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.     

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.     

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.     

Generating precise and homogeneous orbits for Jason-1 and Jason-2

Driven by the GMES (Global Monitoring for Environment and Security) and GGOS (Global Geodetic Observing System) initiatives the user community has a strong demand for high-quality altimetry products. In order to derive such high-quality altimetry products, precise orbits for the altimetry satellites are a necessity. With the launch of the TOPEX/Poseidon mission in 1992 a still on-going time series of high-accuracy altimetry measurements of ocean topography started, continued by the altimetry missions Jason-1 in 2001 and Jason-2/OSTM in 2008. This paper contributes to the on-going orbit reprocessing carried out by several groups and presents the efforts of the Navigation Support Office at ESA/ESOC using its NAPEOS software for the generation of precise and homogeneous orbits referring to the same reference frame for the altimetry satellites Jason-1 and Jason-2. Data of all three tracking instruments on-board the satellites (beside the altimeter), i.e. GPS, DORIS, and SLR measurements, were used in a combined data analysis. About 7 years of Jason-1 data and more than 1 year of Jason-2 data were processed. Our processing strategy is close to the GDR-C standards. However, we estimated slightly different scaling factors for the solar radiation pressure model of 0.96 and 0.98 for Jason-1 and Jason-2, respectively. We used 30 s sampled GPS data and introduced 30 s satellite clocks stemming from ESOC’s reprocessing of the combined GPS/GLONASS IGS solution. We present the orbit determination results, focusing on the benefits of adding GPS data to the solution. The fully combined solution was found to give the best orbit results. We reach a post-fit RMS of the GPS phase observation residuals of 6 mm for Jason-1 and 7 mm for Jason-2. The DORIS post-fit residuals clearly benefit from using GPS data in addition, as the DORIS data editing improves. The DORIS observation RMS for the fully combined solution is with 3.5 mm and 3.4 mm, respectively, 0.3 mm better than for the DORIS-SLR solution. Our orbit solution agrees well with external solutions from other analysis centers, as CNES, LCA, and JPL. The orbit differences between our fully combined orbits and the CNES GDR-C orbits are of about 0.8 cm for Jason-1 and at 0.9 cm for Jason-2 in the radial direction. In the cross-track component we observe a clear improvement when adding GPS data to the POD process. The 3D-RMS of the orbit differences reveals a good orbit consistency at 2.7 cm and 2.9 cm for Jason-1 and Jason-2. Our resulting orbit series for both Jason satellites refer to the ITRF2005 reference frame and are provided in sp3 file format on our ftp server.     

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.     

Detection of Envisat RA2/ICE-1 retracked radar altimetry bias over the Amazon basin rivers using GPS

Altimetry is now routinely used to monitor stage variations over rivers, including in the Amazon basin. It is desirable for hydrologic studies to be able to combine altimetry from different satellite missions with other hydrogeodesy datasets such as leveled gauges and watershed topography. One requirement is to accurately determine altimetry bias, which could be different for river studies from the altimetry calibrated for deep ocean or lake applications. In this study, we estimate the bias in the Envisat ranges derived from the ICE-1 waveform retracking, which are nowadays widely used in hydrologic applications. As a reference, we use an extensive dataset of altitudes of gauge zeros measured by GPS collocated at the gauges. The thirty-nine gauges are spread along the major tributaries of the Amazon basin. The methodology consists in jointly modeling the vertical bias and spatial and temporal slope variations between altimetry series located upstream and downstream of each gauge. The resulting bias of the Envisat ICE-1 retracked altimetry over rivers is 1.044 ± 0.212 m, revealing a significant departure from other Envisat calibrations or from the Jason-2 ICE-1 calibration.     

DORIS-based point mascons for the long term stability of precise orbit solutions

In recent years non-tidal Time Varying Gravity (TVG) has emerged as the most important contributor in the error budget of Precision Orbit Determination (POD) solutions for altimeter satellites’ orbits. The Gravity Recovery And Climate Experiment (GRACE) mission has provided POD analysts with static and time-varying gravity models that are very accurate over the 2002–2012 time interval, but whose linear rates cannot be safely extrapolated before and after the GRACE lifespan. One such model based on a combination of data from GRACE and Lageos from 2002–2010, is used in the dynamic POD solutions developed for the Geophysical Data Records (GDRs) of the Jason series of altimeter missions and the equivalent products from lower altitude missions such as Envisat, Cryosat-2, and HY-2A. In order to accommodate long-term time-variable gravity variations not included in the background geopotential model, we assess the feasibility of using DORIS data to observe local mass variations using point mascons. In particular, we show that the point-mascon approach can stabilize the geographically correlated orbit errors which are of fundamental interest for the analysis of regional Mean Sea Level trends based on altimeter data, and can therefore provide an interim solution in the event of GRACE data loss. The time series of point-mass solutions for Greenland and Antarctica show good agreement with independent series derived from GRACE data, indicating a mass loss at rate of 210 Gt/year and 110 Gt/year respectively.     

