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On 25 February 2013, the Satellite for Argos and AltiKa (SARAL) was launched from the Indian Sriharikota launch site. The AltiKa payload consisted of an altimeter and a radiometer. This paper describes the AltiKa radiometer. This instrument has been studied for several years by CNES, TAS-F, ASTRIUM-F and a set of science laboratories, and AltiKa is the first compact instrument embedding simultaneously the altimeter and radiometer functions. AltiKa radiometer is a dual frequency instrument working in K (23.8 GHz) and Ka band (37 GHz), it is based on the total power principle, with direct detection receivers. On-ground acceptance tests exhibited a very high level of performance: less than 0.2 dB has been estimated for both sensitivity and absolute accuracy in both frequencies. This paper focuses on the in-flight performances that have been observed since the launch. All the instrument observable characterizations are nominal, and in-flight sensitivity has been estimated lower than 0.2 K. [ABSTRACT FROM PUBLISHER]

The SARAL/AltiKa mission is a complement of the Jason altimeter series. A two-channels (23.8 GHz and 37 GHz) microwave radiometer (MWR) is combined to the altimeter in order to correct the altimeter range for the excess path delay (referred as WTC for wet tropospheric correction. First, the in-flight calibration of AltiKa MWR is assessed from a systematic comparison to other radiometers using a complete set of metrics (comparison to simulations and over geophysical targets). Then the “mixed” empirical approach successfully used for Envisat shows nonoptimal performances for the WTC retrieval. In order to find the potential sources of issues, this method is compared to a purely empirical relationship established between measured brightness temperatures (TB) and altimeter backscattering coefficient () on one hand and modeled WTC on the other hand. Various retrieval configurations for both AltiKa MWR and advanced microwave radiometer (AMR) on Jason-2, are for the first time systematically compared with respect to their performances against the variance of sea surface height differences at crossovers. Finally, the issues on the “mixed” approach are attributed to the differences between simulated and measuredat Ka-band. Now, a configuration of the empirical approach proved to have performances closed to what is initially expected with the “mixed” approach. [ABSTRACT FROM PUBLISHER]

Following the successful launch of the SARAL space mission in February 2013, the reliability of the innovative AltiKa altimeter has been demonstrated for deep ocean applications, where Ka-band performances are excellent. With the objective to ensure the complementarity but also the continuity with the altimeter Level-2 products provided in the open ocean, the Prototype for Expertise on AltiKa for Coastal, Hydrology and Ice (PEACHI) project has been set up as an initiative of the French space agency, CNES, to provide a data set of research-grade Level-2 parameters that might be interesting for SARAL secondary objectives on the study of coastal dynamics, inland waters, polar oceans, or continental and sea ices. Thus, the PEACHI prototype has been developed to process and accurately tune dedicated algorithms for the assessment of Ka-band parameters, from the instrument processing to geophysical corrections. As a result, the PEACHI prototype routinely provides end users with new or improved altimeter corrections for scientific applications dedicated to mesoscale monitoring but also synergistic science. [ABSTRACT FROM PUBLISHER]

On 25 February 2013, the SARAL satellite was launched from the Indian Sriharikota launch site. The key feature of the altimetric payload has been the selection of Ka-band. Using Ka-band avoids the need for a second frequency to correct for the ionosphere delay and eases the sharing of the antenna by the altimeter and the radiometer. The use of the Ka-band also allows the improvement of the range measurement accuracy in a ratio close to 2 due to the use of a wider bandwidth and to a better pulse to pulse echo decorrelation. Eventually, Ka-band antenna aperture is reduced, which limits the pollution within useful ground footprint. A summary of the results obtained during the in-flight assessment phase is given. All the tracking modes have also been gone through. Eventually, a new high data rate mode, called “HD mode” is implemented on AltiKa and has been used. The performance assessment is excellent: the range measurement accuracy is close to 1 cm for 1s averaging and the Significant Wave Height (SWH) noise is less than 5 cm (for a 2m SWH at 1?). The tracking success is close to 100% over oceans and 96% over all surfaces. [ABSTRACT FROM PUBLISHER]

