SPOT-2 and TOPEX/Poseidon precise orbit determination from DORIS doppler tracking

The French earth observation satellite SPOT-2 has served as a testbed for precise orbit determination from DORIS doppler tracking in anticipation of the TOPEX/Poseidon mission. Using the most up-to-data gravity field model, JGM-2, a radial orbit accuracy of about 2–9 cm was achieved, with an rms of fit of the tracking data of about 0.64 mm/s. Furthermore, it was found that the coordinates of the ground stations can be determined with an accuracy of the order of 2–5 cm after removal of common rotations, and translations.

Using a slightly different model for atmospheric drag, but the same gravity model, precise orbits of TOPEX/Poseidon from DORIS tracking data were determined with a radial orbit accuracy of the order of 4–5 cm, which is far within the 13 cm mission requirement. This conclusion is based on the analysis of 1-day overlap of successive 11-day orbits, and the comparisons with orbits computed from satellite laser tracking (SLR) and from the combination of SLR and DORIS tracking. Results indicate a consistency between the different orbits of 1–4 cm, 4–20 cm, and 6–13 cm in the radial, cross-track, and along-track directions, respectively. The residual rms is about 4–5 cm for SLR data and 0.56 mm/s for DORIS tracking. These numbers are roughly twice as large as the system noise levels, reflecting the fact that there are still some modeling errors left.     

Gravity field model adjustment from ERS-1 and TOPEX altimetry and SLR tracking using an analytical orbit perturbation theory

The JGM-2 gravity field model has been adjusted using 70 days of ERS-1 and 19 10-day repeat cycles of TOPEX/Poseidon SLR and single satellite altimeter crossover differences. In addition, dual satellite altimeter crossover differences between ERS-1 and TOPEX for the selected 70-day period of ERS-1 and TOPEX repeat cycle 18 have been used in the JGM-2 model adjustment. In the computation of the normal equations, use was made of the analytical Lagrange linear perturbation theory. The single satellite altimeter crossover difference rms was brought down from 15.2 to 14.1 cm for ERS-1 and from 10.6 to 10.5 cm for TOPEX. The dual satellite altimeter crossover difference rms was reduced from 17.7 to 16.9 cm. Furthermore, the weighted rms of fit of SLR measurements was brought down from 16.0 to 14.4 cm for ERS-1, and from 5.4 to 5.0 cm for TOPEX/Poseidon.     

TOPEX/Poseidon precise orbit determination using GPS double difference phase observables

Data from the experimental onboard GPS receiver were used to accurately compute the orbit of TOPEX/Poseidon. This represents a unique opportunity to intercompare with two other classical tracking techniques (SLR and DORIS). A review of the methodology used is given together with current results.     

Dynamic and non-dynamic ERS-1 radial orbit improvement from ERS-1/TOPEX dual-satellite altimetry

Dual-satellite altimeter crossover differences between ERS-1 and TOPEX/Poseidon have been included as supplementary tracking data in ERS-1 orbit computations from SLR and single-satellite crossover differences. It was found that including the dual-satellite crossover differences slightly improves the ERS-1 radial orbit accuracy of about 12 cm for orbits computed with the JGM-2 gravity field and also leads to a better ‘centering’ of the ERS-1 orbit in the terrestrial reference frame defined for TOPEX/Poseidon. In addition to this dynamic orbit improvement technique, a non-dynamic technique has been investigated that removes the larger part of the ERS-1 radial orbit error from the dual-satellite crossover difference residuals. For ERS-1 orbits computed with the GEM-T2 gravity field, it was found that the non-dynamic technique could improve the radial orbit accuracy from 140 cm to the same level of accuracy as the ERS-1 JGM-2 orbits.     

