Precise Orbit Information Research Articles

LabSAR, a one-GCP coregistration tool for SAR–InSAR local analysis in high-mountain regions

The coregistration of single-look complex (SLC) SAR images for InSAR or offset tracking applications is often performed by using an accurate DEM and precise orbital information. However, in cold regions, such DEMs are rare over high-latitude areas or not up-to-date over fast melting glaciers for instance. To overcome this difficulty, we propose in this article a coregistration method preserving InSAR phase information that only requires a 3D point of reference instead of a full DEM. Developed in a Python toolbox called LabSAR, the proposed method only uses orbital information to coregister the images on the sphere centered on the Earth center passing by the ground control point (GCP). Thanks to the use of the orbital information, the so-called orbital fringes are compensated without having to estimate them. This coregistration method is compared to other approaches in two different types of applications, InSAR and offset tracking, on a PAZ Dual-Pol Temporal Stack covering the Mont Blanc massif (western European Alps). First, InSAR measurements from LabSAR are compared with the results of the Sentinel-1 ESA toolbox (SNAP). The LabSAR interferograms exhibit clearer topographical fringes, with fewer parameters to set. Second, offset tracking based on LabSAR coregistated images is used to measure the displacement of the Bossons glacier. The results are compared with those obtained by a conventional approach developed in the EFIDIR tools. By evaluating the uncertainties of both approaches using displacements over stable areas and the temporal closure error, similar uncertainty values are found. However, velocity values differ between the two approaches, especially in areas where the altitudes are different from the altitude of the reference point. The difference can reach up to 0.06 m/day, which is in the range of the glacier velocity measurement uncertainty given in the literature. The impact of the altitude of the reference point is limited: this single GCP can be chosen at the median altitude of the study area. The error margin on the knowledge of this altitude is 1,000 m, which is sufficient for the altitude to be considered as known for a wide range of study area in the world.

A novel pulsar-based template-independent navigation method

AbstractBecause of the high photon flux, the Crab nebula pulsar is widely used as the observation target for X-ray pulsar-based navigation. The built profile of the Crab pulsar will change over time, however, which means that the pre-calibrated template cannot be used for the long term. In this paper, a novel pulsar-based template-independent navigation method is proposed. The detected phase propagation model is given as a term of position of the vehicle, taking the orbital motion into account. A different method of time-of-arrival process between the recovered profiles is introduced. With the aid of orbital transition matrix, a measurement model is derived to be a term of velocity error of the vehicle varying with time. The state errors of the vehicle are transformed into velocity errors by performing multi-segment observations to achieve the navigation system observability. The navigation equations of the system are then established and can be solved directly. Some simulations are performed to verify the method and suggest that the proposed method is feasible, effective and easy to implement. The precise orbit information of the vehicle can be determined. The state estimation accuracy is basically consistent with the traditional filtering algorithms, and the computational cost is still very low.

Effect of Erroneous Observations in GPS-based LEO Satellite Orbit Determination

Performance of an EKF-based Orbit Determination (OD) algorithm using Global Positioning System (GPS) measurements is examined under erroneous observation conditions. A generalized formulation of a dynamic orbit model is designed to predict the satellite states in an Earth-Centered Earth-Fixed (ECEF) frame. A suitable orbit interpolation method is then chosen for interpolating the GPS satellite position at the measurement time tag before finding the geometric range between the GPS and the LEO satellites. Thereafter, the commonly used Extended Kalman Filter (EKF) algorithm is implemented for the nonlinear OD problem. The proposed algorithm is tested for the precision in OD of two LEO satellites, namely GRACE and Megha-Tropiques based on the available raw GPS measurements in the Receiver Independent Exchange (RINEX) observation files. For the GRACE satellite, the efficacy of the present OD algorithm is validated by comparing the estimated and publicly available precise orbit information. The Megha-Tropiques satellite, however, does not have precise orbit information. Furthermore, it is observed that the RINEX file has several erroneous observations as detected by the Fault Detection and Exclusion (FDE) algorithm in-built in the OD algorithm. A comparison of the estimated satellite position with that obtained from the standard software GNSS-Lab Tool (gLAB) shows that the accuracy of the position estimates varies proportionally with the number of erroneous observations of the Megha-Tropiques satellite.

