International Earth Rotation Service Research Articles

Анализ вариаций в движении Северного полюса для повышения точности координатно-временных определений

This article considers a method for predicting the coordinates of the north pole, taking into account the influence of trend, quasi-periodic, regular and high-frequency variations. The necessary graphic material is given, illustrating the picture of the listed variations from 2000 to the present. In particular, a regular change in the amplitude of the Chandler period (СP) is shown. So, from 2006 to 2017, the amplitude of the СP gradually decreases from the maximum to zero, and from 2020 to the present, it has been steadily increasing. Based on the analysis of the behavior of trend changes in the pole coordinates, it is shown that at present the movement of the average pole is directed towards the Greenwich meridian, and not towards North America, as it is given in a number of literary sources. The practical implementation of the proposed method resulted in the creation of programs for predicting the coordinates of the pole and Universal Time for periods from one to 90 days. At the same time, the accuracy of forecasts calculated using these programs exceeds the accuracy of similar forecasts posted on the website of the International Earth Rotation Service. The problem of high-precision prediction of the coordinates of the pole and the Universal Time has now acquired particular relevance, especially in the light of the adopted Federal Target Program for the development of GLONASS up to 2030. In the field of improving the accuracy of ephemeris-time support (ETS).

Comprehensive analysis of superconducting gravimeter data, GPS, and hydrology modelling in support of lunar laser ranging at Apache Point Observatory, New Mexico

SUMMARYThe Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) project began in 2006 using a 3.5 m telescope on a high peak in the Sacramento Mountains of New Mexico. It has been one of the best performing (in terms of returned photons per viewing period and range uncertainty) of a handful of similar sites worldwide. The purpose of installing a superconducting gravimeter (SG) in 2009 was to improve the determination of local deformation, directly affecting the telescope motions, as part of an effort to reduce the error in lunar distance to the mm level. We have now accumulated 10 yr of gravity measurements, together with data from a nearby permanent GPS Plate Boundary Observatory (P027) site. This paper describes the traditional analysis of SG data to produce a local tidal model and to determine a number of necessary dynamic corrections that are specific to the site, of which local atmospheric and hydrology attraction and loading are the most important. Loading corrections are available through the Ecole et Observatoire des Sciences de la Terre (EOST) website as part of the International Earth Rotation Service, and we consider 12 different EOST loading series for global atmosphere and hydrology loading and attraction. Nonetheless, local hydrology can only be addressed at the required level of detail using local models determined directly from the gravity data. We devote a major portion of the paper to the development of several water storage and conceptual tank models that succeed in reducing the large seasonal variance of the SG residuals by more than 90 per cent. Yet there remain episodes in the corrected residuals that depart by up to 4 $\mu {\rm{Gal}}\,$ from the hydrological models, and the mean error of our fit remains between 1.5–2.0 $\mu {\rm{Gal}}\,$ for our two data sets, 2009–2012 and 2013–2018 respectively. Unfortunately the vertical GPS data relies on the UNAVCO processing, and has typically poor vertical accuracy. We find the correlation between the GPS and gravity residuals to be correspondingly weak, and then discuss briefly the incorporation of the displacement and gravity data into the Planetary Ephemeris Program for solving for the lunar distance.

Investigations of Thermal Deformation Based on the Monitoring System of Tianma 13.2 m VGOS Telescope

The thermal expansion of radio telescopes has been recognized as a significant systematic error in Very Long Baseline Interferometry (VLBI) data analysis. Although the thermal expansion model recommended by International Earth Rotation Service (IERS) Conventions 2010 can achieve millimeter accuracy for the International VLBI Service for Geodesy and Astrometry (IVS) routine telescopes, both the International Terrestrial Reference Frame 2020 (ITRF2020) in preparation and the VLBI2010 project encourage the scientific community to reconsider its modeling. To this end, we developed a monitoring system for the Tianma 13.2 m VLBI Global Observing System (VGOS) telescope. Based on the observed data, we refined the IERS expansion model, with results showing that the accuracy of our modified model was improved by 1.9 times. It suggested that the IERS thermal expansion model can achieve the declared millimeter accuracy, and refining its modeling can meet the requirement of 0.3 mm rms stability of the VGOS antenna reference point for the VLBI2010 project.

