Satellite Laser Ranging Network Research Articles

Geodetic Datum Realization Using SLR‐GNSS Co‐Location Onboard Galileo and GLONASS

AbstractModern satellites of Global Navigation Satellite Systems (GNSS) are equipped with laser retroreflector arrays for Satellite Laser Ranging (SLR). Laser range observations to GNSS satellites allow for the co‐location of two space geodetic techniques onboard navigation satellites. We search for the best network constraining strategy for the SLR and GNSS stations to realize the geodetic datum using combined microwave‐GNSS and SLR‐to‐GNSS measurements. We find that consistent imposing of no‐net‐translation and no‐net‐rotation for the unified GNSS and SLR network provides the best quality of station coordinates and the best common realization of the terrestrial reference frame (TRF). We employ the space ties using solely space geodetic techniques for the SLR and GNSS station coordinates and confront them with the ground‐based measurements, that is, local ties, conducted at the SLR‐GNSS co‐located sites. The common processing of the GNSS and SLR observations allows for the realization of the TRF using space ties that are independent of the a priori coordinates and independent of the errors in local tie measurements. The agreement of space ties with ground measurements is at the level of 1 mm in terms of long‐term mean values for the co‐located station in Zimmerwald, Switzerland. We also revise the approach for handling the SLR range biases which now considers the impact of the SLR observations to GNSS and LAGEOS satellites. The updated SLR range biases improve the agreement between space ties and local ties from 4.4 to 2.4 mm for the co‐located station in Wettzell, Germany.

Upgraded modelling for the determination of centre of mass corrections of geodetic SLR satellites: impact on key parameters of the terrestrial reference frame

We present improved modelling to compute centre of mass (CoM) offsets for spherical geodetic satellites and derive new values for the spacecraft LAGEOS, LAGEOS-2, Etalon-1/2, Starlette, Stella, LARES, and Ajisai for all system configurations of the laser ranging network, past and present. The updated CoM values, i.e. the average distances between the centres of mass and the optical reflection points, reveal that a major contributor to the error budget in satellite laser ranging (SLR) for reference frame realisation has been a limited accuracy of the CoM corrections hitherto employed to model some of the systems of the SLR network. Our modelling strategy takes into account target characteristics, detection hardware, and mode of operation. For the optical modelling, we have employed high precision data from the single-photon, high-repetition rate SLR system at Herstmonceux (UK), performing the relevant computations for all laser frequencies ever used for tracking operations in the SLR network. For stations that do not operate in single-photon mode, we performed Monte Carlo simulations to estimate the CoM offsets, in this way taking into consideration previously neglected aspects of the detection process. The newly derived values reduce by up to 50% the biases estimated for the two biggest targets, Etalon and Ajisai, and eliminate the generally positive trend in range biases previously observed across the network. These CoM values, mostly lower than the previous ones, imply higher estimated station heights, with a consequent increase in the scale of the terrestrial reference frame that is realised by SLR on the basis of LAGEOS and Etalon observations. This increase in scale, of about 0.65 ppb, is confirmed by geodetic solutions reported here using nineteen years of observations from the ILRS network of tracking stations. We also examine the consequences for the determination of the geocentric gravitational constant, GM, that result from using the new CoM corrections.

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.

Transitioning the NASA SLR network to Event Timing Mode for reduced systematics, improved stability and data precision

NASA’s legacy Satellite Laser Ranging (SLR) network produces about one-third of the global SLR data to support space geodesy. This network of globally distributed stations has been using Time Interval Units (TIU) for range measurements for the last 25 + years. To improve the reliability of the SLR network and satisfy the need for stable millimeter precision data, a phased replacement of the TIUs in the network with picosecond-precise Event Timer Modules was initiated in 2015. This scheme allowed the time of flight and laser transmit epoch measurement to one picosecond resolution. For a network with global scientific impact, transitioning to a new data generation metrological scheme requires significant data scrutiny and long-term science data validation. Any long-term testing/measurement has the potential to interrupt the station’s daily operational data flow to the International Laser Ranging Service (ILRS) as the station under test will have to put its test data into quarantine. We have demonstrated a very effective way to test and implement the new device without removing the old hardware and without the need for the orbit analysis. This operationally noninvasive scheme performed concurrent test measurements enabling uninterrupted operational data flow to the users, while allowing simultaneous test data capture for short- and long-term systematics and stability analysis. Extensive analysis of the test data was performed by the NASA SLR engineering team and the ILRS Analysis Standing Committee, to uncover biases and any dependencies on the satellite ranges (for nonlinear scale issues). Multi-ETM comparison was also performed at two of the SLR stations through the interchange of hardware to establish the inter-device range biases and stability. Such benchmarked hardware was subsequently sent to the remaining stations to allow traceability and normalize the network performance. The range bias intercomparison performed using the multiyear SLR data analysis agreed well with the engineering changes, thus validating the approach to flush out station-specific ranging systematics affecting precise orbit determination. Such an improvement and rebalancing of the current network will allow an orderly transition of the current NASA SLR network operating at a maximum rate of 10 Hz to the NASA next generation Space Geodesy Satellite Laser Ranging (SGSLR) network operating at 2 kHz (McGarry et al. in J Geod, 2018. https://doi.org/10.1007/s00190-018-1191-6; Merkowitz et al. in J Geod, 2018. https://doi.org/10.1007/s00190-018-1204-5).