Non-cooperative LEO Satellite Orbit Determination Based on Single Pass Doppler Measurements

Precise orbit information is the premise of the wide application of low-Earth-orbit (LEO) satellites. Traditional orbit determination (OD) methods have disadvantages of poor reliability, high hardware cost, and long orbit determination time. To account for these issues, we propose an OD framework, using several distributed facilities to acquire Doppler shift values of signals of opportunity (SOPs) during the same pass.Firstly, we propose a search-least squares (LS) algorithm for initial orbit determination (IOD), providing a reliable accurate initial value for precise orbit determination (POD). Moreover, a non-recursive cubature batch filter (NR-CBF) is proposed for POD to reduce linearization errors, avoid tuning parameters, and save the calculation time. Besides, we modify the measurement model by considering signal propagation delays and provide the corresponding theoretical basis. Finally, the real experiment result verifies the feasibility of the proposed method, whose velocity accuracy is at m/s grade.

Rapid Earthquake Source Description Using Variometric-Derived GPS Displacements toward Application to the 2019 Mw 7.1 Ridgecrest Earthquake

AbstractUsing near-field high-rate Global Positioning System (GPS) displacements to invert for earthquake fault slips in real time has the potential to improve the accuracy of earthquake early warning or tsunami early warning. For such applications, real-time retrieval of high-accuracy GPS displacements is essential. Here, we report on rapid modeling of the 2019 Mw 7.1 Ridgecrest earthquake with real-time GPS displacements derived from a variometric approach with readily available broadcast ephemeris. This method calculates station variations in real time by differencing continuous phase observations and does not rely on precise orbit and clock information. The phase ambiguity is also removed, and thus the method does not suffer from a relatively long convergence time. To improve the accuracy of variometric displacements, we use a local spatial filter to decrease the influence of residual errors that cannot be removed completely by the time difference. We invert for the centroid moment tensor, static fault slips, and fault rupture process from the derived displacements. Our results show that all inverted models are available within about 65 s after the origin time of the earthquake and are comparable with models inverted by real-time precise point positioning displacements. This study highlights the great value of variometric displacements for the rapid earthquake source description with only broadcast ephemeris.

In-orbit aerodynamic coefficient measurements using SOAR (Satellite for Orbital Aerodynamics Research)

The Satellite for Orbital Aerodynamics Research (SOAR) is a CubeSat mission, due to be launched in 2021, to investigate the interaction between different materials and the atmospheric flow regime in very low Earth orbits (VLEO). Improving knowledge of the gas–surface interactions at these altitudes and identification of novel materials that can minimise drag or improve aerodynamic control are important for the design of future spacecraft that can operate in lower altitude orbits. Such satellites may be smaller and cheaper to develop or can provide improved Earth observation data or communications link-budgets and latency. In order to achieve these objectives, SOAR features two payloads: (i) a set of steerable fins which provide the ability to expose different materials or surface finishes to the oncoming flow with varying angle of incidence whilst also providing variable geometry to investigate aerostability and aerodynamic control; and (ii) an ion and neutral mass spectrometer with time-of-flight capability which enables accurate measurement of the in-situ flow composition, density, velocity. Using precise orbit and attitude determination information and the measured atmospheric flow characteristics the forces and torques experienced by the satellite in orbit can be studied and estimates of the aerodynamic coefficients calculated. This paper presents the scientific concept and design of the SOAR mission. The methodology for recovery of the aerodynamic coefficients from the measured orbit, attitude, and in-situ atmospheric data using a least-squares orbit determination and free-parameter fitting process is described and the experimental uncertainty of the resolved aerodynamic coefficients is estimated. The presented results indicate that the combination of the satellite design and experimental methodology are capable of clearly illustrating the variation of drag and lift coefficient for differing surface incidence angle. The lowest uncertainties for the drag coefficient measurement are found at approximately 300 km, whilst the measurement of lift coefficient improves for reducing orbital altitude to 200 km.

