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This study is aimed at investigating the dynamic evolution of the orbits of stellar globular clusters (SGCs). To integrate the orbits backward in time, the authors use models of the time-varying potentials derived from cosmological simulations, which are closest to the potential of the Galaxy. This allows for estimating the probability of close passages ("collisions" herein) of SGCs with respect to each other and the Galactic center (GC) in the Galaxy undergoing dynamic changes in the past. To reproduce the dynamics of the Galaxy in time, five of the 54 potentials previously selected from the IllustrisTNG-100 large-scale cosmological database, which are similar in their characteristics (masses and dimensions of the disk and halo) to the current physical parameters of the Milky Way, are used. With these time-varying potentials, we have reproduced the orbital trajectories of 143 SGCs 10 billion years back in time using our original φ-GPU high-order N-body parallel dynamic computer code. Each SGC was treated as a single physical particle with the assigned position and velocity of the cluster center from the Gaia DR2 observations. For each of the potentials, 1000 initial conditions were generated with randomized initial velocities of SGCs within the errors of the observational data. In this study, we consider close passages to be passages with a relative distance of less than 100 pc and a relative speed of less than 250 km s–1. Clusters that pass at longer distances and/or with higher velocities do not have a substantial dynamic effect on the orbits of SGC. In our opinion, the largest changes in the orbits of clusters can be caused by clusters that pass with low velocities at distances smaller than several fold (for example, fourfold) the sum of the radii of the cluster half-masses. Therefore, the authors regard such close passages separately (for brevity, we will call such passages "collisions"). To select clusters that pass at close distances from the GC, the following criterion is applied based only on the relative distance: it must be less than 100 pc. Applying the above criteria, the authors obtained statistically significant rates of close passages of SGCs with respect to each other and to the GC. It has been determined that SGCs during their evolution have approximately ten intersecting trajectories with each other on the average and approximately three to four close passages near the GC in 1 billion years at a distance of 50 pc for each of the chosen potentials. [ABSTRACT FROM AUTHOR]

Observations at permanent stations of the Global Navigation Satellite System (GNSS) are directly related to global and local movements of the Earth's crust and are also affected by various factors, such as the multipath effect and radio noise in the signal. Currently, the influence of such effects can be analyzed and excluded from further processing of GNSS observations. However, there are a number of GNSS stations that deserve more attention for the monitoring of operational stability, because they define the terrestrial implementation of the reference system. The Chernihiv station (CNIV, DOMES 15501M001), which is attributed to the class A for the definition of the European Reference Frame, is an example of such a GNSS station in the territory of Ukraine. In this article, the coordinate time series based on long continuous coordinate time series and the log file of the GNSS station Chernihiv is analyzed. The coordinates of the station during the operation of various equipment are compared, and conclusions about the stability of the station are made. It was found that the station has been working stably and has not had long interruptions in observations since its commissioning. The most significant changes that occurred at the station are associated with the change of equipment. The equipment at the station was changed three times: Trimble equipment was installed in 2005, it was replaced by NovAtel equipment in 2011, and Leica equipment was installed in 2013, which is still in operation. Analysis of the time series of this permanent station shows that there is a certain jump in the study of changes in coordinates simultaneous for all devices when the Leica equipment was installed, which is associated with the change of equipment, namely with a different system of mounting the antenna on the pole. However, in the study of coordinate residuals separately for each equipment, fluctuations in values in winter and summer are observed, which can be related to the structural deformation of the GNSS antenna. [ABSTRACT FROM AUTHOR]

The current state of the research activities of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (MAO NASU) on the use of GNSS technology is presented. Today, MAO operates a permanent GNSS network consisting of five stations (two stations are temporarily inactive). All stations are included in EPN and the EPOS network, GLSV, KHAR, and UZHL stations are also integrated to the IGS network. MAO Operational Centre also provides automatic data transfer from 12 stations installed by other Ukrainian organizations included in the international networks. Data from approximately 300 Ukrainian permanent GNSS stations are stored in the MAO Local Data Centre. The MAO Local Analysis Centre uses Bernese GNSS Software for different types of data processing of GNSS observations. Rapid processing is performed on a daily basis for monitoring the stability of the permanent stations from some Ukrainian RTK networks. All available observations data from the Ukrainian stations for the period from December 7, 1997, to January 28, 2017, were processed according EPN standards during the MAO's second reprocessing campaign and regular processing. As a result, the coordinates in the IGb08 reference frame and tropospheric zenith delays for 233 permanent stations (202 of them are the Ukrainian stations), as well as the velocities for the 128 stations that have observation period more than 3 years, were estimated. The creation and development of local GNSS networks, as well as the long-term filling of databases with high-precision coordinate solutions and estimates of the displacement velocities of GNSS stations, made possible to conduct geodynamic studies at the local level. The strain ellipses and rotation were found based on estimated coordinates and velocities of the GNSS stations. In 2016–2019, Kharkiv National University of Radio Electronics, in cooperation with MAO NASU, performed a number of studies on high-precision GNSS positioning and navigation. New implementations of modern methods and algorithms for the PPP positioning were created and experimentally tested. The errors of the daily floating PPP solutions were 5...8 mm (with a probability of P ≈ 95%) for static mode and 5...8 cm (P ≈ 68%) for kinematic mode. The decimeter/centimeter error of determining the coordinates of current satellites with low Earth orbits (LEO) in the implementation of kinematic floating and fixed PPP solutions was proven. The development of a high-accuracy multiposition phase system of trajectory measurements for field tests of high dynamic aircrafts (HDA) were proposed. The estimated values of the root-mean-square error (RMSE) in determining the HDA motion variables were in the range from 0.05 to 0.40 m for coordinates and from 0.5 to 1.6 cm/s for the velocity vector components. [ABSTRACT FROM AUTHOR]

Crustal strain for the northern region of Ukraine is determined at the GNSS Data Analysis Center of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine based on the triangular method with GNSS stations as vertices. Crustal strain was calculated using the GPS Triangle Strain Calculator software developed by UNAVCO. [ABSTRACT FROM AUTHOR]

Dynamics of horizontal and vertical motions of East European GNSS stations has been studied at the GNSS Data Analysis Center, Main Astronomical Observatory, National Academy of Sciences of Ukraine (MAO NASU). The GNSS station coordinates have been estimated from regional processing and reprocessing performed with the Bernese GNSS Software ver. 5.2 at the GNSS Data Analysis Center, MAO NASU. The velocity values have been computed for 42 GNSS stations, 15 of them located in Ukraine. Global and local offsets of the horizontal and vertical motions of the GNSS stations have been determined. [ABSTRACT FROM AUTHOR]

Data of GPS-satellite observations from permanent stations located in Ukraine (Main Astronomical Observatory of the National Academy of Sciences) and in Eastern Europe were reprocessed using the Bernese GPS software ver. 5.0. The impact of the models of relative and absolute variations of the phase centers corresponding to the different antenna/radome combinations was examined in determining coordinates of the permanent GPS stations. [ABSTRACT FROM AUTHOR]