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.

Impact of the Atmospheric Non-tidal Pressure Loading on Global Geodetic Parameters Based on Satellite Laser Ranging to GNSS

Atmospheric pressure loading plays a crucial role in precise space geodesy, thus its omission, especially the nontidal loading (ANTL) part, leads to the inconsistency between solutions based on the microwave [Global Navigation Satellite Systems (GNSS)] and optical [satellite laser ranging (SLR)] observations. SLR observations are performed only during the cloudless conditions coincident with high values of air pressure that deforms the earth’s crust downward. The systematic shift of the estimated SLR station coordinates, which arises from the ANTL omission, is called the Blue-Sky effect. The offset is related to the long-term averaging of ANTL for SLR observations which are provided in sparse intervals, unlike GNSS, which observes continuously. Based on the ANTL model applied on SLR observations to GNSS, we determined the values of the Blue-Sky effect. The largest values of the Blue-Sky effect are observed for the inland stations, i.e., 2.3, 2.1, 2.0, and 1.9 mm for Svetloe, Potsdam, Baikonur, and Altay, respectively. We also investigate the impact of the omission of ANTL corrections on geodetic parameters, i.e., earth rotation parameters, geocenter motion, precise GNSS orbits, as well as on the global SLR network. The negligence of ANTL causes a systematic effect on geocenter coordinates with the amplitude of the annual signal at the level of 1.9 mm for the Z-component. The omission of ANTL corrections causes also a systematic shift of the multi-GNSS orbit constellation with the amplitude of the annual signal at the level of 2.7 mm for the Z-component.

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.

The Smithsonian Astrophysical Observatory (SAO) and the Centre National d’Études Spatiales (CNES): contributions to the international laser ranging network

The Smithsonian Astrophysical Observatory (SAO) and the Centre National d’Etudes Spatiales (CNES) worked closely together in the early years of the space program to deploy satellite laser ranging (SLR) systems at overseas sites to enhance global coverage to support specific missions. The data were routinely made available for use by the science community for programs in geodesy, gravity field, atmospheric physics, and ultimately for oceanography and geodynamics. SAO and CNES organized campaigns for international participation and loaned each other equipment to enhance the network. SAO and CNES provided technical expertise and advice to a number of other groups as they planned and deployed SLR systems. In this paper, we will discuss the history and the role of the two institutions in the building of the international SLR network.

Future global SLR network evolution and its impact on the terrestrial reference frame

Satellite laser ranging (SLR) is an important technique that contributes to the determination of terrestrial geodetic reference frames, especially to the realization of the origin and the scale of global networks. One of the major limiting factors of SLR-derived reference frame realizations is the datum accuracy which significantly suffers from the current global SLR station distribution. In this paper, the impact of a potential future development of the SLR network on the estimated datum parameters is investigated. The current status of the SLR network is compared to a simulated potential future network featuring additional stations improving the global network geometry. In addition, possible technical advancements resulting in a higher amount of observations are taken into account as well. As a result, we find that the network improvement causes a decrease in the scatter of the network translation parameters of up to 24%, and up to 20% for the scale, whereas the technological improvement causes a reduction in the scatter of up to 27% for the translations and up to 49% for the scale. The Earth orientation parameters benefit by up to 15% from both effects.

Satellite Laser Ranging in Egypt

The paper concerns on the satellite laser ranging (SLR) in Egypt. The three generations which can be loosely defined by their single shot root mean square precision, are discussed. The laser generators used at the Helwan half automatic and full automatic stations are described. The equipments used for the operation of the satellite laser ranging and their upgrading are presented. The observations carried out from Helwan-SLR stations are mentioned. The importance of the satellite laser ranging from Egypt and their contributions to the SLR network are explained. The modification requested for increasing the performance of the Helwan-SLR station is given.