Matching of TerraSAR-X derived ground control points to optical image patches using deep learning

High resolution synthetic aperture radar (SAR) satellites like TerraSAR-X are capable of acquiring images exhibiting an absolute geolocation accuracy within a few centimeters, mainly because of the availability of precise orbit information and by compensating range delay errors due to atmospheric conditions. In contrast, satellite images from optical missions generally exhibit comparably low geolocation accuracies because of the propagation of errors in angular measurements over large distances. However, a variety of remote sensing applications, such as change detection, surface movement monitoring or ice flow measurements, require precisely geo-referenced and co-registered satellite images. By using Ground Control Points (GCPs) derived from TerraSAR-X, the absolute geolocation accuracy of optical satellite images can be improved. For this purpose, the corresponding matching points in the optical images need to be localized. In this paper, a deep learning based approach is investigated for an automated matching of SAR-derived GCPs to optical image elements. Therefore, a convolutional neural network is pretrained with medium resolution Sentinel-1 and Sentinel-2 imagery and fine-tuned on precisely co-registered TerraSAR-X and Pléiades training image pairs to learn a common descriptor representation. By using these descriptors, the similarity of SAR and optical image patches can be calculated. This similarity metric is then used in a sliding window approach to identify the matching points in the optical reference image. Subsequently, the derived points can be utilized for co-registration of the underlying images. The network is evaluated over nine study areas showing airports and their rural surroundings from several different countries around the world. The results show that based on TerraSAR-X-derived GCPs, corresponding points in the optical image can automatically and reliably be identified with a pixel-level localization accuracy.

Tomography of the Solar Corona with the Wide-Field Imager for the Parker Solar Probe

The Wide-field Imager for the Parker Solar Probe (PSP/WISPR) comprises two telescopes that record white-light total brightness $[B]$ images of the solar corona. Their fields of view cover a widely changing range of heliocentric heights over the 24 highly eccentric orbits planned for the mission. In this work, the capability of PSP/WISPR data to carry out tomographic reconstructions of the three-dimensional (3D) distribution of the coronal electron density is investigated. Based on the precise orbital information of the mission, $B$-images for Orbits 1, 12, and 24 are synthesized from a 3D magnetohydrodynamic model of the corona. For each orbit, the time series of synthetic images is used to carry out a tomographic reconstruction of the coronal electron density and results are compared with the model. As the PSP perihelion decreases, the range of heights that can be tomographically reconstructed progressively shifts to lower values, and the period required to gather the data decreases. For Orbit 1 tomographic reconstruction is not possible. For Orbit 12, tomographic reconstruction is possible in the heliocentric height range ${\approx}\, 5\,\mbox{--}\,15~\mathrm{{R}_{\odot}}$, over a region spanning up to ${\approx}\,160^{\circ}$ in Carrington longitude, with data gathered over a ${\approx}\, 3.4$ day-long period. For Orbit 24, tomographic reconstruction is possible in the heliocentric height range ${\approx}\,3\,\mbox{--}\,10~\mathrm {R}_{\odot}$, over a region spanning up to ${\approx} \,170^{\circ}$ in Carrington longitude, with data gathered over a ${\approx}\, 2.8$ day-long period.

Airborne Pseudolite Distributed Positioning based on Real-time GNSS PPP

Airborne-Pseudolite (A-PL) systems have been proposed to augment Global Navigation Satellite Systems (GNSSs) in difficult areas where GNSS-only navigation cannot be guaranteed due to signal blockages, signal jamming, etc. One of the challenges in realising such a system is to determine the coordinates of the A-PLs to a high accuracy. The GNSS Precise Point Positioning (PPP) technique is a possible alternative to differential GNSS techniques such as those that generate Real-Time Kinematic (RTK) solutions. To enhance the A-PL positioning performance in GNSS challenged areas, it is assumed that inter-PL range measurements are also used in addition to GNSS measurements. When processing these new measurements, cross-correlations among A-PL estimated states introduced during measurement updates need to be accounted for so as to obtain consistent estimated states. In this paper, a distributed algorithm based on a Split Covariance Intersection Filter (SCIF) is proposed. Three commonly used means of implementing the SCIF algorithm are analysed. Another challenge is that real-time GNSS PPP relies on the use of precise satellite orbit and clock information. One problem is that these real-time orbit and satellite clock error corrections may not be always available, especially for moving A-PLs in challenging environments when signal outages occur. To maintain A-PL positioning accuracy using GNSS PPP, it is necessary to predict these error corrections during outages. Different prediction models for orbit and clock error corrections are discussed. A test was conducted to evaluate the A-PL positioning based on GNSS PPP and inter-PL ranges, as well as the performance of error prediction modelling. It was found that GNSS PPP combined with inter-PL ranges could achieve better converged positioning accuracy and a reduction in convergence time of GNSS PPP. However, the performance of GNSS PPP with inter-PL ranges was degraded by observing A-PLs with limited positioning accuracy. Although the performance improvement achieved by the SCIF-based distributed algorithms was smaller than that by the centralised algorithm, greater robustness in dealing with deteriorated observed A-PLs' trajectory data was demonstrated by the distributed algorithms. In addition, short-term prediction models were analysed, and their performance was shown to reduce the effect of error correction outages on A-PL positioning accuracy.