This article deals with the recent improvements of the Global and Regional Mean Sea Level trend using Envisat data. It focuses on the methodology based on comparison with Jason-1, Jason-2, and in situ data and shows that Envisat is now getting more reliable for MSL studies, as a comparison reference for future missions. Notably, Jason-1/Envisat comparisons enabled to detect a weakness in the orbit standard solution, now improved for all altimetric missions including Jason-2. [ABSTRACT FROM AUTHOR]

As long-term stability of altimeter backscatter coefficient (σ0) can impact the accuracy of the Global Mean Sea Level (GMSL) trend through the Sea State Bias (SSB) correction, it is important to accurately monitor its evolution in order to detect potential instabilities. This study aims to characterize long-term uncertainties from several altimeter missions. The originality of our approach is to perform comparisons between wind speed estimations deduced from σ0measurements and from ERA-interim reanalysis. Thanks to the good temporal correlation between the Global Wind Speed (GWS) evolution issued from altimeters and the atmospheric reanalysis, the detection of small drifts is possible in the altimeter time series. In particular, drifts have been detected on Jason-1 GWS between mid-2004 and 2005 and on Envisat prior to 2004. They are associated to σ0jumps of approximately 0.03 dB. Larger anomalies have been detected on TOPEX σ0data with variations reaching 0.2 dB. These low altimeter wind speed variations originate from small σ0changes are within the instrumental stability requirements. However, via the SSB calculation, their impact on the GMSL evolution is estimated close to 0.1 mm/yr over the altimetry 20-year period. In addition, they strongly affect inter-annual GMSL variations with associated errors reaching 2 mm. [ABSTRACT FROM AUTHOR]

This study presents a new method of precise altimeter absolute calibration using a dedicated microwave transponder, which acts as an altimeter signal repeater that can be deployed at any sub-track position both in coastal regions and inland. The Austrian Academy of Sciences operates an altimeter transponder at the Gavdos calibration/validation facility located beneath a Jason cross-over point. We discuss the capabilities and strengths of the transponder technique in general and in the particular case of a dedicated calibration campaign carried out in 2011. For the accomplishment of this campaign, including 26 ascending and descending Jason-2 passes, the onboard Poseidon-3 altimeter had to be switched to the DIODE/DEM mode for every overflight. Four different methods have been developed to analyze the transponder generated waveforms, provided by S-IGDR and S-GDR products, respectively. The resulting biases of the altimetric ranges have proven to be stable (3 mm rms) and agree to a large extent among the proposed algorithms. However, the absolute bias value of 25.8 ± 0.3 cm derived from the transponder calibration technique shows a significant deviation of several centimeters compared with the ones resulting from conventional techniques. Possible causes of this behavior are under further investigation. [ABSTRACT FROM AUTHOR]

This article focuses on the determination and the validation of the new Mean Sea Surface from the CNES/CLS. This new model merges multiple satellite altimeters over 16 years of observations. Particular attention was paid on the analysis of the oceanic variability. Then a novel method was applied to remove this variability and to take into account different kinds of errors that affect the data. Several types of validations were performed to estimate the quality of the shortest topographic structures and to quantify the accuracy of the mean oceanic content at a larger scale. [ABSTRACT FROM AUTHOR]

Altimeter missions have provided accurate measurements of sea surface height since 1992 not only with TOPEX/Poseidon but also with Jason-1, Envisat, and recently Jason-2. The overall quality assessment of altimeter data can be performed by analyzing their internal consistency and the cross-comparison between all missions. In this study, in situ measurements are used as an external, independent reference to enable further quality assessment of the altimeter sea level. The most up-to-date altimeter data are assessed and compared with those from tide gauges and Argo profiling floats. The first focus is on detection of global and regional drifts in altimeter sea surface height compared with in situ measurements. A second point is that the method can assess the impact of new altimeter standards (e.g., orbit solution, instrumental correction, retracking algorithm) thanks to in situ observations. Finally, the study shows how multiple and reliable altimeter products are used to detect potential anomalies in tide gauges. The results demonstrate the close link between these three steps of the method: while the detection of altimeter drifts using in situ measurements is corrected by computing new altimeter standards whose impact can then be estimated, the improved altimeter sea level time series are used as input for controlling the quality of in situ observations. [ABSTRACT FROM AUTHOR]