The contribution of local gravimetric geoid models to the calibration of satellite altimetry data and an outlook of the latest GOCE GGM performance in Gavdos

The use of geoid heights has been one of the available methodologies utilized for the independent calibration/validation of altimeters on-board satellites. This methodology has been employed for long in the Gavdos dedicated cal/val facility (Crete, Greece), where calibration results for the Jason satellites have been estimated, both for ascending and descending passes. The present work gives a detailed overview of the methodology followed in order to estimate a high-resolution and accuracy gravimetric geoid model for the wider Gavdos area, in support of the on-going calibration work. To estimate the geoid model, the well-known remove-compute-restore method is used while residual geoid heights are estimated through least-squares collocation so that associated errors are determined as well. It is found that the estimated formal geoid errors from LSC along passes 018 and 109 of Jason satellites, used for the bias estimation, range between ±0.8–1.6 cm. The so-derived geoid heights are employed in the determination of the Jason-2 altimeter bias for all available cycles (cycles 1-114, spanning the period from July 2008 to August 2011) together with the RioMed DOT model. From the results acquired the Jason-2 bias has been estimated to be +196.1 ± 3.2 mm for pass 109 and +161.9 ± 5.1 mm for pass 018. Within the same frame, the GOCE/GRACE-based geopotential model GOCO02s has been used to estimate the mean dynamic ocean topography and the steady-state circulation in the area around Gavdos. The so-derived DOT model was used to estimate the Jason-2 bias in an effort to evaluate the performance of satellite-only geoid models and investigate whether their spatial resolution and accuracy provides some improvement w.r.t. traditional local gravimetric geoids. From the results acquired with geoid heights from GOCO02s, the estimated Jason-2 bias deviates significantly from that of the local gravimetric model, which can be attributed to a possible mean offset and the low resolution of GOCE-based GGMs. On the other hand, when the newly estimated GOCE-based DOT was employed with geoid heights from the local gravimetric geoid model, the Jason-2 bias has been estimated to be +185.1 ± 3.2 mm for pass 109 and +130.2 ± 5.1 mm for pass 018.     

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.     

A demonstration of the potential of Cryosat-2 to contribute to mesoscale observation

Cryosat-2 was designed for its primary scientific objectives, i.e. for cryosphere science. As far as oceanography is concerned, various mission design choices make it less accurate than missions designed to comply with ocean surface topography requirements such as Jason-2 or ENVISAT. Cryosat-2-specific errors are equivalent to more than 50% of the sea surface height variability over 40% of the oceans. Cryosat-2’s sampling pattern is also suboptimal for mesoscale observation because the satellite tracks from any consecutive period of 2 to 20 days (e.g. the most recent and most valuable data for near real time mesoscale observation) are aggregated in 500 km wide bands which are interleaved with 500 km wide observation gaps.     

Altimetry,bathymetry and geoid variations at the Gavdos permanent Cal/Val facility

The aim of this work has been to examine the relationship of steep bathymetry in the coastal areas around the permanent Cal/Val facility of Gavdos, and their influence on the produced calibration values for the Jason-2 satellite altimeter. The paper describes how changes in seafloor topography (from 200 to 3500 m depth over a distance of 10 km) are reflected on the determined altimeter parameters using different reference surfaces for satellite calibration. Finally, it describes the relation between these parameter trends and the region’s local characteristics.     

First quality assessment of the Cryosat-2 altimetric system over ocean

This paper presents the results of a Calval analysis performed for the Cryosat-2 mission over ocean. The data set used in this analysis consists of products generated by the Cryosat-2 Processor Prototype developed by CNES (Center National d’Etudes Spatiales). This data set has been analysed focusing on LRM (Low Resolution Mode) mode only. One major objective of this paper is to illustrate the potential of Cryosat-2 data over ocean, mainly for waves and Sea Level Anomaly applications. All the results indicate very good performances of the SIRAL (SAR/Interferometric Altimeter) altimeter over ocean. Crossover standard deviation is close to 6.5 cm over the analysed period (3 months) which is close to the Jason-2 and ENVISAT performance. All these results confirm that Cryosat’s altimeter can provide data almost as valuable as other flying altimetric missions, and that it has the potential to contribute to oceanography (e.g. multi-mission climate record, mesoscale monitoring in near real time) and to geodesy (e.g. mean sea surface, bathymetry).     