Multi-GNSS precise orbit positioning for airborne gravimetry over Antarctica

Accurate position, velocity, and acceleration information are critical for airborne gravimetry. In Antarctica, there is a sparse distribution of IGS stations, and the rough terrain makes it impractical to set up nearby reference stations. Therefore, traditional differential GPS techniques may be difficult to implement. Precise point positioning (PPP) is independent of reference stations and provides an unlimited operating distance. However, it is highly dependent on precise satellite orbit and clock information, and may be vulnerable to discontinuities of orbit or clock offsets. An extended PPP method, called precise orbit positioning (POP), is implemented towards multi-GNSS. This approach introduces a widely spaced network of stations and is independent of precise clock information, as it estimates satellite clock offsets and drifts "on-the-fly" and only relies on precise orbit information. The advantage of being independent of clock information is that the IGS ultra-rapid (predicted) products can be applied to real-time POP since the orbits can achieve an accuracy of about 5 cm. This becomes significant when applied to airborne gravimetry, as gravity results calculated from gravity measurements and GNSS solutions can be investigated in real time. By means of processing of 5 IGS stations over Antarctica, it turns out that the PPP solution is greatly affected by discontinuities of the IGS orbit and clock offsets at the day boundaries, accompanied with some biases as large as several decimeters in the vertical component. However, the POP solution remains robust and almost no large positioning errors appear, and the accuracy improves by about 50% in the north, east, and up coordinate components. The aircraft positions derived from relative positioning, PPP, and POP during the kinematic flight period generally agree at the decimeter level because of the lack of observed satellites with elevation angles higher than 60°. Nevertheless, the potential for POP to generate centimeter-level kinematic vertical positions over long baselines is illustrated. Moreover, the POP and double-difference derived velocities and accelerations agree with each other well in both static and kinematic flight periods. After a low-pass filter, the GNSS-based accelerations are of the order of 1 mgal and are considered useful for separating the disturbing kinematic accelerations from the gravity measurements.

ANALYSIS OF PERFORMANCE OF FLAT EARTH PHASE REMOVAL METHODS

Abstract. Interferometric synthetic aperture radar (InSAR) has been widely used in remote sensing field, which can reflect actual topographic trend or possible surface deformation. Due to the orbit attitude influence, the flat-earth phase usually causes the interferogram dense and difficult to be used in further procedures. Before phase unwrapping, interferogram must be flattened to derive accurate topographic or deformation information. In this paper, analysis of performance of two methods of flat-earth removal is done. First method uses imaging geometry and second method uses precise orbital information. Further, 3-degree, 5-degree and 7-degree polynomials are fitted in the method using precise orbital information. Validation is done both visually and empirically using entropy as the evaluation index.

Development of Operational Applications for TerraSAR-X

In the course of the TerraSAR-X mission, various new applications based on X-Band Synthetic Aperture Radar (SAR) data have been developed and made available as operational products or services. In this article, we elaborate on proven characteristics of TerraSAR-X that are responsible for development of operational applications. This article is written from the perspective of a commercial data and service provider and the focus is on the following applications with high commercial relevance, and varying operational maturity levels: Surface Movement Monitoring (SMM), Ground Control Point (GCP) extraction and Automatic Target Recognition (ATR). Based on these applications, the article highlights the successful transition of innovative research into sustainable and operational use within various market segments. TerraSAR-X’s high orbit accuracy, its precise radar beam tracing, the high-resolution modes, and high-quality radiometric performance have proven to be the instrument’s advanced characteristics, through, which reliable ground control points and surface movement measurements are obtained. Moreover, TerraSAR-X high-resolution data has been widely exploited for the clarity of its target signatures in the fields of target intelligence and identification. TerraSAR-X’s multi temporal interferometry applications are non-invasive and are now fully standardised autonomous tools to measure surface deformation. In particular, multi-baseline interferometric techniques, such as Persistent Scatter Interferometry (PSI) and Small Baseline Subsets (SBAS) benefit from TerraSAR-X’s highly precise orbit information and phase stability. Similarly, the instrument’s precise orbit information is responsible for sub-metre accuracy of Ground Control Points (GCPs), which are essential inputs for orthorectification of remote sensing imagery, to locate targets, and to precisely georeference a variety of datasets. While geolocation accuracy is an essential ingredient in the intelligence field, high-resolution TerraSAR-X data, particularly in Staring SpotLight mode has been widely used in surveillance, security and reconnaissance applications in real-time and also by automatic or assisted target recognition software.