Comparison of non-tidal loading data for application in a secular terrestrial reference frame

The Deutsches Geodätisches Forschungsinstitut der Technischen Universität München (DGFI-TUM) is one of the three Combination Centres of the International Earth Rotation and Reference Systems Service for the International Terrestrial Reference System (ITRS). In its upcoming realization of the ITRS, the DTRF2020, DGFI-TUM will again correct for non-tidal loading (NTL) effects at the normal equation level. Next to the dedicated NTL data set for the ITRS 2020 realization provided by the Global Geophysical Fluid Center (GGFC), we also considered the data provided by the Earth System Modelling group of the Deutsches GeoForschungsZentrum (ESMGFZ). Besides also comprising all NTL components (atmospheric, oceanic, hydrological) and being mass conserving, the ESMGFZ data has the advantage of daily availability and is already in use at DGFI-TUM. The decision for one or the other data set depends on their suitability for a secular terrestrial reference frame like the DTRF2020, which will be assessed in this work. Although we also compare the site displacements induced by NTL to the residuals of station positions of the Global Navigation Satellite Systems, we will not evaluate the quality of the underlying geophysical models per se. The two data sets differ w.r.t. the underlying hydrological models and the treatment of non-tidal oceanic loading, but the most relevant difference is given in terms of trends in the displacement time-series. After a close investigation of the latter, we finally decided to apply the GGFC contribution to the ITRS 2020 realization in the DTRF2020.Graphical

Prediction of Earth polar motion in short time interval

The accuracy characteristics of mathematical models of the Earth pole motion in a short time intervals have been investigated. The modeling was carried out using two approaches: a numerical-analytical model of the Earth’s rotational motion relative to the center of mass, based on the weighted least squares method and a neural network model for predicting the Earth pole motion. A comparison is made between the standard deviations of short-term forecasts of the Earth pole motion for 1–2 days according to the developed models and the standard deviations of forecasts published by the International Earth Rotation Service in Bulletin A. The models under consideration did not use data smoothing. The model based on neural network approach showed a higher accuracy of short-term forecasting.

The ocean pole tide loading and its effect on GPS position time-series

SUMMARY The changes in the centrifugal force induced by polar motion perturb the ocean, causing an ocean pole tide. The induced displacements due to the ocean pole tide, namely ocean pole tide loading (OPTL), previously ignored, raises the concerns in the GPS data processing with the increasing accuracy of Global Position System (GPS) technique. Though the amplitude of this effect is small, it is worth to demonstrate the magnitude and its impact on GPS solutions of the processing choices for correcting it from observations or onto estimated coordinates. For OPTL modelling, subdaily polar motion can introduce annual variations in OPTL with the small magnitudes in micrometres. The new secular pole model can cause OPTL vertical velocity difference up to 0.03 mm yr−1 compared with the mean pole model of International Earth Rotation and Reference System Service Conventions (2010). The OPTL deformation is dominated by the annual (∼365.25 d) and Chandler (∼433 d) periods, and the largest peak-to-peak variations can reach 0.70, 0.84 and 2.38 mm for East, North and Up components, respectively. We then investigate the effects of OPTL correction on GPS daily positions of 133 stations from 2014 to 2018. The root mean square of OPTL induced GPS daily displacements can reach submillimetre level. In most cases, a posteriori OPTL correction with daily averaged values applied onto the coordinates is acceptable considering the small deviation between the solution removing OPTL from coordinates and the solution correcting OPTL from observations. However, this does not hold when GPS coordinates have been aligned to a secular reference frame, as the annual/Chandler variations of OPTL would be biased. When reference frame alignment is required, either OPTL correction at observation level or a posteriori OPTL correction before reference frame alignment is recommended. Furthermore, we have demonstrated that the effects of OPTL on the annual variations of GPS position time-series can reach as much as 0.4 mm in the verticals. It has limited effect on the linear velocity and reduction of the residuals scatter (annual variations excluded), within Global Geodetic Observing System requirements for the future terrestrial reference frame at millimetre level.