First laser measurements to space debris in Poland

The Borowiec Satellite Laser Ranging station (BORL 7811, Borowiec) being a part of the Space Research Centre of the Polish Academy of Sciences (SRC PAS) went through modernization in 2014–2015. One of the main tasks of the modernization was the installation of a high-energy laser module dedicated to space debris tracking. Surelite III by Continuum is a Nd:YAG pulse laser with 10?Hz repetition rate, a pulse width of 3–5?ns and a pulse energy of 450?mJ for green (532?nm). This new laser unit was integrated with the SLR system at Borowiec performing standard satellite tracking. In 2016 BORL 7811 participated actively to the observational campaigns related to the space debris targets from LEO region managed by the Space Debris Study Group (SDSG) of the International Laser Ranging Service (ILRS).Currently, Borowiec station regularly tracks 36 space debris from the LEO regime, including typical rocket bodies (Russian/Chinese) and cooperative targets like the inactive TOPEX/Poseidon, ENVISAT, OICETS and others. In this paper the first results of space debris laser measurements obtained by the Borowiec station in period August 2016 – January 2017 are presented. The results gained by the SRC PAS Borowiec station confirm the rotation of the defunct TOPEX/Poseidon satellite which spins with a period of approximately 10?s. The novelty of this work is the presentation of the sample results of the Chinese CZ-2C R/B target (NORAD catalogue number 31114) which is equipped (probably) with retroreflectors. Laser measurements to space debris is a very desirable topic for the next years, especially in the context of the Space Surveillance and Tracking (SST) activity. Some targets are very easy to track like defunct ENVISAT or TOPEX/Poseidon. On the other hand, there is a big population of different LEO targets with different orbital and physical parameters, which are challenging for laser ranging like small irregular debris and rocket boosters.     

Minimum time span of TOPEX/Poseidon,Jason-1 and Jason-2 global altimeter data to detect a significant trend and acceleration in sea level change

The present study aims to estimate a minimum time span of the global mean sea level time series (from TOPEX/Poseidon, Jason-1 and Jason-2 satellite altimetry) which is sufficient to detect a statistically meaningful trend in global sea level variation. In addition, the objective of this paper is also to seek a minimum time span required to detect a significant acceleration in sea level change.     

Towards development of a consistent orbit series for TOPEX,Jason-1, and Jason-2

The TOPEX/Poseidon, Jason-1 and Jason-2 set of altimeter data now provide a time series of synoptic observations of the ocean that span nearly 17 years from the launch of TOPEX in 1992. The analysis of the altimeter data including the use of altimetry to monitor the global change in mean sea level requires a stable, accurate, and consistent orbit reference over the entire time span. In this paper, we describe the recomputation of a time series of orbits that rely on a consistent set of reference frames and geophysical models. The recomputed orbits adhere to the IERS 2003 standards for ocean and earth tides, use updates to the ITRF2005 reference frame for both the SLR and DORIS stations, apply GRACE-derived models for modeling of the static and time-variable gravity, implement the University College London (UCL) radiation pressure model for Jason-1, use improved troposphere modeling for the DORIS data, and apply the GOT4.7 ocean tide model for both dynamical ocean tide modeling and for ocean loading. The new TOPEX orbits have a mean SLR fit of 1.79 cm compared to 2.21 cm for the MGDR-B orbits. These new TOPEX orbits agree radially with independent SLR/crossover orbits at 0.70 cm RMS, and the orbit accuracy is estimated at 1.5–2.0 cm RMS over the entire TOPEX time series. The recomputed Jason-1 orbits agree radially with the Jason-1 GDR-C orbits at 1.08 cm RMS. The GSFC SLR/DORIS dynamic and reduced-dynamic orbits for Jason-2 agree radially with independent orbits from the CNES and JPL at 0.70–1.06 cm RMS. Applying these new orbits, and using the latest altimeter corrections for TOPEX, Jason-1, and Jason-2 from September 1992 to May 2009, we find a global rate in mean sea level of 3.0 ± 0.4 mm/yr.     