Estimated SLR station position and network frame sensitivity to time-varying gravity

This paper evaluates the sensitivity of ITRF2008-based satellite laser ranging (SLR) station positions estimated weekly using LAGEOS-1/2 data from 1993 to 2012 to non-tidal time-varying gravity (TVG). Two primary methods for modeling TVG from degree-2 are employed. The operational approach applies an annual GRACE-derived field, and IERS recommended linear rates for five coefficients. The experimental approach uses low-order/degree $$4\times 4$$ coefficients estimated weekly from SLR and DORIS processing of up to 11 satellites (tvg4x4). This study shows that the LAGEOS-1/2 orbits and the weekly station solutions are sensitive to more detailed modeling of TVG than prescribed in the current IERS standards. Over 1993–2012 tvg4x4 improves SLR residuals by 18 % and shows 10 % RMS improvement in station stability. Tests suggest that the improved stability of the tvg4x4 POD solution frame may help clarify geophysical signals present in the estimated station position time series. The signals include linear and seasonal station motion, and motion of the TRF origin, particularly in Z. The effect on both POD and the station solutions becomes increasingly evident starting in 2006. Over 2008–2012, the tvg4x4 series improves SLR residuals by 29 %. Use of the GRGS RL02 $$50\times 50$$ series shows similar improvement in POD. Using tvg4x4, secular changes in the TRF origin Z component double over the last decade and although not conclusive, it is consistent with increased geocenter rate expected due to continental ice melt. The test results indicate that accurate modeling of TVG is necessary for improvement of station position estimation using SLR data.

ITRF2005: A new release of the International Terrestrial Reference Frame based on time series of station positions and Earth Orientation Parameters

Unlike the past International Terrestrial Reference Frame (ITRF) versions where global long‐term solutions were combined, the ITRF2005 uses as input data time series (weekly from satellite techniques and 24‐h session‐wise from Very Long Baseline Interferometry) of station positions and daily Earth Orientation Parameters (EOPs). The advantage of using time series of station positions is that it allows to monitor station non‐linear motion and discontinuities and to examine the temporal behavior of the frame physical parameters, namely the origin and the scale. The ITRF2005 origin is defined in such a way that it has zero translations and translation rates with respect to the Earth center of mass, averaged by the Satellite Laser Ranging (SLR) time series spanning 13 years of observations. Its scale is defined by nullifying the scale and its rate with respect to the Very Long Baseline Interferometry (VLBI) time series spanning 26 years of observations. The ITRF2005 orientation (at epoch 2000.0) and its rate are aligned to the ITRF2000 using 70 stations of high geodetic quality. The estimated level of consistency of the ITRF2005 origin (at epoch 2000.0) and its rate with respect to the ITRF2000 is respectively 0.1, 0.8, 5.8 mm and 0.2, 0.1, 1.8 mm/yr along the X, Y and Z‐axis. We estimate the formal errors on these components to be 0.3 mm and 0.3 mm/yr. We believe that this low level of agreement between the two frame origins is most probably due to the poor SLR network geometry and its degradation over time. The ITRF2005 combination involving 84 co‐location sites revealed a scale inconsistency of 1 ppb (6.3 mm at the equator), at epoch 2000.0, and 0.08 ppb/yr between the SLR and VLBI long‐term solutions as obtained by the stacking of their respective time series. Possible causes of this inconsistency may include the poor SLR and VLBI networks and their co‐locations, local tie uncertainties, systematic effects and possible inconsistent model corrections used in the data analysis of both techniques. For the first time of the ITRF history, the ITRF2005 rigorous combination provides self‐consistent series of EOPs, including Polar Motion from VLBI and satellite techniques and Universal Time and Length of Day from VLBI only. A velocity field of 152 sites with an error less than 1.5 mm/yr is used to estimate absolute rotation poles of 15 tectonic plates that are consistent with the ITRF2005 frame. This new absolute plate motion model supersedes and significantly improves that of the ITRF2000 which involved six major tectonic plates.