The space\u2013time references of BeiDou Navigation Satellite System

The BeiDou Navigation Satellite System (BDS) is essentially a precise time measurement and time synchronization system for a large-scale space near the Earth. General relativity is the basic theoretical framework for the information processing in the master control station of BDS. Having introduced the basic conceptions of relativistic space–time reference systems, the space–time references of BDS are analyzed and the function and acquisition method of the Earth Orientation Parameters (EOP) are briefly discussed. The basic space reference of BDS is BeiDou Coordinate System (BDCS), and the time standard is the BDS Time (BDT). BDCS and BDT are the realizations of the Geocentric Terrestrial Reference System (GTRS) and the Terrestrial Time (TT) for BDS, respectively. The station coordinates in the BDCS are consistent with those in International Terrestrial Reference Frame (ITRF)2014 at the cm–level and the difference in scale is about 1.1 times 10^{ - 8}. The time deviation of BDT relative to International Atomic Time (TAI) is less than 50 ns and the frequency deviation is less than 2 times 10^{ - 14}. The Geocentric Celestial Reference System (GCRS) and the solar Barycentric Celestial Reference System (BCRS) are also involved in the operation of BDS. The observation models for time synchronization and precise orbit determination are established within the GCRS framework. The coordinate transformation between BDCS and GCRS is consistent with the International Earth Rotation and Reference Systems Service (IERS). In the autonomous operation mode without the support of the ground master control station, Earth Orientation Parameters (EOP) is obtained by means of long-term prediction and on-board observation. The observation models for the on-board astrometry should be established within the BCRS framework.

VLBI and GPS inter- and intra-technique combinations on the observation level for evaluation of TRF and EOP

We study the effects of combination on the observation level (COL) of different space-geodetic techniques and of networks of the same technique and present the corresponding improvement for the determination of station positions and earth orientation parameters. Data from the continuous geodetic very long baseline interferometry (VLBI) campaign CONT17 are used in a batch least-squares (LSQ) estimator. This campaign includes 15 days of observations with two legacy S/X networks, namely Legacy-1 (L1) and Legacy-2 (L2). For this study the VLBI L1 network is used as the base and reference solution. Data from the L1 network are combined first with data from co-located Global Positioning System (GPS) stations by estimating common tropospheric parameters. The derived station positions repeatabilities of the VLBI and GPS networks are evaluated with respect to single-technique solutions. In terms of precision, we find a 25% improvement for the vertical repeatability of the L1 network, and a 10% improvement for the horizontal one. The GPS network also benefits by 20% and 10% in the horizontal and vertical components, respectively. Furthermore, a combined solution using data of the L1 and L2 network is performed by estimating common earth orientation parameters. The combined L1&GPS and L1&L2 solutions are compared to the reference solution by investigating UT1 and polar motion estimates. UT1 is evaluated in terms of mean bias and formal errors with respect to the International Earth Rotation Service (IERS) C04 products which were used as a priori values. The L1&GPS solution has the lowest formal error and mean bias for UT1 with a 30% improvement. The weighted root mean square (WRMS) and weighted mean offset (WMO) differences between the obtained polar motion estimates and the ones derived by the International GNSS Service (IGS) are also compared. We find that the L1&GPS solution gives the lowest WRMS and WMO, exhibiting an average 40% improvement with respect to the reference solution. The presented results highlight the potential of COL for ongoing transition to multi-space geodetic analysis, e.g., Global Navigation Satellite Systems (GNSS) with the next-generation VLBI system.Graphic

ВЛИЯНИЕ РОТАЦИОННОГО ПОСТЭФФЕКТА НА РАЗРЯДКУ В КОРОВОМ СЛОЕ

This work represents experiments which have been performed in an attempt to establish a correla-tion between the constantly changing rotational regime of the planet and the discharge in the crustal layer. From the displacement of the TAI, UTC, and UT1 time scales taken from the site of the Interna-tional Earth Rotation Service (IERS), the average annual and monthly angular rotation rates were cal-culated for the period from 1962 to 2018, and a catalog of earthquakes with 1962 to 2018. The com-piled algorithm and the written program found partial derivatives of the total deforming potential and the distribution of annual number of earthquakes over the Earth's surface per one square kilometer. The article presents the results of analytical analysis and calculations for further investigation of the rotational regime of the Earth and other planets.