Improvement of the SLR Borowiec station position in the global network ITRF91

This paper describes briefly two station coordinates solutions, the first one computed at Space Research Centre (SRC) using tracking data from Lageos satellite, and the second one computed at European Space Operations Centre (ESOC) using tracking data from Lageos 1, ERS-1 and TOPEX/Poseidon in a multi-arc solution. In particular the solution computed for the Borowiec station in ITRF91 system is described extensively. The Borowiec station position was estimated simultaneousely, considering or not the existence of range biases, with other geophysical parameters such as: daily polar motion parameters x_p, y_p, ocean tide coefficients, earth gravitational constant GM, etc.     

Simulation of long-term rotational dynamics of large space debris: A TOPEX/Poseidon case study

Accurate knowledge of the rotational dynamics of a large space debris is crucial for space situational awareness (SSA), whether it be for accurate orbital predictions needed for satellite conjunction analyses or for the success of an eventual active debris removal mission charged with stabilization, capture and removal of debris from orbit. In this light, the attitude dynamics of an inoperative satellite of great interest to the space debris community, the joint French and American spacecraft TOPEX/Poseidon, is explored. A comparison of simulation results with observations obtained from high-frequency satellite range measurements is made, showing that the spacecraft is currently spinning about its minor principal axis in a stable manner. Predictions of the evolution of its attitude motion to 2030 are presented, emphasizing the uncertainty on those estimates due to internal energy dissipation, which could cause a change of its spin state in the future. The effect of solar radiation pressure and the eddy-current torque are investigated in detail, and insights into some of the satellite’s missing properties are provided. These results are obtained using a novel, open-source, coupled orbit-attitude propagation software, the Debris SPin/Orbit Simulation Environment (D-SPOSE), whose primary goal is the study of the long-term evolution of the attitude dynamics of large space debris.     

Evaluation of JGM 2 geopotential errors from Geosat, TOPEX/Poseidon and ERS-1 crossover altimetry

World-ocean distribution of the crossover altimetry data from Geosat, TOPEX/Poseidon (T/P) and the ERS 1 missions have provided strong independent evidence that NASA's/CSR's JGM 2 geopotential model (70 × 70 in spherical harmonics) yields accurate radial ephemerides for these satellites. In testing the sea height crossover differences found from altimetry and JGM 2 orbits for these satellites, we have used the sea height differences themselves (of ascending minus descending passes averaged at each location over many exact repeat cycles) and the Lumped Latitude Coefficients (LLC) derived from them. For Geosat we find the geopotential-induced LLC errors (exclusive of non-gravitational and initial state discrepancies) mostly below 6 cm, for TOPEX the corresponding errors are usually below 2 cm, and for ERS 1 (35-day cycle) they are generally below 5 cm. In addition, we have found that these observations agree well overall with predictions of accuracy derived from the JGM 2 variance-covariance matrix; the corresponding projected LLC errors for Geosat, T/P, and ERS 1 are usually between 1 and 4 cm, 1 – 2 cm, and 1 – 4 cm, respectively (they depend on the filtering of long-periodic perturbations and on the order of the LLC). This agreement is especially impressive for ERS 1 since no data of any kind from this mission was used in forming JGM 2.

The observed crossover differences for Geosat, T/P and ERS 1 are 8, 3, and 11 cm (rms), respectively. These observations also agree well with prediction of accuracy derived from the JGM 2 variance-covariance matrix; the corresponding projected crossover errors for Geosat and T/P are 8 cm and 2.3 cm, respectively. The precision of our mean difference observations is about 3 cm for Geosat (approx. 24,000 observations), 1.5 cm for T/P (approx. 6,000 observations) and 5 cm for ERS 1 (approx. 44,000 observations). Thus, these “global” independent data should provide a valuable new source for improving geopotential models. Our results show the need for further correction of the low order JGM 2 geopotential as well as certain resonant orders for all 3 satellites.     

