valenciana (SEJIGENT/2021/001), the European Union–Next Generation EU (ZAMBRANO 21-04) and Ministerio de Ciencia e Innovación (Spanish Project PID2020-119383GB-I00).

2019 marked the 20th anniversary of the International VLBI Service for Geodesy and Astrometry (IVS). This service is the largest and most authoritative organization that coordinates international activities in radio astrometry and VLBI sub-system of space geodesy. Currently, about 60 antennas located in many countries on all continents participate in the IVS observing programs. The IVS Data Centers have accumulated more than 18 million observations obtained during more than 17'000 sessions, including more than 10,000 Intensive sessions for rapid determination of Universal Time. The paper traces the dynamics of IVS development based on statistical processing of the array of observations collected in the IVS Data Centers for the period of 1979-2018. Various statistics by the years, stations, baselines, and radio sources are provided. The evolution of the IVS observational data and the accuracy of results obtained from processing VLBI observations is considered., Comment: Incorporated corrections submitted and accepted for publication in Astronomy Reports

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich. This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively. IAU Commission 19 began in 1919 with the birth of the IAU at the Brussels Conference, where Standing Committee 19 on Latitude Variations was established as one of 32 standing committees. At the first IAU General Assembly in 1922, Standing Committee 19 became Commission 19 “Variation of Latitude”. In the beginning, the main topic of the Commission was the investigation of polar motion. Later, its activities included observations and theory of Earth rotation and connections between Earth orientation variations and geophysical phenomena. As a result, in 1964 at the XII IAU General Assembly, the Commission was renamed “Rotation of the Earth”. The investigation of Earth orientation variations is primarily based on observations of natural and artificial celestial objects. Therefore, maintenance of the international terrestrial and celestial reference frames, as well as the coordinate transformation between the frames and the improvement of the model of precession/nutation, have always been among the primary Commission topics. In 1987, the IAU through Commissions 19 and 31 “Time” established, jointly with the International Union of Geodesy and Geophysics, what is now known as the International Earth Rotation and Reference Systems Service. Commission 19 continued to work to develop methods to improve the accuracy and understanding of Earth orientation variations and related reference systems and frames as well as theoretical studies of Earth rotation. In 2015, Commission 19 was renewed as Commission A2 “Rotation of the Earth” continuing Commission 19’s functions and linking the astronomical community to other scientific organizations such as the International Association of Geodesy, International VLBI Service for Geodesy and Astrometry, International GNSS Service, International Laser Ranging Service and International DORIS Service. During its entire history, IAU Commission 19/A2 has always worked in close cooperation with these and other related services to improve the accuracy and consistency of the Earth orientation parameters and celestial and terrestrial reference frames.

A comparison was made between $Gaia$ magnitudes and magnitudes obtained from ground-based observations for astrometric radio sources . The comparison showed that these magnitudes often not agree well. There may be several reasons for this disagreement. Nevertheless, such an analysis can serve as an additional filter for verification of the object cross-identification. On the other hand, it can help to detect possible errors in optical magnitudes of astrometric radio sources coming from unreliable or inconsistent data sources., Presented at the XIX Serbian astronomical conference (19 SAC), Belgrade, 13-17 October 2020; accepted in the Conference Proceedings

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) provides the geodetic infrastructure needed to monitor the Earth system.. The understanding of forced temporal variations of celestial pole motion (CPM) could bring us significantly closer to meeting the GGOS goals (i.e. 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the ITRF defining parameters). Besides astronomical forcing, CPM excitation depends on the processes in the fluid core and the core-mantle boundary. The same processes are responsible for the variations of the geomagnetic field (GMF). This study investigates the interconnection between the celestial pole offset (CPO) and effective geophysical processes that contribute to the Earth's rotational variation. We use the CPO time series obtained from very long baseline interferometry (VLBI) observations together with the latest GMF data such as geomagnetic jerk and magnetic dipole moment, and a state-of-the-art geomagnetic field model to explore the correlation between CPM and GMF. Our results confirm the findings of previous studies, revealing that substantial free core nutation (FCN) disturbance occurred at the epochs close to the GMJ events. The results also reveal some common features in the FCN and GMF variation, which show the potential to improve knowledge regarding the GMF's contribution to the Earth's rotation.