The Earth Orientation Parameters Inaccuracy and Spacecraft Motion Prediction Errors. 1. LAGEOS-1

The Earth Orientation Parameters (EOP) uncertainty influence on the gravitational acceleration calculating during the LAGEOS-1 spacecraft orbit prediction is a matter of the article. This influence is compared with the error due to geopotential model variability. The EGM-96 and EGM-2008 geopotential models are considered, with and without taking account of the actual displacement of the Earth’s pole and its rotation irregularity according to final С04 bulletins of the International Earth Rotation and Reference Systems Service (IERS).

The Earth Orientation Parameters Inaccuracy and Spacecraft Motion Prediction Errors. 2. GPS Satellites

The influence of Earth orientation parameters inaccuracy on the Earth’s gravitational acceleration calculation error when predicting spacecraft motion is analyzed, for the example of GPS satellites. This influence is compared with the error introduced by the EGM-96 and EGM-2008 geopotential models, with and without taking account of the actual data of the polar motion and the Earth`s rotation irregularity according to final С04 bulletins of the International Earth Rotation and Reference Systems Service (IERS). Parameters of the GPS spacecraft real orbit are compared with that formed by integration of the perturbed motion equations. The contribution of uncertain factors to the error of modeling the trajectory of navigation system satellites is demonstrated. The results obtained demonstrate the impact level of EOPs errors on the accuracy of navigation satellite orbits prediction and expand the scope for creating models of errors in the ephemeris of global satellite systems due to uncontrolled factors of various nature.

Учет параметров вращения Земли для высокоточного имитационного моделирования движения спутников ГЛОНАСС в интересах гражданских потребителей

The purpose of the study was to consider the parameters of the Earth’s rotation in solving the problem of high-precision simulation of the GLONASS navigation spacecraft motion. The paper introduces an algorithm for the operation of a program-algorithm complex for predicting the motion of the GLONASS satellite, taking into account the mathematical models recommended by the International Earth Rotation Service. The study presents the results of estimating the influence of the Earth’s pole motion, uneven rotation of the Earth, precession and nutation on the GLONASS satellite orbit in the form of errors for longitudinal range, vertical and lateral range. The values of the disturbing accelerations and the degree of their influence on the motion of the GLONASS satellite were estimated. The results of deviations of the orbital parameters are obtained: semi-major axis, eccentricity, inclination, longitude of the ascending node, the argument of the pericenter and the focal parameter at an interval of 30 days under the influence of the parameters of the Earth’s orientation.

Earth Pole Motion Due to Nonstacionary Perturbations

A numerical analytical refined model of a short term forecast of the Earth’s pole motion is proposed. The model allows to increase the accuracy of predicting the coordinates of the pole with observed irregular effects in its motion. A numerical simulation of the oscillatory motion of the Earth’s pole is carried out in comparison with the data of observations and measurements of the International Earth Rotation Service, and the accuracy characteristics of the model are investigated.

Calculation of Tidal Displacements and Tilts for a Nonelastic Rotating Earth

A technique for calculating tidal displacements and tilts for the Earth both with the ocean and without it is presented in this work. This technique mainly follows the technique described in the International Earth Rotation Service Conventions and differs from it in small corrections in formulas for the calculation of displacements for diurnal (21) and semidiurnal (22) waves, as well as in an another set of usual and load Love numbers. Values of the gamma factors for the Earth without the ocean are also presented. Our results can be used for the processing of GNSS and tilt-measuring observations. The obtained results are compared with works by other authors, and the calculation technique is implemented in the new version of the ATLANTIDA3.1_2017 program for predicting parameters of Earth tides.