A point-wise least squares spectral analysis (LSSA) of the Caspian Sea level fluctuations,using TOPEX/Poseidon and Jason-1 observations

The Caspian Sea has displayed considerable fluctuations in its water level during the past century. Knowledge of such fluctuation is vital for understanding the local hydrological cycles, climate of the region, and construction activities within the sea and along its shorelines. This study established a point-wise satellite altimetry approach to monitor the fluctuations of the Caspian Sea using a complete dataset of TOPEX/Poseidon for the period 1993 to the middle of 2002, and its follow-on Jason-1 for the period 2002 to August 2009. Therefore, 280 virtual time-series were constructed to monitor the fluctuations. The least squares spectral analysis (LSSA) method is, then employed to find the most significant frequencies of the time-series, while the statistical method of principle component analysis (PCA) is applied to extract the dominant variability of level variations. The study also used the observations of TOPEX/Poseidon and Jason-1 over the Volga River along with 5 years of Volga’s water discharge to study its influence on the Caspian Sea level changes. The LSSA results indicate that the lunar semidiurnal (M2) and the Sun semidiurnal (S2) frequencies are the main tidal frequencies of the Caspian Sea with the mean amplitude of 4.2 and 2.8 cm, respectively. A statistically significant long-term frequency (12.5-years period) is also found from altimetry and tide gauge observations. A phase lag, related to the inter-annual frequencies of the Volga River was detected from the point-wise time-series showing level propagation from the northwest to the southeast of the sea. The cross-correlation between the power spectrum of Volga and that of the northern-most, middle, and southern-most points within the Caspian Sea were respectively 0.63, 0.51 and 0.4 of zero-lag correlation, corroborating the influence of the Volga River. The result of PCA also shows that different parts of the Caspian Sea exhibit different amplitudes of level variations, indicating that the point-wise approach, when employing all available satellite measurements could be a suitable method for a preliminary monitoring of this inland water resource as it gives accurate local fluctuations.     

Accuracy assessment of TDRSS-based TOPEX/Poseidon orbit determination

Orbit determination results are obtained for the Ocean Topography Experiment (TOPEX)/Poseidon spacecraft by the Goddard Space Flight Center (GSFC) Flight Dynamics Division (FDD) using a batch-least-squares estimator available in the Goddard Trajector Determination System (GTDS) to process Tracking and Data Relay Satellite (TDRS) System (TDRSS) measurements. The GTDS orbit solutions are compared with the definitive Precision Orbit Determination (POD) orbit solutions. The root-mean-square (RMS) solution difference in the radial component is 28 centimeters.     

ERS-1 radial position refinement by dual crossover analysis with TOPEX/Poseidon

Achieving orbital accuracies in the radial direction for ERS-1 commensurate with those for TOPEX/Poseidon is of utmost importance for the integration of the two altimeter data sets. This paper outlines a procedure whereby the radial orbit error for ERS-1 is recovered as a time series expansion in the form of a finite Fourier series with additional terms for atmospheric drag, solar radiation pressure, and initial state vector mismodelling. Using a least squares collocation method with constraints derived from the JGM2 gravity field co-variance matrix, the radial error is recovered using both dual crossovers and ERS-1 single satellite crossovers. Aggregate arcs are then used to derive the ERS-1 orbit error over the repeat period of 35 days. The results are presented in the improvement of fit in the dual crossover, ERS-1 crossover and altimetry data sets as well as the recovery of an altimeter bias for the two satellites.     

Precision orbit determination for TOPEX/Poseidon using TDRSS doppler tracking data

