Satellite Laser Ranging Site Research Articles

The miniSLR: a low-budget, high-performance satellite laser ranging ground station

Satellite Laser Ranging (SLR) is an established technique providing very accurate position measurements of satellites in Earth orbit. However, despite decades of development, it remains a complex and expensive technology, which impedes its further growth to new applications and users. The miniSLR implements a complete SLR system within a small, transportable enclosure. Through this design, costs of ownership can be reduced significantly, and the process of establishing a new SLR site is greatly simplified. A number of novel technical solutions have been implemented to achieve a good laser ranging performance despite the small size and simplified design. Data from the initial six months of test operation have been used to generate a first estimation of the system performance. The data include measurements to many of the important SLR satellites, such as Lageos, Etalon and most of the geodetic and Earth observation missions in LEO. It is shown that the miniSLR achieves sub-centimetre accuracy, comparable with conventional SLR systems. The miniSLR is an engineering station in the International Laser Ranging Service and supplies data to the community. Continuous efforts are undertaken to further improve the system operation and stability.

Precise Orbit Determination of BDS-2 and BDS-3 Using SLR

The BeiDou Navigation Satellite System (BDS) of China is currently in the hybrid-use period of BDS-2 and BDS-3 satellites. All of them are equipped with Laser Retroreflect Arrays (LRAs) for Satellite Laser Ranging (SLR), which can directly obtain an independent, sub-centimetre level of distance measurement. The main purpose of this contribution is to use the solely SLR Normal Points (NPs) data to determinate the precise orbit of BDS-2 and BDS-3 satellites, including one Geostationary Earth Orbit (GEO), three Inclined Geo-Synchronous Orbits (ISGO), and one Medium Earth Orbit (MEO) of BDS-2 satellites, as well as four MEO of BDS-3 satellites, from 1 January to 30 June 2019. The microwave-based orbit from Wuhan University (WUM) are firstly validated to mark and eliminate the bad SLR observations in our preprocessing stage. Then, the 3-, 5-, 7-, and 9-day arc solutions are performed to investigate the impact of the different orbital arc lengths on the quality of SLR-derived orbits and test the optimal solution of the multi-day arc. Moreover, the dependency of SLR-only orbit determination accuracy on the number of SLR observations and the number of SLR sites are discussed to explore the orbit determination quality of the 3-,5-, 7-, and 9-day arc solutions. The results indicate that (1) during the half-year time span of 2019, the overall Root Mean Square (RMS) of SLR validation residuals derived from WUM is 19.0 cm for BDS-2 GEO C01, 5.2–7.3 cm for three BDS-2 IGSO, 3.4 cm for BDS-2 MEO C11, and 4.4–5.7 cm for four BDS-3 MEO satellites respectively. (2) The 9-day arc solutions present the best orbit accuracy in our multi-day SLR-only orbit determination for BDS IGSO and MEO satellites. The 9-day overlaps median RMS of BDS MEO in RTN directions are evaluated at 3.6–5.7, 12.4–21.6, and 15.6–23.9 cm respectively, as well as 5.7–9.6, 15.0–36.8, and 16.5–35.2 cm for the comparison with WUM precise orbits, while these values of BDS IGSO are larger by a factor of about 3–10 than BDS MEO orbits in their corresponding RTN directions. Furthermore, the optimal average 3D-RMS of 9-day overlaps is 0.49 and 1.89 m for BDS MEO and IGSO respectively, as well as 0.55 and 1.85 m in comparison with WUM orbits. Owing to its extremely rare SLR observations, the SLR-only orbit determination accuracy of BDS-2 GEO satellite can only reach a level of 10 metres or worse. (3) To obtain a stable and reliable SLR-only precise orbit, the 7-day to 9-day arc solutions are necessary to provide a sufficient SLR observation quantity and geometry, with more than 50–80 available SLR observations at 5–6 SLR sites that are evenly distributed, both in the Northern and Southern Hemispheres.

Future TRFs and GGOS – where to put the next SLR station?