Determination of the local tidal parameters for the Borowiec station using Satellite Laser Ranging data

The values of regional tidal parameters h2, l2 associated with the tidal variations of ground stations were estimated for the Polish Satellite Laser Ranging (SLR) station Borowiec using SLR data. The study is based on satellite observations taken by the global network of ground stations during the period from January 1, 1999 until January 1, 2019 for monthly orbital arcs of the satellites LAGEOS-1 and LAGEOS-2. The adjusted regional values for h2 equalling 0.7308 ± 0.0008 and l2 equalling 0.1226 ± 0.0003 are discussed and compared with the nominal values of h2 and l2 given in the the International Earth Rotation and Reference Systems Service (IERS) standards and with other estimations of these parameters. Furthermore, the influence of the tidal parameters changes on estimation of the Borowiec station coordinates in the ITRF2014 reference frame was investigated. The analysis was carried out in two variants. The first one consisted in the determination of the Borowiec station coordinates with the use of the nominal values of the tidal parameters: h2 = 0.6078 and l2 = 0.0847 (IERS recommended values). In the second one, the Borowiec station coordinates were determined using the local tidal parameters estimated in this paper (h2 = 0.7308 ± 0.0008 and l2 = 0.1226 ± 0.0003). The differences between X, Y ,Z for Variant 1 and Variant 2 are −3.5, 3.3 and 4.2 mm, respectively.

The ILRS: approaching 20 years and planning for the future

The International Laser Ranging Service (ILRS) was established by the International Association of Geodesy (IAG) in 1998 to support programs in geodesy, geophysics, fundamental constants and lunar research, and to provide the International Earth Rotation Service with data products that are essential to the maintenance and improvement in the International Terrestrial Reference Frame (ITRF), the basis for metric measurements of changes in the Earth and Earth–Moon system. Other scientific products derived from laser ranging include precise geocentric positions and motions of ground stations, satellite orbits, components of Earth’s gravity field and their temporal variations, Earth Orientation Parameters, precise lunar ephemerides and information about the internal structure of the Moon. Laser ranging systems are already measuring the one-way distance to remote optical receivers in space and are performing very accurate time transfer between remote sites in the Earth and in Space. The ILRS works closely with the IAG’s Global Geodetic Observing System. The ILRS develops (1) the standards and specifications necessary for product consistency, and (2) the priorities and tracking strategies required to maximize network efficiency. The service collects, merges, analyzes, archives and distributes satellite and lunar laser ranging data to satisfy a variety of scientific, engineering, and operational needs and encourages the application of new technologies to enhance the quality, quantity, and cost effectiveness of its data products. The ILRS works with (1) new satellite missions in the design and building of retroreflector targets to maximize data quality and quantity, and (2) science programs to optimize scientific data yield. Since its inception, the ILRS has grown to include forty laser ranging stations distributed around the world. The ILRS stations track more than ninety satellites from low Earth orbit (LEO) to the geosynchronous orbit altitude as well as retroreflector arrays on the surface of the Moon. Applications have been expanded to include time transfer, asynchronous ranging for targets at extended ranges, free space quantum telecommunications, and the tracking of space debris. Laser ranging technology is moving to lower energy, higher repetition rates (kHz), single-photon-sensitive detectors, shorter pulse widths, shorter normal point intervals for faster data acquisition, and increased pass interleaving, automated to autonomous operation with remote access, and embedded software for real-time updates and decision making. An example of pass interleaving is presented for the Yarragadee station (see Fig. 4); tracking of LEO satellites is often accommodated during break in LEO and GNSS passes. New satellites arrays provide more compact targets and work continues on the development of lighter less expensive arrays for satellites and the moon. The service now provides operational ITRF products including daily/weekly station positions and daily resolution Earth orientation products; the flow of weekly combination of satellite orbit files for LAGEOS/Etalon-1 and -2 has recently been established. New products are under testing through a pilot project on systematic error monitoring currently underway. The article will give an overview of activities underway within the service, paths forward presently envisioned, and current issues and challenges.