An analysis of the source position differences between VLBI-based ICRF and $Gaia$-CRF catalogues is a key step in assessing their systematic errors and determining their mutual orientation. One of the main factors that limits the accuracy of determination of the orientation parameters between two frames is the impact of outliers. To mitigate this effect, a new method is proposed based on pixelization data over the equal-area cells, followed by median filtering of the data in each cell. After this, a new data set is formed, consisting of data points near-uniformly distributed over the sphere. The vector spherical harmonics (VSH) decomposition is then applied to this data to finally compute the orientation parameters between ICRF and $Gaia$ frames. To validate the proposed approach, a comparison was made of the ICRF3-SX and $Gaia$~DR2 catalogues using several methods for outliers removal. The results of this work showed that the proposed method is practically insensitive to outliers and thus provides much more robust results of catalogues comparison than the methods used so far. This conclusion was confirmed by analogous test comparison of the $Gaia$~DR2 and OCARS catalogues., Comment: Accepted in MNRAS

Despite continuous increasing of the number of ICRF sources, their sky coverage is still not satisfactory. The goal of this study is to discuss some new considerations for extending the ICRF source list. Statistical analysis of the ICRF catalog allows us to identify less populated sky regions where new ICRF sources or additional observations of the current ICRF sources are most desirable to improve both the uniformity of the source distribution and the uniformity of the distribution of the position errors. It is also desirable to include more sources with high redshift in the ICRF list. These sources may be of interest for astrophysics. To select prospective new ICRF sources, the OCARS catalog is used. The number of sources in OCARS is about three times greater than in the ICRF3, which gives us an opportunity to select new ICRF sources that have already be tested and detected in astrometric and geodetic VLBI experiments., Comment: Presented at the 25th Working Meeting of the European VLBI Group for Geodesy and Astrometry (EVGA), 15-18 March 2021 (virtual). Submitted to the Conference Proceedings. arXiv admin note: text overlap with arXiv:1808.10035

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, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry. The ICRF3 includes positions at 8.4 GHz for 4536 sources, supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources, for a total of 4588 sources. A subset of 303 sources among these, uniformly distributed on the sky, are identified as "defining sources" and as such serve to define the axes of the frame. Source positions 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 (mas/yr). The frame 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 at 8.4 GHz, in the range of 0.03 to 0.06 mas. Comparing ICRF3 with the Gaia Celestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames. 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., Comment: 28 pages, 22 figures

A new method SREAG (spherical rectangular equal-area grid) is proposed to divide a spherical surface into equal-area cells. The method is based on dividing a sphere into latitudinal rings of near-constant width with further splitting each ring into equal-area cells. It is simple in construction and use, and provides more uniform width of the latitudinal rings than other methods of equal-area pixelization of a spherical surface. The new method provides a rectangular grid cells with the latitude- and longitude-oriented boundaries, near-square cells in the equatorial rings, and the closest to uniform width of the latitudinal rings as compared with other equal-area isolatitudinal grids. The binned data is easy to visualize and interpret in terms of the longitude-latitude rectangular coordinate system, natural for astronomy and geodesy. Grids with arbitrary number of rings and, consequently, wide and theoretically unlimited range of cell size can be built by the proposed method. Comparison with other methods used in astronomical research showed the advantages of the new approach in sense of uniformity of the ring width, a wider range of grid resolution, and simplicity of use., Accepted in AJ. Supporting Fortran routines are available at http://www.gaoran.ru/english/as/ac_vlbi/index.htm#SREAG

The third realisation of the International Celestial Reference Frame (ICRF-3) is based on high precision Very Long Baseline Interferometric (VLBI) measurements of positions of 4,588 extragalactic radio sources and 4,536 sources of them are observed at 2.3 and 8.4 GHz. This includes the 303 defining sources which determine the orientation of the frame axes. These reference sources often show extended emissions on milliarcsecond scales that can vary with time, frequency and baseline projection. The effect of source structure on astrometric VLBI positions can be significant and it is therefore important to map the structures of these sources on a regular basis. However, due to the limited number of radio telescopes in the south, it has always been difficult to run dedicated imaging programmes and to monitor source structure, especially for sources below -30$^{\circ}$ declination. We present results from our efforts to image and monitor the intrinsic source structures of ICRF sources using ongoing astrometric and geodetic VLBI observations in the south.