Abstract. Satellite Laser Ranging (SLR) is one of the four geodetic space techniques contributing to the realisation of terrestrial reference frames (TRFs) as well as to the determination of Earth Rotation Parameters (ERPs). The current SLR tracking network suffers from an insufficient network geometry due to a lack of stations especially in the southern hemisphere. Previous simulation studies have shown that the extension of the global SLR tracking network is indispensable for reaching the target accuracy of future TRFs according to user requests and the ambitious goals of the Global Geodetic Observing System (GGOS). The simulation study presented here puts the focus on a determination of the locations where additional SLR stations are most valuable for an improved estimation of the geodetic parameters. Within the present study, we perform a simulation of a set of stations distributed homogeneously over the globe and compare different solutions, always adding one of these simulated stations to the real SLR station network. This approach has been chosen in order to be able to investigate the deficiencies of the existing SLR network and to judge in which regions on the globe an additional SLR station would be most valuable for the improvement of certain geodetic parameters of SLR-derived reference frames. It is shown that the optimum location of a future SLR station depends on the parameter of interest. In case of the ERPs, the main potential for improvement by a single additional station can be shown for locations in polar regions (improvement for ypole up to 7 %) and for locations along the equator for the lengh of day (LOD, improvement up to 1.5 %). The TRF parameters would benefit from an additional station around the pierce points of the axes of the terrestrial reference frame (improvement for ty up to 4 %), the Arctic and the Pacific Ocean region (tz improved by up to 4.5 %), and the Antarctic and the Indian Ocean region (scale improved by up to 2.2 %). As outcome of this study, it is concluded that an additional SLR site in the Antarctic region might be of first priority, enabling improvements in the pole coordinates and the scale of the TRF; potential further sites are recommended in the equatorial region, especially beneficial for the origin of the realised TRF as well as for LOD.

Impact of network constraining on the terrestrial reference frame realization based on SLR observations to LAGEOS

The Satellite Laser Ranging (SLR) network struggles with some major limitations including an inhomogeneous global station distribution and uneven performance of SLR sites. The International Laser Ranging Service (ILRS) prepares the time-variable list of the most well-performing stations denoted as ‘core sites’ and recommends using them for the terrestrial reference frame (TRF) datum realization in SLR processing. Here, we check how different approaches of the TRF datum realization using minimum constraint conditions (MCs) and the selection of datum-defining stations affect the estimated SLR station coordinates, the terrestrial scale, Earth rotation parameters (ERPs), and geocenter coordinates (GCC). The analyses are based on the processing of the SLR observations to LAGEOS-1/-2 collected between 2010 and 2018. We show that it is essential to reject outlying stations from the reference frame realization to maintain a high quality of SLR-based products. We test station selection criteria based on the Helmert transformation of the network w.r.t. the a priori SLRF2014 coordinates to reject misbehaving stations from the list of datum-defining stations. The 25 mm threshold is optimal to eliminate the epoch-wise temporal deviations and to provide a proper number of datum-defining stations. According to the station selection algorithm, we found that some of the stations that are not included in the list of ILRS core sites could be taken into account as potential core stations in the TRF datum realization. When using a robust station selection for the datum definition, we can improve the station coordinate repeatability by 8%, 4%, and 6%, for the North, East and Up components, respectively. The global distribution of datum-defining stations is also crucial for the estimation of ERPs and GCC. When excluding just two core stations from the SLR network, the amplitude of the annual signal in the GCC estimates is changed by up to 2.2 mm, and the noise of the estimated pole coordinates is substantially increased.

Determination of Global Geodetic Parameters Using Satellite Laser Ranging Measurements to Sentinel-3 Satellites

Sentinel-3A/3B (S3A/B) satellites are equipped with a number of precise instruments dedicated to the measurement of surface topography, sea and land surface temperatures and ocean and land surface color. The high-precision orbit is guaranteed by three instruments: Global Positioning System (GPS) receiver, laser retroreflector dedicated to Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) antenna. In this article, we check the possibility of using SLR observations and GPS-based reduced-dynamic orbits of active S3A/B satellites for the determination of global geodetic parameters, such as geocenter motion, Earth rotation parameters (ERPs) and the realization of the terrestrial reference frame, based on data from 2016-2018. The calculation process was preceded with the estimation of SLR site range biases, different network constraining tests and a different number of orbital arcs in the analyzed solutions. The repeatability of SLR station coordinates based solely on SLR observations to S3A/B is at the level of 8-16 mm by means of interquartile ranges even without network constraining in 7-day solutions. The combined S3A/B and LAGEOS solutions show a consistency of estimated station coordinates better than 13 mm, geocenter coordinates with a RMS of 6 mm, pole coordinates with a RMS of 0.19 mas and Length-of-day with a RMS of 0.07 ms/day when referred to the IERS-14-C04 series. The altimetry observations have to be corrected by the geocenter motion to obtain unbiased estimates of the mean sea level rise. The geocenter motion is typically derived from SLR measurements to passive LAGEOS cannonball-like satellites. We found, however, that SLR observations to active Sentinel satellites are well suited for the determination of global geodetic parameters, such as Earth rotation parameters and geocenter motion, which even further increases the potential applications of Sentinel missions for deriving geophysical parameters.