Precision orbit determination on the TOPEX/Poseidon (T/P) altimeter satellite is now being routinely achieved with sub-5cm radial and sub-15 cm total positioning accuracy using state-of-the-art modeling with precision tracking provided by a combination of: (a) global Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), or (b) the Global Positioning System (GPS) Constellation which provides pseudo-range and carrier phase observations. The geostationary Tracking and Data Relay Satellite System (TDRSS) satellites are providing the operational tracking and communication support for this mission. The TDRSS Doppler data are of high precision (0.3 mm/s nominal noise levels). Unlike other satellite missions supported operationally by TDRSS, T/P has high quality independent tracking which enables absolute orbit accuracy assessments. In addition, the T/P satellite provides extensive geometry for positioning a satellite at geostationary altitude, and thus the TDRSS-T/P data provides an excellent means for determining the TDRS orbits. Arc lengths of 7 and 10 days with varying degrees of T/P spacecraft attitude complexity are studied. Sub-meter T/P total positioning error is achieved when using the TDRSS range-rate data, with radial orbit errors of 10.6 cm and 15.5 cm RMS for the two arcs studied. Current limitations in the TDRSS precision orbit determination capability include mismodeling of numerous TDRSS satellite-specific dynamic and electronic effects, and in the inadequate treatment of the propagation delay and bending arising from the wet troposphere and ionosphere.     

Calibration of Envisat radar altimeter over Lake Issykkul

This study presents the results of calibration/validation (C/V) of Envisat satellite radar altimeter over Lake Issykkul located in Kyrgyzstan, which was chosen as a dedicated radar altimetry C/V site in 2004. The objectives are to estimate the absolute altimeter bias of Envisat and its orbit based on cross-over analysis with TOPEX/Poseidon (T/P), Jason-1 and Jason-2 over the ocean. We have used a new method of GPS data processing in a kinematic mode, developed at the Groupe de Recherche de Geodesie Spatiale (GRGS), which allows us to calculate the position of the GPS antenna without needing a GPS reference station. The C/V is conducted using various equipments: a local GPS network, a moving GPS antenna along the satellites tracks over Lake Issykkul, In Situ level gauges and weather stations. The absolute bias obtained for Envisat from field campaigns conducted in 2009 and 2010 is between 62.1 and 63.4 ± 3.7 cm, using the Ice-1 retracking algorithm, and between 46.9 and 51.2 cm with the ocean retracking algorithm. These results differ by about 10 cm from previous studies, principally due to improvement of the C/V procedure. Apart from the new algorithm for GPS data processing and the orbit error reduction, more attention has been paid to the GPS antenna height calculation, and we have reduced the errors induced by seiche over Lake Issykkul. This has been assured using cruise data along the Envisat satellite track at the exact date of the pass of the satellite for the two campaigns. The calculation of the Envisat radar altimeter bias with respect to the GPS levelling is essential to allow the continuity of multi-mission data on the same orbit, with the expected launch of SARAL/Altika mission in 2012. Implications for hydrology in particular, will be to produce long term homogeneous and reliable time series of lake levels worldwide.     

GSFC DORIS contribution to ITRF2008

The NASA GSFC DORIS analysis center has provided weekly DORIS solutions from November 1992 to January 2009 (839 SINEX files) of station positions and Earth Orientation Parameters for inclusion in the DORIS contribution to ITRF2008. The NASA GSFC GEODYN orbit determination software was used to process the orbits and produce the normal equations. The weekly SINEX gscwd10 submissions included DORIS data from Envisat, TOPEX/Poseidon, SPOT-2, SPOT-3, SPOT-4, SPOT-5. The orbits were mostly seven days in length (except for weeks with data gaps or maneuvers). The processing used the GRACE-derived EIGEN-GL04S1 gravity model, updated modeling for time-variable gravity, the GOT4.7 ocean tide model and tuned satellite-specific macromodels for SPOT-2, SPOT-3, SPOT-4, SPOT-5 and TOPEX/Poseidon. The University College London (UCL) radiation pressure model for Envisat improves nonconservative force modeling for this satellite, reducing the median residual empirical daily along-track accelerations from 3.75 × 10⁻⁹ m/s² with the a priori macromodel to 0.99 × 10⁻⁹ m/s² with the UCL model. For the SPOT and Envisat DORIS satellite orbits from 2003 to 2008, we obtain average RMS overlaps of 0.8–0.9 cm in the radial direction, 2.1–3.4 cm cross-track, and 1.7–2.3 cm along-track. The RMS orbit differences between Envisat DORIS-only and SLR & DORIS orbits are 1.1 cm radially, 6.4 cm along-track and 3.7 cm cross-track and are characterized by systematic along-track mean offsets due to the Envisat DORIS system time bias of ±5–10 μs. We obtain a good agreement between the geometrically-determined geocenter parameters and geocenter parameters determined dynamically from analysis of the degree one terms of the geopotential. The intrinsic RMS weekly position repeatability with respect to the IDS-3 combination ranges from 2.5 to 3.0 cm in 1993–1994 to 1.5 cm in 2007–2008.     