Transponder calibration of the Envisat RA-2 altimeter Ku band sigma naught

Although the history of spaceborne altimeters goes back to the early seventies, the absolute calibration of the backscattering coefficient has never been deeply investigated. This information has been primarily used to infer the wind speed via an empirical model, and the intercalibration among different satellite altimeters has revealed to be suitable for this purpose, being the wind retrieval based on an empirical relationship. As far as Ku band system is concerned, the sigma naught absolute calibration of the Envisat altimeter (RA-2) has been performed using an active reference target provided by a transponder. This has been exploited during the 6-month Commissioning phase to generate early calibration results. In order to monitor the RA-2 backscatter calibration during the Envisat lifetime, a continuous calibration effort has been carried out by operating the transponder as much as possible. This paper aims to review the entire effort for calibrating the RA-2 sigma naught measurements, which lasted for almost seven years. It presents in detail the adopted methodology and the final outcome of the activity, providing the users with the correction (bias) to get the calibrated sigma naught and analyzing its stability during almost the entire Envisat lifetime. Specifically, it is concluded that the RA-2 backscatter measurements were quite stable, even if a bias of about 1 dB should be considered with respect to the actually released product. Some small changes in the bias as function of time can be identified during most of the Envisat lifetime, consisting in a slight increase in the first two years, followed by a more stable period and a final drop observed at the end of 2009, until the conclusion of the calibration activity (corresponding to the change in Envisat orbit).     

Using altimetry and seafloor pressure data to estimate vertical deformation offshore: Vanuatu case study

Measuring ground deformation underwater is essential for understanding Earth processes at many scales. One important example is subduction zones, which can generate devastating earthquakes and tsunamis, and where the most important deformation signal related to plate locking is usually offshore. We present an improved method for making offshore vertical deformation measurements, that involve combining tide gauge and altimetry data. We present data from two offshore sites located on either side of the plate interface at the New Hebrides subduction zone, where the Australian plate subducts beneath the North Fiji basin. These two sites have been equipped with pressure gauges since 1999, to extend an on-land GPS network across the plate interface. The pressure series measured at both sites show that Wusi Bank, located on the over-riding plate, subsides by 11 ± 4 mm/yr with respect to Sabine Bank, which is located on the down-going plate. By combining water depths derived from the on-bottom pressure data with sea surface heights derived from altimetry data, we determine variations of seafloor heights in a global reference frame. Using altimetry data from TOPEX/Poseidon, Jason-1, Jason-2 and Envisat missions, we find that the vertical motion at Sabine Bank is close to zero and that Wusi Bank subsides by at least 3 mm/yr and probably at most 11 mm/yr.     

Integrating Non-Tidal Sea Level data from altimetry and tide gauges for coastal sea level prediction

The main objective of this paper is to integrate Non-Tidal Sea Level (NSL) from the joint TOPEX, Jason-1 and Jason-2 satellite altimetry with tide gauge data at the west and north coast of the United Kingdom for coastal sea level prediction. The temporal correlation coefficient between altimetric NSLs and tide gauge data reaches a maximum higher than 90% for each gauge. The results show that the multivariate regression approach can efficiently integrate the two types of data in the coastal waters of the area. The Multivariate Regression Model is established by integrating the along-track NSL from the joint TOPEX/Jason-1/Jason-2 altimeters with that from eleven tide gauges. The model results give a maximum hindcast skill of 0.95, which means maximum 95% of NSL variance can be explained by the model. The minimum Root Mean Square Error (RMSe) between altimetric observations and model predictions is 4.99 cm in the area. The validation of the model using Envisat satellite altimetric data gives a maximum temporal correlation coefficient of 0.96 and a minimum RMSe of 4.39 cm between altimetric observations and model predictions, respectively. The model is furthermore used to predict high frequency NSL variation (i.e., every 15 min) during a storm surge event at an independent tide gauge station at the Northeast of the UK (Aberdeen).     

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.     