Estimation of SLR station coordinates by means of SLR measurements to kinematic orbit of LEO satellites

Generally, the dynamical model is used to estimate coordinates of satellite laser ranging (SLR) stations. In this paper, we put forward a kinematic method to estimate the coordinates of SLR stations by using global navigation satellite system (GNSS) technique onboard a low Earth orbiting (LEO) satellite. We applied it to the processing of SLR and GNSS observations of the GRACE-A satellite from January to December 2012. We found that the LEO satellite as a space-based connection between SLR and GNSS techniques allowed the precise estimation of SLR site positions with the agreement level of 2–3 cm with respective to SLRF2014. The SLR network can be attached to the IGS network at the centimeter level. The results show that the proposed method is feasible for determining the coordinates of SLR stations as an idea for the satellite-based integration of different space geodetic techniques.

Characteristics of GOCE orbits based on Satellite Laser Ranging

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) was the first European Space Agency’s (ESA) Earth Explorer core mission. Through its extremely low, about 260 km above the Earth, circular, sun-synchronous orbit, the satellite gained high spatial resolution and accuracy gravity gradient, and ocean circulation data. Global Positioning System (GPS) receivers, mounted on the spacecraft, allowed the determination of reduced-dynamic and kinematic GOCE orbits, whereas Laser Retroreflector Array (LRA) dedicated to Satellite Laser Ranging (SLR) allowed an independent validation of GPS-derived orbits. In this paper, residuals between different GPS-based orbit types and SLR observations are used to investigate the sensitivity and the influence of solar, geomagnetic, and ionospheric activities on the quality of kinematic and reduced-dynamic GOCE orbits. We also analyze the quality of data provided by individual SLR sites, by detecting time biases using ascending and descending sun-synchronous GOCE orbit passes, and the residual analysis of the measurement characteristics, i.e., the dependency of SLR residuals as a function of nadir and horizontal angles. Results show a substantial vulnerability of kinematic orbit solutions to the solar F10.7 index and the ionospheric activity measured by the variations of the Total Electron Content (TEC) values. The sensitivity of kinematic orbits to the three-hour-range KP index is rather minor. The reduced-dynamic orbits are almost insensitive to indices describing ionospheric, solar, and geomagnetic activities. The investigation of individual SLR sites shows that some of them are affected by time bias errors, whereas other demonstrate systematics, such as a dependency between observation residuals and the satellite nadir angle or the horizontal azimuth angle from the SLR station to the direction of the satellite.

Identification and calibration of one-way delays in satellite laser ranging systems

We are reporting on identification and calibration of one-way delays in satellite laser ranging systems. Satellite Laser Ranging (SLR) is a standard technique to measure the distance of satellites as a function of time with millimeter precision and a few millimeters accuracy. For one-way laser ranging, laser time transfer ground to space and for bi- and multi-static laser ranging to space objects identification and measurement of system delays related separately to transmitting and receiving parts of the system are needed. The epochs of transmission and reception of optical signals have to be referred to the coordinated time scale with the accuracy reaching one nanosecond level or better for one-way ranging and space debris multi-static ranging. For transponder ranging and laser time transfer an even higher accuracy of 50ps or better is needed. These accuracy requirements are by several orders of magnitude higher in comparison to standard SLR applications. A new procedure of calibration of one-way delays related to the SLR systems has been developed and tested. The necessary hardware components needed for calibration measurements were designed and developed in a form of a Calibration Device. It consists of a photon counting detector, an epoch timing device and a dedicated signal cable. The signal propagation delays of these components were determined with an accuracy of better than 20ps. The signal propagation delay stability of the Calibration Device is on a level of units of picoseconds over days of operation. The Calibration Device and calibration procedure were tested in real measurements at the SLR site in Graz, Austria. The time needed to complete a calibration of one-way delays of the SLR system is less than two days. The one-way system delays were determined with the accuracy better than 50ps. The measurement principle, Calibration Device and the first results are presented.