Lidia Dmitrievna Kostina and Natalia Romanovna Persiyaninova left a bright mark in the history of the Pulkovo Observatory, as well as in the history of the domestic and international latitude services. In the first place, they were absolute leaders in the latitude observations with the famous zenith telescope ZTF-135. In 1954-2001, they obtained together more than 66'000 highly accurate latitudes, which make about 2/3 of all the observations made by 23 observers with the ZTF-135 after the WW2. They also provided a large contribution to investigation of the instrumental errors, methods of the data analysis, developing of the observing programs. Their results in studies of the latitude variations and polar motion were also highly recognized by the community., Comment: Presented at the Journees 2019 "Astrometry, Earth rotation and Reference systems in the Gaia era", Paris, France, 7-9 Oct 2019

The final clock product provided by the IGS (International GNSS Service) with a 30/300 s sampling interval for satellites and stations respectively is characterized by accuracy of 75 ± 20 ps RMS (Root Mean Square). Since mid-2012, the MGEX (Multi-GNSS Experiment) experiment provides full products containing all available GNSS (Global Navigation Satellite Systems) satellite signals. In this paper, the phase, frequency and stability of Galileo and GPS (Global Positioning System) on-board oscillators based on the MGEX data are analysed. Moreover, the authors also determined the noise type for specific on-board oscillators depending on GNSS type and frequency standard. The analysis showed different characteristics of clock correction phase and frequency for both satellite systems. Also, each system and clock type has different noise for each averaging time. In order to distinguish more types of noises, three different variants of stability analysis were conducted: Allan deviation (ADEV), modified Allan deviation (MDEV), and Hadamard deviation (HDEV).

A new version of the Optical Characteristics of Astrometric Radio Sources (OCARS) catalog is presented. The catalog includes a list of radio sources observed in astrometric and geodetic VLBI programs since 1979, their redshifts, photometric data in 13 bands in the visible and near infrared, and a table indicating identifications btween the OCARS objects and objects in other catalogs. The main sources of information for the OCARS catalog are the NED and SIMBAD databases, as well as a variety of publications. Targeted observing programs designed to supplement the optical data for the astrometric radio sources have also been organized. The catalog currently contains 9956 sources, of which 5449 have redshifts and 7473 have photometric data. The catalog is updated, on average, once every several weeks, and is continuously augmented with new sources and new optical data.

Three combined celestial pole offset (CPO) series computed at the Paris Observatory (C04), the United States Naval Observatory (USNO), and the International VLBI Service for Geodesy and Astrometry (IVS), as well as six free core nutation (FCN) models, were compared from different perspectives, such as stochastic and systematic differences, and FCN amplitude and phase variations. The differences between the C04 and IVS CPO series were mostly stochastic, whereas a low-frequency bias at the level of several tens of $$\upmu $$ as was found between the C04 and USNO CPO series. The stochastic differences between the C04 and USNO series became considerably smaller when computed at the IVS epochs, which can indicate possible problems with the interpolation of the IVS data at the midnight epochs during the computation of the C04 and USNO series. The comparison of the FCN series showed that the series computed with similar window widths of 1.1–1.2 years were close to one another at a level of 10–20 $$\upmu $$ as, whereas the differences between these series and the series computed with a larger window width of 4 and 7 years reached 100 $$\upmu $$ as. The dependence of the FCN model on the underlying CPO series was investigated. The RMS differences between the FCN models derived from the C04, USNO, and IVS CPO series were at a level of approximately 15 $$\upmu $$ as, which was considerably smaller than the differences among the CPO series. The analysis of the differences between the IVS, C04, and USNO CPO series suggested that the IVS series would be preferable for both precession-nutation and FCN-related studies.