Influence of use of different values of tidal parameters h2, l2 on determination of coordinates of SLR stations

The paper presents the results of coordinates determination for some satellite laser ranging (SLR) stations in the new ITRF2014 system based on LAGEOS-1 and LAGEOS-2 satellite data. The analysis was conducted in two variants. In the first one, coordinates of the SLR stations were estimated with the use of the nominal values of the tidal parameters: h2 = 0.6078 and l2 = 0.0847 (i.e. the standard International Earth Rotation and Reference Systems Service recommended values). In the second, coordinates of the SLR stations were calculated with the use of values of the tidal parameters estimated in the author’s previous paper: h2 = 0.6140 and l2 = 0.0876 (determined from the LAGEOS-1 and LAGEOS-2 data). The influence of the tidal parameters changes on the computation of the stations’ coordinates was investigated. The maximum differences (X, Y, Z(Variant 1) — X, Y, Z(Variant 2)) of about 4 mm, were achieved for Z component for Yarragadee (station ID 7090) and Monument Peak (7110) and of about 3 mm for Y component for Yarragadee (7090) and Changchun (7237) stations. All calculations related to determining the satellite orbits and the SLR stations’ coordinates were carried out with the use of the GEODYN II NASA/GSFC software.

Tectonic plates parameters estimated in International Terrestrial Reference Frame ITRF2008 based on DORIS stations

This paper concerns an analysis of the accuracy of the estimated parameters Ω (Φ, Λ, ω) which define the tectonic plate motions. The study is based on the velocities of station positions in the IERS (International Earth Rotation and Reference Systems Service) which has published new realization of the International Terrestrial Reference System—ITRF2008 for Doppler Orbitography by Radiopositioning Integrated on Satellite DORIS technique. Eurasian, African, Australian, North American, Australian, Pacific, Antarctic and South American plates were used in the analysis. The influence of the number and localization of stations on the plate surface on the estimation accuracy of the tectonic plate motion parameters were discussed. The results were compared with the APKIM 2005 IGN model and our earlier estimation for the SLR technique. In general, a remarkable concurrent agreement between the present and the APKIM 2005 solutions was found.

On the 485-day Mode in the Atmospheric Angular Momentum: Spectral Analysis of IERS Data

The modification of spectral analysis especially intended for studying the disturbing functions of the atmosphere and ocean, as well as the observed polar motion (Wiener–Liouville spectrum), is used. The time series of the atmospheric disturbing functions obtained by the U.S. National Centers for Environmental Prediction (NCEP) of the International Earth Rotation and Reference Systems Service (IERS) for the period from January 1, 1980 to June 20, 2014 ( http://www.iers.org/.cs1?pid=43-1100116 ) are analyzed. It is shown that the baric disturbing function contains a regular mode with a period of ~16 months; the contribution of this mode in the polar motion is estimated.

Influence of subdaily polar motion model on nutation offsets estimated by very long baseline interferometry

This paper studies the connection between the subdaily model for polar motion used in the processing of very long baseline interferometry (VLBI) observations and the estimated nutation offsets. By convention accepted by the International Earth Rotation Service, the subdaily model for polar motion recommended for routine processing of geodetic observations does not contain any daily retrograde terms due to their one-to-one correlation with the nutation. Nevertheless, for a 24-h VLBI solution a part of the signal contained in the polar motion given by the used subdaily model is numerically mistaken for a retrograde daily sidereal signal. This fictitious retrograde daily signal contributes to the estimated nutation, leading to systematic differences between the nutation offsets from VLBI solutions computed with different subdaily polar motion models. We demonstrate this effect using solutions for all suitable 24-h VLBI sessions over a time span of 11 years (2000--2011). By changing the amplitudes of one tidal term in the underlying subdaily model for polar motion and comparing the estimated parameters to the solutions computed with the unchanged subdaily model, the paper shows and explains theoretically the effects produced by the individual subdaily terms on the VLBI nutation estimates.