Sea level monitoring in the Arctic Ocean can provide useful information in the context of a rapid change of several parts of the Arctic climate system. Satellite altimetry systems are affected by various problems at high latitudes. As a consequence, no precise and reliable mean sea level record is available yet from altimetry products. After identifying some of the issues that affect satellite altimetry in the Arctic Ocean region, we describe the tailored processing that has been applied to along-track mono-mission altimetry data. We generate a new dataset of weekly gridded sea level anomaly fields over the Arctic region for the period spanning from 1993 to 2009 based on multisatellite altimetry missions. We demonstrate the improvements achieved by this new dataset, among which a better data coverage. The grids are used to describe some features of mean sea level variability in the Arctic Ocean both at basin-wide and local scales. The regional trend estimated for the Arctic Ocean mean sea level over all latitudes from 66°N to 82°N is 3.6 mm/yr with an uncertainty of 1.3 mm/yr (90% confidence) and without any glacial isostatic adjustment (GIA) correction applied. The record displays large inter-annual variability, but no strong correlation with climate indices was found. Spatial patterns in sea level trends and variability over the Arctic region are also investigated. [ABSTRACT FROM AUTHOR]

The ageing of Jason-1, the risk of losing control of the satellite, and the collision risk with TOPEX/Poseidon (still in orbit and no longer maneuverable) initiated a reflection on a so-called “extension of life phase” (EoL) phase that would involve moving Jason-1 to a new orbit to mitigate collision risks while optimizing its science return. This paper describes three practical consequences of any such EoL phase: 1) the ability to build an unprecedented low inclination and high precision geodetic dataset, 2) the loss of coordination with Jason-2 and the associated mesoscale (and sea state) sampling degradation, and 3) the increased topography height error budget stemming from the use of a gridded mean sea surface in place of the classical repeat track analysis that operational systems have been using and improving for almost two decades. More than 17,000 potential orbits were analyzed to identify desirable altitude ranges that could host a Jason-1 EoL phase. The objective was to minimize the sampling degradation of ocean observations (primary objective of Jason-1) while securing a good geodetic EoL dataset (secondary objective of Jason-1). After a first automated screening and scoring process, the final orbit candidates are analyzed through an end-to-end Observing System Simulation Experiment (OSSE) protocol, assessing the multimission observational capability of the EoL phase in a DUACS/AVISO-like system. All EoL orbits are shown to be largely inferior to the interleaved orbit as far as oceanography is concerned. Yet some EoL options are shown to be more desirable than others because their sampling patterns blend well with Jason-2. Good geodetic orbit options could provide a unique bathymetry-oriented dataset and help improve gridded mean sea surfaces (MSS), while repetitive options with a short cycle could cancel some additional EoL errors if a conservative repeat track strategy is preferred. [ABSTRACT FROM AUTHOR]

Calibration of the wind speed is an issue not only for the provision of an accurate metocean parameter but also for the stability of the whole altimetry system across several flying missions. Wind speed is one of the parameters used to derive the sea state bias correction and therefore affects the sea surface height measurement. The analysis performed in this paper benefits from an in-house reprocessing exercise of 1 year of Jason-2 data with both the 3- and 4-parameter Maximum Likelihood Estimator (MLE-3, and MLE-4) retracking algorithms. It took into account the new altimeter characterization file that will be used in the upcoming official reprocessing activity to generate version C of the Jason-2 Geophysical Data Record products. Updated comparison of MLE-3 and MLE-4 data confirms that the MLE-3 algorithm gives less high-frequency variations in σ0. A calibration of the Jason-2 wind speed estimates has been performed based on a bias-adjustment of the radar backscatter data (+0.32 dB for MLE-3 data and +0.28 dB for MLE-4 ones) between Jason-2 and Jason-1. This allows us to re-assess the sea state bias differences between these two instruments from recomputed solutions and to show that there is no SSB difference between the two altimeters at cm-level. [ABSTRACT FROM PUBLISHER]