The International VLBI Service for Geodesy and Astrometry (IVS) regularly produces high-quality Earth orientation parameters from observing sessions employing extensive networks or individual baselines. The master schedule is designed according to the telescope days committed by the stations and by the need for dense sampling of the Earth orientation parameters (EOP). In the pre-2011 era, the network constellations with their number of telescopes participating were limited by the playback and baseline capabilities of the hardware (Mark4) correlators. This limitation was overcome by the advent of software correlators, which can now accommodate many more playback units in a flexible configuration. In this paper, we describe the current operations of the IVS with special emphasis on the quality of the polar motion results since these are the only EOP components which can be validated against independent benchmarks. The polar motion results provided by the IVS have improved continuously over the years, now providing an agreement with IGS results at the level of 20–25 $$\upmu $$ as in a WRMS sense. At the end of the paper, an outlook is given for the realization of the VLBI Global Observing System.

The Allan variance (AVAR) was introduced 50 years ago as a statistical tool for assessing the frequency standards deviations. For the past decades, AVAR has increasingly been used in geodesy and astrometry to assess the noise characteristics in geodetic and astrometric time series. A specific feature of astrometric and geodetic measurements, as compared with clock measurements, is that they are generally associated with uncertainties; thus, an appropriate weighting should be applied during data analysis. In addition, some physically connected scalar time series naturally form series of multidimensional vectors. For example, three station coordinates time series $X$ , $Y$ , and $Z$ can be combined to analyze 3-D station position variations. The classical AVAR is not intended for processing unevenly weighted and/or multidimensional data. Therefore, AVAR modifications, namely weighted AVAR (WAVAR), multidimensional AVAR (MAVAR), and weighted multidimensional AVAR (WMAVAR), were introduced to overcome these deficiencies. In this paper, a brief review is given of the experience of using AVAR and its modifications in processing astrogeodetic time series.

A new version of the Optical Characteristics of Astrometric Radio Sources (OCARS) catalog is presented. This compiled catalog includes radio sources observed in different VLBI programs and experiments that result in source position determination, their redshift, and photometry in the visible and near-infrared bands. A cross-identification table between the OCARS and other catalogs is also provided. The status of the catalog as of 2018 September 7 is described in this paper. The OCARS catalog currently contains 6432 sources, of which 3895 have redshift data and 5479 have photometric data. Compared with the previous version, the current version has been enriched with extended redshift and photometry information, and cross-identification with several catalogs in radio, optical, infrared, ultraviolet, X-ray, and gamma-ray bands. The OCARS catalog is updated every few weeks on average to incorporate new data that appear in the NASA/IPAC Extragalactic Database (NED), SIMBAD database, and in the literature.

The activities and some research progress of IAU Commission 19 (C19) - Rotation of the Earth - in the past triennial term (2012-2015) is reported in this paper, including the scientific session and business meeting of C19, as well as a business meeting of the IAU/IAG Joint Working Group of “Theory of Earth Rotation” (JWG_ThER) during the XXIX IAU General Assembly in Hawaii, USA. Three reports of JWG_ThER progress, IERS and IAG, eleven reports of national projects and individual institutions, a short summary of the history and heritage of C19, and an Overview of the status and outlook of new Commission A2 are also presented.

Corrections to the IAU 2000/2006 parameters of the theory of precession and nutation are calculated using five different series—two individual series and three combined series that have been used in the literature for this purpose. A comparison of the sets of corrections obtained using the different datasets indicates significance systematic differences between them, which often substantially exceed the corresponding random errors. At the same time, existing studies have usually used data obtained from one or two series chosen by the authors without special justification. When refining the theory of precession and nutation, it is necessary to consider and compare various available series of VLBI data if one wishes to reduce the systematic errors in an improved model.

According to the decision of the International Astronomical Union (IAU), since 1998 the International Celestial Reference System has been realized by the ICRF catalogue of extragalactic radio source positions obtained from VLBI observations. Over the past years, the accuracy of the ICRF catalogue data has been increased only through an increase in the number and quality of observations and an improvement in the methods of their processing. Both the first ICRF version and the new ICRF2 version adopted by the IAU in 2009 are based on the catalogues obtained at the same VLBI data processing center. However, the experience of classical astrometry shows that a significant increase in the accuracy of the International Celestial Reference Frame can be achieved by creating combined catalogues, such as the fundamental catalogues of star positions. The same approach was applied to improve the ICRF catalogue. Even the first experience of such a combined solution has shown its high efficiency. Here, a new combined catalogue of radio source positions PUL(2013)C02 is presented. Mainly classical methods based on the expansion of the systematic differences between the input catalogues into series of orthogonal functions with additional improvements have been applied for its creation. Comparison of the combined catalogue obtained with the ICRF2 catalogue has shown that the latter is most likely not devoid of systematic errors at a level of 15–20 μas.