Signal Biases Calibration for Precise Orbit Determination of the Chinese Area Positioning System using SLR and C-Band Transfer Ranging Observations

In C-Band transfer measuring systems, the Precise Orbit Determination (POD) precision of Geostationary Earth Orbit (GEO) satellites is limited by signal biases such as the station delay biases, transponder delay biases, the ionospheric delay model bias, etc. In order to improve the POD precision, the signal biases of the Chinese Area Positioning System (CAPS) are calibrated using Satellite Laser Ranging (SLR) and C-Band Transfer Ranging (CBTR) observations. Since the Changchun SLR site and C-Band station are close to each other, the signal biases of the Changchun C-Band station are calibrated using the co-location comparison method. Then the signal biases of the other two CAPS C-Band stations, located in Linton and Kashi, are calibrated using the combined POD method, with the signal biases of the Changchun C-Band station being fixed. After the signal biases are calibrated, the RMS of the line-of-sight residuals of the Changchun SLR observations decrease by 0·4 m, with the percentage improvement being 75·19%.

Evaluation of C-Band Precise Orbit Determination of Geostationary Earth Orbit Satellites based on the Chinese Area Positioning System

Geostationary Earth Orbit (GEO) satellites play a significant role in the space segment of the Chinese Area Navigation System. The C-Band transfer ranging method developed by the National Time Service Center (NTSC) has been widely used in the Chinese Area Positioning System (CAPS), with its advantages of separating satellite ranging from time synchronization and being unaffected by weather. The explicit ranging correction models for the C-Band transfer ranging method are introduced in detail in this article for the first time. Precise Orbit Determination (POD) using C-Band pseudo-range observation of GEO satellite 2010-001A in July 2012 has been conducted. The residual Root Mean Square (RMS) of each site and POD are analysed with orbit difference over overlaps of adjacent orbit arcs. Moreover, the orbit of the GEO satellite has been evaluated by Satellite Laser Ranging (SLR) data from both domestic and foreign SLR sites for the first time. The residual RMS of POD using C-Band observation is better than 0·1 m, and the orbit difference over overlaps of adjacent orbit arcs is better than 3 m. In addition, the residual RMS in line-of-sight for a SLR site in China are better than 1 m, while the RMS for the Yarragadee site in Australia is about 3·4 m. It has been shown that the GEO satellite orbit accords very well with the C-Band observation. Also, the distribution of CAPS stations affects the orbit precision. All sites in CAPS are now located in China with low and medium latitudes. The residual RMS of the SLR site in the southern hemisphere is larger than that of the site in China.

Precision and accuracy of GPS-derived station displacements

Space geodetic techniques like Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) provide valuable input for, e.g., studies of Global Isostatic Adjustment (GIA). This paper discusses the current precision and accuracy of GPS-derived vertical and horizontal station displacements. The precision is evaluated by repeatabilities and solutions computed from different subintervals of the data available. However, due to systematic effects, the precision is often much better than the accuracy. The accuracy is evaluated by comparisons of the space geodetic techniques amongst each other and comparisons with geophysical models for atmospherical and hydrological loading. Besides the analysis of time series, co-located GNSS, SLR, and VLBI sites allow for a comparison of velocities estimated in Terrestrial Reference Frame (TRF) solutions of the different techniques.