Self-Shadowing of a Spacecraft in the Computation of Surface Forces. An Example in Planetary Geodesy

Abstract We describe in details the algorithms used in modelling the self-shadowing between spacecraft components, which appears when computing the surface forces as precisely as possible and especially when moving parts are involved. This becomes necessary in planetary geodesy inverse problems using more and more precise orbital information to derive fundamental parameters of geophysical interest. Examples are given with two Mars orbiters, which show significant improvement on drag and solar radiation pressure model multiplying factors, a prerequisite for improving in turn the determination of other global models.

Observations of the Quadrantid meteor shower from 2008 to 2012: Orbits and emission spectra

The activity of the Quadrantids in January during several years (2008, 2010, 2011 and 2012) has been investigated in the framework of the SPanish Meteor Network (SPMN). For this purpose, an array of high-sensitivity CCD video devices and CCD all-sky cameras have been used to obtain multi-station observations of these meteors. These allowed to obtain precise radiant and orbital information about this meteoroid stream. This paper presents a large set of orbital data (namely, 85 orbits) of Quadrantid meteoroids. Most meteors produced by these particles were recorded during the activity peak of this shower. Besides, we discuss four Quadrantid emission spectra. The tensile strength of Quadrantid meteoroids has been also obtained.

Orbits and emission spectra from the 2014 Camelopardalids

We have analyzed the meteor activity associated with meteoroids of fresh dust trails of Comet 209P/LINEAR, which produced an outburst of the Camelopardalid meteor shower (IAU code #451, CAM) in May 2014. With this aim, we have employed an array of high-sensitivity CCD video devices and spectrographs deployed at 10 meteor observing stations in Spain in the framework of the Spanish Meteor Network (SPMN). Additional meteoroid flux data were obtained by means of two forward-scatter radio systems. The observed peak zenithal hourly rate (ZHR) was much lower than expected, of around 20 meteors h-1. Despite of the small meteor flux in the optical range, we have obtained precise atmospheric trajectory, radiant and orbital information for 11 meteor and fireball events associated with this stream. The ablation behaviour and low tensile strength calculated for these particles reveal that Camelopardalid meteoroids are very fragile, mostly pristine aggregates with strength similar to that of the Orionids and the Leonids. The mineral grains seem to be glued together by a volatile phase. We also present and discuss two unique emission spectra produced by two Camelopardalid bright meteors. These suggest a non-chondritic nature for these particles, which exhibit Fe depletion in their composition.

Analysis of Land Deformation on Slope Area using PS InSAR. Case Study: Malang Area

The geographical position of Indonesia located between two continents and oceans is strategic, but at large risk of experiencing various disasters. Climate change and vulnerable location (surrounded by plates and geological faults in the Earth's crust) creates an earthquake-prone region and causes land/mudslides. In this paper, PS InSAR method (Persistent Scatterer Interferometric Synthetic Aperture Radar) is implemented to Phased Array type L-band Synthetic Aperture Radar (PALSAR) data to study the potential damage caused by the earthquake or volcanic eruption in Malang vicinity. By comparing the amplitude images periodically, shifting soil can be determined using precise orbital information. The analysis showed a significant decrease of land deformation on slope area in Klojen district in Malang city, reached up to −7.128 mm/year.