Aims. The IVS Working Group on Galactic Aberration (WG8) was established to investigate issues related to incorporating the effect of Galactic aberration in IVS analysis. The circular motion of the solar system barycenter around the Galactic center causes a change in aberration, which in the case of geodetic VLBI observing is over time scales of several decades. One of the specific goals was to recommend a Galactic aberration model to be applied by the IAU ICRF3 working group in the generation of ICRF3 as well as in other IVS analysis. Studies made by working group members have shown that the three-dimensional acceleration vector of the solar system barycenter can be estimated from VLBI delay observations. Methods. Among the working group members, three methods were used to estimate the acceleration vector. One is to directly estimate the acceleration vector as a global parameter. The second is to estimate the acceleration vector from source proper motions determined from estimated source position time series. A third method estimated a global reference frame scale parameter for each source and derived the acceleration vector from these estimates. The acceleration vector estimate consists of a galactocentric component along with the non-galactocentric components. Results. The geodetic reference frame VLBI estimates of the galactocentric aberration constant from the different working group members are in the range 5.1–6.4 μas yr−1. These estimates are relatively close to independent estimates of 4.8–5.4 μas yr−1 that can be derived from astrometric measurements of proper motions and parallaxes of masers in the Milky Way galaxy. Based on the most recent geodetic VLBI solutions, we find an upper bound of 0.8 μas yr−1 for the non-galactocentric component of the secular aberration. Conclusions. The working group made a recommendation only for the galactocentric component of the observed acceleration vector. For the recommended galactocentric aberration constant, the working group chose a geodetic value to be consistent with geodetic VLBI applications. The recommended value 5.8 μas yr−1 was estimated directly in a global solution that used the ICRF3 solution data set: 1979–May 2018.

Variations in free core nutation (FCN) are associated with different processes in the Earth's fluid core and core–mantle coupling. The same processes are generally caused the variations in the geomagnetic field (GMF) particularly the geomagnetic jerks (GMJs), which are rapid changes in GMF secular variations. Therefore, the joint investigation of variations in FCN and GMF can elucidate the Earth's interior and dynamics. In this paper, we investigated the FCN amplitude and phase variations derived from VLBI observations. Comparison of the epochs of the changes in the FCN amplitude and phase with the epochs of the GMJs indicated that the observed extremes in the FCN amplitude and phase variations were closely related to the GMJ epochs. In particular, the FCN amplitude begins to grow one to three years after the GMJs. Thus, processes that cause GMJs are assumed as sources of FCN excitation.

Although the calculation of the weighted mean of several individual values (one of the most frequently used operations in scientific analysis) is straightforward, the calculation of the corresponding uncertainty does not always receive the attention it requires. The application of methods of classical statistics to real observational data is often not justified, since the assumptions lying at the basis of these methods are not satisfied. The presence of systematic uncertainties in the averaged measurements and underestimation of the corresponding random errors used to define the weights are typical examples. A new approach to calculating the uncertainty of weighted mean based on a combination of known methods is considered. The proposed method makes it possible to automatically take into account both the random errors and scatter of the input data, making the method suitable for determining realistic errors for mean values in the case of observational data displaying both good and poor consistency.

During the XXVIII IAU General Assembly in Beijing IAU Commission 19 - Rotation of the Earth - held a business meeting and a scientific meeting. The business meeting was held on Wednesday, 29 August 2012 during session 1 (08:30-10:00). It was attended by about 35 participants, and six reports were given. First the activities of IAU Commission 19 during the past triennium (2009–2012) were highlighted by the Commission president. Afterwards, the Commission secretary presented the results of the elections for the next triennium (2012–2015) and a list of new members of the Commission. The designated Commission president provided an outlook into the next triennium, before the representatives of the international bodies and services IAG (International Association of Geodesy), IVS (International VLBI Service for Geodesy and Astrometry), and IERS (International Earth Rotation and Reference Systems Service) gave reports about recent activities. A summary of the business meeting is given below in Section 2. The scientific meeting was held on Thursday, 20 August 2012 during sessions 1 and 2 (08:30-12:30). Eleven presentations were given, and about 40 participants attended the sessions. Summaries of the presentations are provided below in Section 3.