The effect of geocenter motion on Jason-2 orbits and the mean sea level

We compute a series of Jason-2 GPS and SLR/DORIS-based orbits using ITRF2005 and the std0905 standards ( Lemoine et al., 2010). Our GPS and SLR/DORIS orbit data sets span a period of 2 years from cycle 3 (July 2008) to cycle 74 (July 2010). We extract the Jason-2 orbit frame translational parameters per cycle by the means of a Helmert transformation between a set of reference orbits and a set of test orbits. We compare the annual terms of these time-series to the annual terms of two different geocenter motion models where biases and trends have been removed. Subsequently, we include the annual terms of the modeled geocenter motion as a degree-1 loading displacement correction to the GPS and SLR/DORIS tracking network of the POD process. Although the annual geocenter motion correction would reflect a stationary signal in time, under ideal conditions, the whole geocenter motion is a non-stationary process that includes secular trends. Our results suggest that our GSFC Jason-2 GPS-based orbits are closely tied to the center of mass (CM) of the Earth consistent with our current force modeling, whereas GSFC’s SLR/DORIS-based orbits are tied to the origin of ITRF2005, which is the center of figure (CF) for sub-secular scales. We quantify the GPS and SLR/DORIS orbit centering and how this impacts the orbit radial error over the globe, which is assimilated into mean sea level (MSL) error, from the omission of the annual term of the geocenter correction. We find that for the SLR/DORIS std0905 orbits, currently used by the oceanographic community, only the negligence of the annual term of the geocenter motion correction results in a – 4.67 ± 3.40 mm error in the Z-component of the orbit frame which creates 1.06 ± 2.66 mm of systematic error in the MSL estimates, mainly due to the uneven distribution of the oceans between the North and South hemisphere.     

Bioelectrochemical degradation of urea at platinized boron doped diamond electrodes for bioregenerative systems

The recovery of potable water from space mission wastewater is critical for the life support and environmental health of crew members in long-term missions. NASA estimates reveal that at manned space missions 1.91 kg/person day of urine is produced, with urea and various salts as its main components. In this research we explore the utilization of urease (EC 3.5.1.5, 15,000 U/g) along with a platinized boron doped diamond electrode (Pt-BDD) to degrade urea. Urea is directly degraded to nitrogen by the in situ utilization of the reaction products as a strategy to increase the amount of clean water in future space expeditions. The biochemical reaction of urease produces ammonia and carbon dioxide from urea. Thereafter, ammonia is electrooxidized at the interface of the Pt-BDD producing molecular nitrogen. The herein presented system has been proven to have 20% urea conversion efficiency. This research has potential applications for future long-term space missions since the reaction byproducts could be used for a biomass subsystem (in situ resource recovery), while generating electricity from the same process.     

Influence of time variable geopotential models on precise orbits of altimetry satellites,global and regional mean sea level trends

During the last decade a significant progress has been reached in the investigation of the gravity field of the Earth. Besides static, also time variable geopotential models have been recently created. In this paper we investigate the impact of the recent time variable geopotential models on altimetry satellite orbits and such altimetry products based on these orbits, as global and regional mean sea level trends. We show that the modeling of time variable gravity improves the orbit solutions, at least for the GRACE period where time variable gravity is sufficiently accurately observed by this mission. Our analysis includes six geopotential models jointly developed by GFZ German Research Centre for Geosciences and Space Geodesy Research Group (CNES/GRGS) Toulouse: the stationary model EIGEN-GL04S, a stationary version of EIGEN-6S (EIGEN-6S_stat), a corrected version of EIGEN-6S and three enhanced versions of EIGEN-6S called EIGEN-6S2, EIGEN-6S2A and EIGEN-6S2B. By “stationary” we mean “containing periodic parameters such as annual and semi-annual variations, but no secular (drift) terms”. We computed precise orbits for the radar altimetry satellites ERS-1, ERS-2, TOPEX/Poseidon, and Envisat over 20 years between 1991 and 2011. The orbit, single-mission and multi-mission altimetry crossover analyses show that the time variable models EIGEN-6S_corrected, EIGEN-6S2 and its two precursors EIGEN-6S2A/B perform notably better than the stationary models for the GRACE period from 2003 onwards. Thus, using EIGEN-6S2 and EIGEN-6S2A/B we have got 3.6% smaller root mean square fits of satellite laser ranging observations for Envisat, as when using EIGEN-GL04S. However, for the pre-GRACE period 1991–2003, the stationary geopotential models EIGEN-GL04S and EIGEN-6S_stat as well as EIGEN-6S2 having no drift terms for degree 3–50 at this time interval perform superior compared to EIGEN-6S_correct and EIGEN-6S2A/B which contain drifts for this period. We found, that the time variable geopotential models have a low (0.1–0.2 mm/yr) impact on our results for the global mean sea level trend. However, we found strong East/West differences up to 3 mm/yr in the regional mean sea level trends when using orbits of all four satellites based on time variable and stationary geopotential models. We show that these differences are related to the relative drifts of the centers-of-origin between the orbit solutions based on the time variable and stationary geopotential models. From the results of our detailed study, we conclude that the final version of the time variable gravity field model EIGEN-6S2 performs best for the four satellites tested. This model provides the most reliable and mission-consistent sea level estimates for the whole time period from 1992 to 2010. This model is of maximum spherical harmonic degree and order 260 and contains time series for drifts as well as annual and semiannual variations of the spherical harmonic coefficients for degree 2–50.