Atmospheric pressure loading displacement of SLR stations

This paper addresses the local displacement at ground stations of the world-wide Satellite Laser Ranging (SLR) network induced by atmospheric pressure variations. Since currently available modelling options do not satisfy the requirements for the target application (real-time availability, complete coverage of SLR network), a new representation is developed. In a first step, the 3-dimensional displacements are computed from a 6-hourly grid of 1°×1° global pressure data obtained from the ECMWF, for the period 1997–2002. After having been converted into pressure anomalies, this pressure grid is propagated into horizontal and vertical station displacements using Green’s functions and integrating contributions covering the entire globe; oceans are assumed to follow the inverted barometer (IB) approximation. In the next step, a linear regression model is developed for each station that approximates the time-series of the predicted vertical displacements as well as possible; this regression model relates the vertical displacement of a particular station to the local (and instantaneous) pressure anomaly. It is shown that such a simple model may represent the actual vertical displacements with an accuracy of better than 1 mm; horizontal displacements are shown to be negligible. Finally, the regression model is tested on actual SLR data on the satellites LAGEOS-1 and LAGEOS-2, covering the period January 2002 until April 2003 (inclusive). Also, two model elements are shown to be potential risk factors: the global pressure field representation (for the convolution method) and the local reference pressure (for the regression method). The inclusion of the atmospheric pressure displacement model gives improvements on most of the elements of the computations, although the effects are smaller than expected since the nominal effect is absorbed by solved-for satellite parameters.

Determination of Chinese mobile SLR sites?coordinates

The geocentric coordinates of three mobile sites including Fangshan in Beijing, Urumqi in the Xinjiang Uygur Autonomous Region and Lhasa in Tibet are determined in the ITRF2000 frame by means of both the global SLR data and the mobile (SLR) data obtained by a mobile satellite laser ranging (SLR) apparatus from Aug. 2000 to Dec. 2001. The results showed that the geometrical performance of Chinese SLR network has been significantly improved.

Current status and future plans for the Chinese Satellite Laser Ranging Network

The Chinese SLR Network consists of 5 fixedstations located in Shanghai, Changchun, Beijing, Wuhan and Kunming respectively and 2 mobilesystems. The single shot precision for Lageos for these stations is in therange of 12–30 mm. Improvements of system performance for all stations are under way.The Chinese SLR Network participates in the national project “CrustalMovement Observation Network of China”, and is actively supporting the global and regionalprojects, such as ILRS, IERS, WPLTN (Western Pacific Laser Tracking Network) andAPSG (Asia-Pacific Space Geodynamics). The operation center and the datacenter for the Chinese SLR Network is located at Shanghai Observatory. The Lageos dataanalysis report for the global stations has been published every two weeks at the web siteof Shanghai Observatory since September 1999.

Crustal deformations in the mediterranean area computed from SLR and GPS observations

Space geodesy provides an efficient technique to determine contemporary crustal deformations. This paper discusses the computation of tectonic motions in the Mediterranean area from observations acquired by Satellite Laser Ranging (SLR) and Global Positioning System (GPS) instruments. The SLR contribution is based on measurements on LAGEOS-1, taken in the period September 1983 to December 1992. For this analysis, the period was divided into 38 consecutive intervals. An independent station coordinates solution was computed for each subinterval. During this 9.3 year interval, the WEGENER project organized four mobile SLR observation campaigns in the Mediterranean region, resulting in three or four independent position solutions for the majority of the sites in this area. The GPS contribution is based on observations taken during the Euref 89 campaign, which took place from May 16 to 28, 1989, and during the WEGENER/GPS-92 campaign, which took place from July 29 to August 3, 1992, as a sub-campaign for the IGS Epoch'92 campaign. Most of the space-geodetic reference points in the Mediterranean area were occupied with GPS receivers during both campaigns, providing two valuable additional position solutions. The time-series of position solutions for each station are converted into individual station motion vectors. The horizontal components of the latter reveal a clear and consistent picture of the deformations taking place in the area: the northward motion of Arabia, the lateral escape to the west of Anatolia, the NE-SW expansion in the Aegean Basin and the northward motion of Africa being transduced into the central part of the Mediterranean. The results are in agreement with the qualitative kinematical models for this region, derived from geophysical evidence. The vertical station motion solutions are generally very small and insignificant from a statistical point of view. The internal and mutual consistency of the independent SLR and GPS network solutions is also investigated. The agreement is shown to be at the level of 5–20 mm.

Testing geopotential models

The actual accuracy of the geopotential value on the geoid computed from satellite altimetry recently asW0 = (62 636 857.5 ± 1.0) m2 s−2 makes it possible to adopt this quantity as geopotential models testing (GMT) value. However, GMT network should be established consisting of points situated near the gauge stations and of other points at small sea level heights, globally distributed. As numerical example illustrating the GMT method suggested, the recent Satellite Laser Ranging network points have been used.