GPS-based sea level measurements to help the characterization of land contamination in coastal areas

The Corsica site has been established in 1996 to perform altimeter calibration on TOPEX/Poseidon and then on its successors Jason-1 and Jason-2. The first chosen location was under the #85 ground track that overflight the Senetosa Cape. In 2005, it was decided to develop another location close to Ajaccio, to be able to perform the calibration of Envisat and in a next future of SARAL/AltiKa that will flight over the same ground tracks. Equipped with various instruments (tide gauges, permanent GPS, GPS buoy, weather station…) the Corsica calibration site is able to quantify the altimeter Sea Surface Height bias but also to give an input on the origin of this bias (range, corrections, orbits, …). Due to the size of Corsica (not a tiny island), the altimeter measurement system (range and corrections) can be contaminated by land. The aim of this paper is to evaluate this land contamination by using GPS measurements from a fixed receiver on land and from another receiver onboard a life buoy. Concerning the altimeter land contamination, we have quantify that this effect can reach 8 mm/km and then affects the Sea Surface Height bias values already published in the framework of the Corsica calibration site by 5–8 mm for TOPEX and Jason missions. On the other hand, the radiometer measurements (wet troposphere correction) are also sensitive to land and we have been able to quantify the level of improvement of a dedicated coastal algorithm that reconciles our results with those coming from other calibration sites. Finally, we have also shown that the standard deviation of the GPS buoy sea level measurements is highly correlated (∼87%) with the Significant Wave Height derived from the altimeters and can be used to validate such parameter.     

From TOPEX/Poseidon to Jason-2/OSTM in the Amazon basin

A major interest of radar altimetry over rivers is to monitor water resources and associated risk in basins where there is little or no conventional in situ data. The objective of the present study is to calibrate altimetry data in a place where conventional data are available, and use the results to estimate the potential error committed in the estimation of water levels in an ungauged or poorly gauged basin. The virtual stations extracted with Jason-2 in this study concern a very broad sample of river channel width and complexity. Minimum channel width has been estimated at 400 m. Unlike TOPEX/Poseidon (T/P), Jason-2 seems to have the capability to distinguish the river bed from its floodplain. The quality of the results obtained with Jason-2 is incomparably better than that obtained with T/P. Despite the fact that no absolute calibration has been assessed for river in this study, the bias calculated converge around 0, 35 m, which could be then the error estimated on the water stage derived from Jason-2 ranges, when no other validation is available. ICE3 algorithm seems to be performing as well as ICE1, and further research is needed to design retracking algorithm specifically for continental water.     

GPS monitoring of vertical seafloor motion at Platform Harvest

We describe results from two decades of monitoring vertical seafloor motion at the Harvest oil platform, NASA’s prime verification site for the TOPEX/Poseidon and Jason series of reference altimeter missions. Using continuous GPS observations, we refine estimates of the platform subsidence—due most likely to fluid withdrawal linked to oil production—and describe the impact on estimates of stability for the altimeter measurement systems. The cumulative seafloor subsidence over 20 yrs is approximately 10 cm, but the rate does not appear constant. The apparent non-linear nature of the vertical motion, coupled with long-period GPS errors, implies that the quality of the seafloor motion estimates is not uniform over the 20-yr period. For the Jason-1 era (2002–2009), competing estimates for the subsidence show agreement to better than 1 mm yr⁻¹. Longer durations of data are needed before the seafloor motion estimates for the Jason-2 era (2008–present) can approach this level of accuracy.