For more than 13 years, the multisatellite DUACS system has been providing the altimetry community with Near Real Time and Delayed Time products ranging from reduced GDR (also known as CorrSSH) to along-track Sea Level Anomalies (SLA) and multimission Maps of Sea Level Anomalies (MSLA). A post-Jason-2 description of the DUACS system is given, with input data, processing and products, and a focus on the DT-2010 reprocessing involving a total of almost 60 years worth of altimetry data from GEOSAT to Jason-2. Less than one month after launch, Jason-2 proved to be a strong asset for multisatellite applications as it was able to replace Jason-1 as the reference mission in DUACS. Furthermore, the new Jason-2/Jason-1 tandem configuration provides an unprecedented duo for mesoscale and circulation observation. More generally, the quality of Jason-2 has a large impact in DUACS on a number of fronts: in the continuity of the reference mission for climate applications exploiting DUACS products, in the new multi-reference orbit error reduction scheme, or for new metrics derived from a Degrees of Freedom of Signal analysis applied to the multimission mapping. This paper gives an overview of the many impacts of the integration of Jason-2 into DUACS. [ABSTRACT FROM PUBLISHER]

OSTM/Jason-2 (Ocean Surface Topography mission, named Jason-2 in this paper) satellite was successfully launched on June 20, 2008, and was put on its nominal orbit on July 4, 2008. Up to January 26, 2009, Jason-2 was flying in formation with Jason-1 only 55 seconds apart before Jason-1 was moved to its new interleaved orbit. The objectives of this paper are to assess Jason-2 data quality and to estimate the altimetry system performance from the beginning of the mission, using Geophysical Data Records (GDRs) products. Our study will focus on Jason-1/Jason-2 cross-calibration using the opportunity that the missions were on the same ground track separated by 55 seconds during the formation flight phase. This allows us to precisely assess parameter discrepancies and Sea Surface Height (SSH) consistency between both missions in order to detect geographically correlated biases, jumps or drifts. From the results presented in this paper, it is demonstrated that the Jason-2 mission fulfils the requirements of high precision altimetry, which is crucial for future oceanographic studies and applications. In particular, it allows continuation of the observation of the Mean Sea Level (MSL) variations at the same (or better) accuracy as Jason-1 and TOPEX/Poseidon, which was one of the challenges of the Jason-2 mission. Potential improvements and open issues are also identified, with the objective of still making progress in terms of altimetry data quality. [ABSTRACT FROM AUTHOR]

For all altimeter missions, precise estimates of geophysical parameters are obtained thanks to an algorithm called “retracking” that fits an analytical model to the measured waveforms. The Brown model provides a good representation of the return echo over deep ocean surfaces and is commonly used. Many different chains can be considered (and have already been tested) for this processing. An unweighted Least Square Estimate derived from a Maximum Likelihood Estimator (MLE) (Dumont 1985; Rodriguez 1988) is implemented in most altimeter ground processing approaches (TOPEX, Jason-1, Jason-2, and Envisat). The aim of this paper is to evaluate the performance of two retracking algorithms based on the same least square principle: The MLE3 algorithm estimates three parameters (range, significant wave height, and power) whereas the MLE4 estimates four parameters (the three previous ones and the slope of the waveform trailing edge). MLE3 was used on Jason-1 before star tracker problems occurred. The MLE4 algorithm has been used for Jason-1 Version B products and onward and for Jason-2 products from the start of the mission. Both algorithms are compared in the paper. Advantages and drawbacks of both algorithms are pointed out showing notable benefits provided by MLE4 especially for waveforms that do not conform to the Brown model. [ABSTRACT FROM AUTHOR]

We present a Jason-2 sea state bias (SSB) model from the first year of GDR products along with updates of Jason-1, TOPEX-A, and TOPEX-B solutions. Our re-evaluation of the Jason-1 SSB focused on data from the new version (C) of the GDR while for TOPEX, the updated solutions benefited from the availability of both a set of new precise orbits that are consistent with Jason-2 and an updated set of state-of-the-art geophysical corrections. Therefore, derived from homogeneous sea surface height (SSH) data to minimize geographically correlated SSH differences between missions, these empirical SSB models contribute to the consistency between Jason-2 and Jason-1 SSH data and between TOPEX and Jason-1 data during their respective tandem phase and thus to the stability of the whole altimetry system over the past 17-year period. [ABSTRACT FROM AUTHOR]