Challenges in Assessing PPP Performance

AbstractThe Precise Point Positioning (PPP) GPS data processing technique has developed over the past 15 years to become a standard method for growing categories of positioning and navigation applications. The technique relies on single receiver point positioning combined with precise satellite orbit and clock information, pseudorange and carrier-phase observable filtering, and additional error modelling. Uniquely addressed is the current accuracy of the technique, and explains the limits of performance, which will be used to define paths for future improvements of the technology.PPP processing of over 300 International GNSS Service (IGS) stations over one week results in few millimetre positioning rms error in the north and east components and centimetre-level in the vertical (all one sigma values). These results are categorised into quality classes in order to analyse the root causes of the resultant errors: "best", "worst", multipath, antenna displacement effects, satellite availability and geometry, etc. Also of interest in PPP performance is solution convergence period. Static, conventional solutions are slow to converge, with approximately 20 minutes required for 95% of solutions to reach a horizontal accuracy of 20 cm or better.From the above analysis, the limitations of PPP and the source of these limitations are isolated, including site displacement modelling, geometric measurement strength, pseudorange multipath and noise, etc. It is argued that new ambiguity resolution and multi-GNSS PPP processing will only partially address these limitations. Improved modelling is required for: site displacement effects, pseudorange noise and multipath, and pseudorange and carrier-phase biases. As well, more robust undifferenced carrier phase ambiguity validation and improved stochastic modelling is required for the pseudorange and carrier-phase observables to allow for more realistic position uncertainties.

On the activity of the γ-Ursae Minorids meteoroid stream in 2010 and 2011

Accurate orbital data obtained for the recently discovered γ -Ursae Minorids meteoroid stream during the 2010 and 2011 Spanish Meteor Network and Finnish Fireball Network observing campaigns are presented. In particular, we focus on an outburst detected in 2010 and on the analysis of the first emission spectrum recorded for a member of this meteoroid stream. An array of high-sensitivity CCD video devices operating from different locations in Spain and Finland was used to perform this study. We have obtained precise trajectory, radiant and orbital information for seven members of this stream. Considerations about its likely parent body based on orbital dissimilarity criteria are made. We also present an estimation of the tensile strength for these meteoroids and a unique emission spectrum of a γ -Ursae Minorid fireball that reveals that the main rocky components have chondritic abundances.

Precise orbit determination of Haiyang-2 using satellite laser ranging

With the successful launch and official commissioning of China’s first dynamic ocean environment satellite Haiyang-2 (HY-2), China’s capabilities for oceanic environment monitoring and oceanic resource detecting have been further improved and enhanced. Precise tracking and orbit determination are not only key technical concerns in the ocean dynamic environment satellite project but also necessary conditions for carrying out related oceanic science research using observational data obtained using spaceborne instruments including radar altimeter. In this study, the current available status of international satellite laser ranging (SLR) monitoring on HY-2 was introduced. Six-months of SLR data from HY-2 were processed to obtain precise satellite orbit information using the dynamic orbit determination method. We carried out a detailed assessment of the SLR orbit accuracy by internal evaluation, comparisons with the orbit derived by the French Doppler orbitography and radio-positioning integrated by satellite (DORIS) system, and station-satellite distance validation. These assessments indicate that the three-dimensional orbital accuracy of HY-2 is about 12.5 cm, and the radial accuracy is better than 3 cm. It provides a good example of the application of international SLR monitoring and precise orbit determination in China’s earth observation satellite project.

TerraSAR-X precise orbit determination with real-time GPS ephemerides

For active and future Earth observation missions, the availability of near real-time precise orbit information is becoming more and more important. The latency and quality of precise orbit determination results is mainly driven by the availability of precise GPS ephemerides and clocks. In order to have high-quality GPS ephemerides and clocks available at real-time, the German Space Operations Center (GSOC) has developed the real-time clock estimation system RETICLE. The system receives data streams with GNSS observations from the global tracking network of the International GNSS Service (IGS) in real-time. Using the known station position, RETICLE estimates precise GPS satellite clock offsets and drifts based on the most recent available ultra rapid predicted orbits provided by the IGS. The clock offset estimates have an accuracy of better than 0.3ns and are globally valid. The latency of the estimated clocks is approximately 7s after the observation epoch.Another limiting factor is the frequency of satellite downlinks and the latency of the data transfer from the ground station to the operations center. Therefore a near real-time scenario using GPS observation data from the TerraSAR-X mission is examined in which the satellite has about one ground station contact per orbit or respectively one contact in 90min. This test campaign shows that precise orbits can be obtained in near real-time. With the use of estimated clocks an orbit accuracy of better than 10cm (3D-RMS) can be obtained. The evaluation of satellite laser ranging (SLR) observations shows residuals of 2.1cm (RMS) for orbits using RECTICLE and residuals of 4.2cm (RMS) for orbits using the IGS ultra rapid ephemerides and clocks products. Hence the use of estimated clocks improves the orbit determination accuracy significantly (∼factor 2) compared to using predicted clocks.