UT1 estimates obtained from the very long baseline interferometry (VLBI) Intensives data depend on the station displacement model used during processing. In particular, because of seasonal variations, the instantaneous station position during the specific intensive session differs from the position predicted by the linear model generally used. This can cause systematic errors in UT1 Intensives results. In this paper, we first investigated the seasonal signal in the station displacements for the 5 VLBI antennas participating in UT1 Intensives observing programs, along with the 8 collocated GPS stations. It was found that a significant annual term is present in the time series for most stations, and its amplitude can reach 8 mm in the height component, and 2 mm in horizontal components. However, the annual signals found in the displacements of the collocated VLBI and GPS stations at some sites differ substantially in amplitude and phase. The semiannual harmonics are relatively small and unstable, and for most stations no prevailing signal was found in the corresponding frequency band. Then two UT1 Intensives series were computed with and without including the seasonal term found in the previous step in the station movement model. Comparison of these series has shown that neglecting the seasonal station position variations can cause a systematic error in UT1 estimates, which can exceed 1 $$\upmu $$ s, depending on the observing program.

Variations in free core nutation (FCN) are connected with various processes in the Earth’s fluid core and core-mantle coupling, which are also largely responsible for the geomagnetic field variations, particularly the geomagnetic jerks (GMJs). A previous study (Malkin, 2013) revealed that the epochs of the observed extremes in the FCN amplitude and phase variations are close to the GMJ epochs. In this paper, a new evidence of this connection was found. The large FCN amplitude and phase disturbance occurred at the epoch close to the newly revealed GMJ 2011. This event occurred to be the second large change in the FCN amplitude and phase after the 1999 disturbance that is also associated with the GMJ 1999. Moreover, the long-time FCN phase drift had changed suddenly in 1998–1999, immediately before the GMJ 1999, and seemed to change again at the epoch immediately preceding the GMJ 2011. The FCN amplitude showed a general long-time decrease before GMJ 1999, and it subsequently grew until GMJ 2011, and then seemed to decrease again. A smaller FCN change can be observed at the epoch around 2013, which is also suspected as the GMJ epoch. The latter confirms the suggestion that a rapid change in the FCN amplitude and/or phase can be used as an evidence of the GMJ that is not clearly detected from the geomagnetic observations. ARTICLE INFO

Allan variance (AVAR) was first introduced more than 40 years ago as a estimator of the stability of frequency standards, and now it is actively used for investigations of time series in astronomy, geodesy and geodynamics. This method allows one to effectively explore the noise characteristics for various data, such as variations of station and source coordinates, etc. Moreover, this technique can be used to investigate the spectral and fractal structure of the noise in measured data. To process unevenly weighted and multidimensional data, which are usual for many astronomy and geodesy applications, AVAR modifications are proposed by the author. In this paper, a brief overview is given of using of classical and modified AVAR method in astronomy and geodynamics., Comment: 10 pp., corresponds to the journal printed version with several misprints corrected

Investigations of the anomalies in the Earth rotation, in particular, the polar motion components, play an important role in our understanding of the processes that drive changes in the Earth's surface, interior, atmosphere, and ocean. This paper is primarily aimed at investigation of the Chandler wobble (CW) at the whole available 163-year interval to search for the major CW amplitude and phase variations. First, the CW signal was extracted from the IERS (International Earth Rotation and Reference Systems Service) Pole coordinates time series using two digital filters: the singular spectrum analysis and Fourier transform. The CW amplitude and phase variations were examined by means of the wavelet transform and Hilbert transform. Results of our analysis have shown that, besides the well-known CW phase jump in the 1920s, two other large phase jumps have been found in the 1850s and 2000s. As in the 1920s, these phase jumps occurred contemporarily with a sharp decrease in the CW amplitude., Comment: 8 pages, submitted to EPS

The Editor-in-Chief is retracting this article [1] because data from the RFC catalog, http://astrogeo.org/rfc , were used without permission from the copyright holder. The author admitted the decision to retract.