Defining the translational velocity of the reference frame of Earth

SUMMARY Earth’s centre is fundamental to geodesy and geoscience because motions of sites on the surface are estimated relative to it. International Terrestrial Reference Frames ITRF2000 and ITRF2005 are defined by the centre of mass of Earth’s system (CM), consisting of solid Earth, the ice sheets, the oceans, and the atmosphere. Satellite LAGEOS rotates about CM; satellite laser ranging (SLR) is used to estimate the velocity of CM relative to sites on the surface. However, ITRF2000 and ITRF2005 differ by 1.8 mm yr −1 , suggesting that the velocity of CM is constrained poorly by SLR. In this study, we define Earth’s reference frame with the centre of mass of solid Earth (CE). Site velocities estimated using SLR, VLBI, GPS and DORIS are corrected for a postglacial rebound model and inverted for the rotational velocities of the plates and the rotational and translational velocities of the four space techniques. Because the postglacial rebound predictions are relative to CE, the velocity of CE relative to sites on the surface is estimated. Because the input SLR site velocities are relative to CM, the output SLR translational velocity is the velocity of CM relative to CE. The estimated velocity of CE does not depend strongly on the postglacial rebound model corrected for. Equal within uncertainties and having a root mean square of 0.5 mm yr −1 are estimates of the velocity of CE determined assuming that plate interiors are deforming radially as predicted by three postglacial rebound models and an estimate of the velocity of CE determined assuming that parts of plate interiors neither beneath nor along the margins of the late Pleistocene ice sheets are not deforming laterally. The velocity of CE equals within uncertainties (probability greater than 5 per cent) the velocity of CM in ITRF2000. The velocity of CE differs significantly (0.05 per cent probability) from the velocity of CM in ITRF2005. Earth’s reference frame (and, we believe, ITRF’s) should be defined with the tightly constrained velocity of CE, not with the poorly constrained velocity of CM. Because CE is believed to be moving relative to CM no faster than 0.5 mm yr −1 , the velocity of CE estimated in this study is likely to be nearer the true velocity of CM than is the velocity of CM estimated using SLR.

Contemporary global horizontal crustal motion

SUMMARY An analysis of Satellite Laser Ranging (SLR) data to the LAGEOS satellite has yielded improved estimates of the horizontal motion for a subset of 34 tracking sites within the global tracking network. The analysis, called SL8.3, utilized data acquired between 1980 January and 1993 June by the global network composed of 71 sites. The solution design provides for the simultaneous estimation of site positions and their velocities within a pre-defined kinematic frame. The solution is statistically rigorous and retains the full correlation information content. Least-squares estimates of relative poles of rotation, which are used to model the motion of one plate relative to another, were made based on the SLR estimated velocities for sites known to be well away from deformation zones. The resulting SLR-based relative rotation poles differ slightly from those of NUVEL-1, but in general, indicate that the magnitude of the SLR implied velocities is slower than those implied by NUVEL-1, consistent with the 4-5 per cent slowing in relative spherical rates noted in earlier comparisons. Spherical rates between sites in western North America support models of extension in the Basin and Range Province and the rotation of the Sierra Nevada microplate. An analysis of the spherical rates crossing the North Atlantic shows that SL8.3 estimated extension between North America-Eurasia sites is generally smaller than those implied by NUVEL-1; meanwhile SL8.3 rates between North America-Africa sites are in better agreement with NUVEL-1, although they are not so well determined. The maintenance and ongoing monitoring of global SLR site kinematics provides a well-defined global reference which will aid in combination global kinematic solutions where information from other technologies are merged (e.g. Very Long Baseline Interferometry and Global Positioning System) and in providing the context for densification studies of regional kinematics derived from terrestrial and Global Positioning System observations.

The role of laser determined orbits in geodesy and geophysics

Since the US National Aeronautics and Space Administration (NASA) launch of the LAGEOS (LAser GEOdynamics Satellite) in May of 1976, a wealth of information enhancing the knowledge of geodesy and geophysics has become available to the scientific community. Both the quality and quantity of data have improved. The precision of satellite laser ranging (SLR) is now at the sub-centimeter level, and cooperation with other nations in the purchase and deployment of SLR systems and laser satellites has led to an extensive SLR data base. Scientists are now able to make precise estimates of the product of the gravitational constant and the Earth's mass (GM), polar motion, Earth rotation, laser station coordinates, distances between stations, and tectonic plate motions. SLR also contributes strongly to improved estimates of the Earth's gravitational field. Recent NASA solutions using the laser ranging data indicate the value of GM is 398600.4408 ± 0.0006 km 3/s 2. The one sigma precision of the other geodetic parameters obtained are on average 1.9 marc sec polar motion, 0.09 ms length of day, 35 mm center of mass geodetic positioning, 20 mm baselines, and 5 mm/yr tectonic plate rates. The precision of a number of these quantities has been confirmed by comparison of an independent data set obtained by very long baseline interferometry antennas located near several of the SLR sites.