From observations with the 6-m BTA telescope at SAO RAS, we have determined spectroscopic redshifts of seven optical objects whose coordinates coincide with those of radio sources from the list of IVS (International VLBI Service for Geodesy and Astrometry). When compared to radio data, the obtained spectra and redshifts provide evidence for reliable identification of four observed objects; the other three require further study. The distances to the sources derived from our measurements will make it possible to refine the current estimates for parameters of cosmological models based on proper motions of these objects, which are determined from geodetic VLBI observations.

A new method is proposed for predicting universal time based on constructing a general harmonic model for the earth’s rotation and using a modified autoregression technique. A comparison of the predictions made at the Siberian State Research Institute of Metrology (SNIIM) and the International Earth Rotation and Reference Systems Service (IERS) during 2008–2009 shows that the former are more accurate. The efficiency of the new prediction technique for time support of the GLONASS system is demonstrated.

During close angular approaches of solar system planets to astrometric radio sources, the apparent positions of these sources shift due to relativistic effects and, thus, these events may be used for testing the theory of general relativity; this fact was successfully demonstrated in the experiments on the measurements of radio source position shifts during the approaches of Jupiter carried out in 1988 and 2002. An analysis, performed within the frames of the present work, showed that when a source is observed near a planet's disk edge, i.e., practically in the case of occultation, the current experimental accuracy makes it possible to measure the relativistic effects for all planets. However, radio occultations are fairly rare events. At the same time, only Jupiter and Saturn provide noticeable relativistic effects approaching the radio sources at angular distances of about a few planet radii. Our analysis resulted in the creation of a catalog of forthcoming occultations and approaches of planets to astrometric radio sources for the time period of 2008-2050, which can be used for planning experiments on testing gravity theories and other purposes. For all events included in the catalog, the main relativistic effects are calculated both for ground-based and space (Earth-Moon) interferometer baselines., Comment: 8 pages, 5 tables

Free core nutation (FCN) is the main factor that limits the accuracy of the modeling of the motion of Earth's rotational axis in the celestial coordinate system. Several FCN models have been proposed. A comparative analysis is made of the known models including the model proposed by the author. The use of the FCN model is shown to substantially increase the accuracy of the modeling of Earth's rotation. Furthermore, the FCN component extracted from the observed motion of Earth's rotational axis is an important source for the study of the shape and rotation of the Earth's core. A comparison of different FCN models has shown that the proposed model is better than other models if used to extract the geophysical signal (the amplitude and phase of FCN) from observational data., Comment: 8 pages, 3 figures; minor update of the journal published version

A new international Pilot Project for the re-determination of the ICRF was initiated by the International VLBI Service for Geodesy and Astrometry (IVS) in January 2005. The purpose of this project is to compare the individual CRF solutions and to analyze their systematic and random errors with focus on the selection of the optimal strategy for the next ICRF realization. Eight radio source catalogues provided by the IVS Analysis Centers GA, SHAO, DGFI, GIUB-BKG, JPL, MAO NANU, GSFC, USNO were analyzed. In present study, four analytical models were used to investigate the systematic differences between solutions: solid rotation, rotation and deformation (IERS method), and expansion in orthogonal functions: Legendre-Fourier polynomials and spherical functions. It was found that expansions by orthogonal function describe the differences between individual catalogues better than the two former models. Finally, the combined CRF was generated. Using the radio source positions from this combined catalogue for estimation of EOP has shown improvement of the uncertainty of the celestial pole offset time series.

In this paper, we propose to use the scatter of celestial pole offset (CPO) series obtained from VLBI observations as a measure of the accuracy of the celestial reference frame (CRF) realizations. Several scatter indices (SIs) including those proposed for the first time are investigated. The first SI is based on analysis of residuals of CPO series w.r.t. Free Core Nutation (FCN) model. The second group of SIs includes Allan deviation and its extensions, which allow one to treat unequal and multidimensional observations. Application of these criteria to several radio source catalogues showed their ability to perform a preliminary assessment of the quality of radio source catalogues, and 2D Allan deviation estimate seems to be a most sensitive measure. However, in common case, the sensitivity of tested criteria is yet not sufficient to distinguish between radio source catalogues of the highest quality. Proposed extensions of Allan deviation, weighted and multidimensional, can be effectively used also for statistical analysis of astro-geodetic and other time series.

The highly-accurate optical reference frame GCRF (Gaia Celestial Reference Frame) is expected to be available in several years. By the same time, a new version of radio reference frame ICRF (International Celestial Reference Frame) will be also published. The link of GCRF to ICRF will be defined by means of computation of the orientation angles between the two frames using the common extragalactic objects observed in both radio (VLBI) and optics (Gaia). Taking into account the expected accuracy of ICRF and GCRF of the first tens microarcseconds, the link between them should be defined at a microarcsecond level, which requires using the most accurate algorithms and models. One of such models is the Galactic aberration in proper motions, which is not included in the data processing yet. In this paper its impact on the ICRF-Gaia link is estimated. Preliminary results showed that this impact is at a level of about 1 microarcsecond., Presented at the All-Russian Astrometric Conference "Pulkovo-2015", 21-25 September 2015, Pulkovo Observatory, St. Petersburg, Russia; in updated version, misprint in the title is corrected and one reference added

In this National Report are given major results of researches conducted by Russian geodesists in 2015-2018 on the topics of the International Association of Geodesy (IAG) of the International Union of Geodesy and Geophysics (IUGG). This report is prepared by the Section of Geodesy of the National Geophysical Committee of Russia. In the report prepared for the XXVII General Assembly of IUGG (Canada, Montreal, 8-18 July 2019), the results of principal researches in geodesy, geodynamics, gravimetry, in the studies of geodetic reference frame creation and development, Earth's shape and gravity field, Earth's rotation, geodetic theory, its application and some other directions are briefly described. For some objective reasons not all results obtained by Russian scientists on the problems of geodesy are included in the report.

Source position time series produced by International VLBI Service for Geodesy and astrometry (IVS) Analysis Centers were analyzed. These series was computed using different software and analysis strategy. Comparison of this series showed that they have considerably different scatter and systematic behavior. Based on the inspection of all the series, new sources were identified as sources with irregular (non-random) position variations. Two statistics used to estimate the noise level in the time series, namely RMS and ADEV were compared.

In this study, we compared results of determination of the orientation angles between celestial reference frames realized by radio source position catalogues using three methods of accounting for correlation information: using the position errors only, using additionally the correlations between the right ascension and declination (RA/DE correlations) reported in radio source position catalogues published in the IERS format, and using the full covariance matrix. The computations were performed with nine catalogues computed at eight analysis centres. Our analysis has shown that using the RA/DE correlations only slightly influences the computed rotational angles, whereas using the full correlation matrices leads to substantial change in the orientation parameters between the compared catalogues.

We present the results of processing the VLBI observations performed at the Svetloe Observatory of the Institute of Applied Astronomy (IAA), Russian Academy of Sciences, in the period 2003–2005 within the framework of geodynamical programs of the International VLBI Service (IVS) for geodesy and astrometry. We analyzed the observations at the Svetloe Observatory, together with the observations at other stations of the global IVS network, at the IAA using a modified OCCAM package. The package uses new reduction models that decrease the systematic errors of the results. The motion of the stations, primarily of the Svetloe Observatory, is investigated to study the global geotectonic processes. Highly accurate estimates of the coordinate and baseline length variations have been obtained for the first time in Russia from observations at a Russian VLBI station. We determined the coordinates and velocity of the Svetloe VLBI station with errors of ∼2 mm and 3 mm yr−1, respectively, and the baseline lengths between the stations with a sufficiently long observational history with an accuracy of 1–3 mm. The results are shown to be in good agreement with currently available models for the motion of tectonic plates.

The definition of UTC was implemented in 1972, principally to accommodate celestial navigation and follows recommendation 460 of the International Radio Consultative Committee (CCIR) in 1970. Since 1972 the use of electronic means to navigate has overtaken celestial navigation. This fact along with increasing public dissatisfaction with the possible disruption to modern electronic communications and navigation systems caused by the insertion of a leap second has called into question the current definition of UTC. An extensive review of the background and issues relating to the leap second can be found in Nelson, et al (2001).