The use of a nadir altimeter radar with a rotating knife-beam antenna pattern isconsidered for improved measurements of the sea surface wind and wave parameters over awide swath. Theoretical calculations suggest the antenna beam rotating about the verticalaxis is able to provide wide swath of order 250-350 km. Processing of the signals using timeor Doppler sampling techniques results in the division of the antenna footprint intoelementary scattering cells of the order of 14x14 km. The theoretical algorithms developedhere indicate that the system may be used to retrieve the variance of large-scale slopes, thedirection of wave propagation and the wind speed in each cell. The possibility of measuringsignificant wave height is also analyzed. The combination of linear motion of the radar andthe rotation of the knife-beam antenna can be exploited to build up a two-dimensional mapof the surface, which enables better understanding of wave processes and to study theirstructure and temporal dynamics using repeated observations. Full article

GANDER – for Global Altimeter Network Designed to Evaluate Risk – was anidea that was probably ahead of its time. Conceived at a time when ocean observingsatellites were sometimes 10 years in the planning stage, the concept of affordable fastersampling through the use of altimeter-carrying microsats was primarily advanced as a wayof detecting and tracking storms at sea on a daily basis. But, of course, a radar altimetermonitors changes in sea-level as well as surface wave height and wind speed. Here then is asystem which, flown with more precise missions such as JASON 2, could meet the needs ofocean modellers by providing the greater detail required for tracking mesoscale eddies,whilst servicing forecasting centres and units at sea with near real-time sea state information.A tsunami mode, instantly activated when an undersea earthquake is detected by the globalnetwork of seismic stations, could also be incorporated. Full article

The multiple altimeter missions have not only advanced our knowledge of oceancirculation, ice sheet topography, and global climate, but also improved the accuracy ofaltimetric measurements by cross-calibration and validation. In this paper, one year’ssimultaneous maps of sea level anomaly (MSLA) data obtained from four altimeters,Envisat, Geosat Follow-On (GFO), Jason-1, and TOPEX/Poseidon (T/P), have beencompiled for a preliminary comparison. First, the discrepancy in global geographicaldistribution of each product relative to the merged MSLA field is analyzed and its signalretrieval capability is discussed. Second, the space/time variability of each discrepancy inthe Atlantic Ocean, Indian Ocean, Pacific Ocean, Northern Hemisphere, SouthernHemisphere, and global ocean is studied. Third, each discrepancy as a function of latitude,longitude, and merged MSLA is presented. The results show that Jason-1 is the best single-mission for mapping large scale sea level variation, while T/P in its new orbit presents thepoorest estimation of SLA due to the short period (from cycle 369 to 403) used to determinethe mean profile. A clear understanding of each product discrepancy is necessary for ameaningful combination or merging of multi-altimeter data, optimal product selection, aswell as for their assimilation into numerical models. Full article

This paper describes the Ka-band altimetry payload and system that has beenstudied for several years by CNES, ALCATEL SPACE and some science laboratories.Altimetry is one of the major elements of the ocean observing system to be madesustainable through the GEOSS (Global Earth Observation System of Systems) and GMES(Global Monitoring of the Environment and Security) programs. A short review of somemission objectives to be fulfilled in terms of mesoscale oceanography in the frame of theGEOSS and GMES programs is performed. To answer the corresponding requirements, theapproach consisting in a constellation of nadir altimeter is discussed. A coupled Ka-bandaltimeter-radiometer payload is then described; technical items are detailed to explain howthis payload shall meet the science and operational requirements, and expectedperformances are displayed. The current status of the payload development and flightperspectives are given. Full article

The authors report the first results in studying the polarization anisotropy of themicrowave backscatter from nadir observations provided by Jason-1 altimeter in both Ku-and C-band. A small but clear wind direction signal for wind speeds above 6 m/s is revealed.These azimuthal variations of radar cross-section increase with increasing wind speed up to14 m/s. The signatures then level off at higher winds. These results extend, for the first time,recent theoretical improved scattering approximation, and point some similarities betweenscattering and emission mechanisms at nadir. The observed directional effect can thus beinterpreted as a signature of the curvature anisotropy of wind-generated short-scale waves.Sensitivities to both wind speed and sea state are also reported in the present analysis. Full article

The main limitations of standard nadir-looking radar altimeters have been knownfor long. They include the lack of coverage (intertrack distance of typically 150 km for theT/P / Jason tandem), and the spatial resolution (typically 2 km for T/P and Jason), expectedto be a limiting factor for the determination of mesoscale phenomena in deep ocean. In thiscontext, various solutions using off-nadir radar interferometry have been proposed byRodriguez and al to give an answer to oceanographic mission objectives. This paperaddresses the performances study of this new generation of instruments, and dedicatedmission. A first approach is based on the Wide-Swath Ocean Altimeter (WSOA) intended tobe implemented onboard Jason-2 in 2004 but now abandoned. Every error domain has beenchecked: the physics of the measurement, its geometry, the impact of the platform andexternal errors like the tropospheric and ionospheric delays. We have especially shown thestrong need to move to a sun-synchronous orbit and the non-negligible impact of propagation media errors in the swath, reaching a few centimetres in the worst case. Some changes in the parameters of the instrument have also been discussed to improve the overall error budget. The outcomes have led to the definition and the optimization of such an instrument and its dedicated mission. Full article

This study addresses the impact of satellite altimetry data processing on sea levelstudies at regional scale, with emphasis on the influence of various geophysical correctionsand satellite orbit on the structure of the derived interannual signal and sea level trend. Thework focuses on the analysis of TOPEX data for a period of over twelve years, for threeregions in the North Atlantic: Tropical (0^o≤φ≤25^o), Sub-Tropical (25^o≤φ≤50^o) and Sub-Arctic (50^o≤φ≤65^o). For this analysis corrected sea level anomalies with respect to a meansea surface model have been derived from the GDR-Ms provided by AVISO by applyingvarious state-of-the-art models for the geophysical corrections. Results show that sea leveltrend determined from TOPEX altimetry is dependent on the adopted models for the majorgeophysical corrections. The main effects come from the sea state bias (SSB), and from theapplication or not of the inverse barometer (IB) correction. After an appropriate modellingof the TOPEX A/B bias, the two analysed SSB models induce small variations in sea leveltrend, from 0.0 to 0.2 mm/yr, with a small latitude dependence. The difference in sea leveltrend determined by a non IB-corrected series and an IB-corrected one has a strong regionaldependence with large differences in the shape of the interannual signals and in the derivedlinear trends. The use of two different drift models for the TOPEX Microwave Radiometer(TMR) has a small but non negligible effect on the North Atlantic sea level trend of about0.1 mm/yr. The interannual signals of sea level time series derived with the NASA and theCNES orbits respectively, show a small departure in the middle of the series, which has noimpact on the derived sea level trend. These results strike the need for a continuousimprovement in the modelling of the various effects that influence the altimetermeasurement. Full article

Nearly three years of Envisat altimetric observations over ocean are available inGeophysical Data Record (GDR) products. The quality assessment of these data is routinelyperformed at the CLS Space Oceanography Division in the frame of the CNES Segment SolAltimétrie et Orbitographie (SSALTO) and ESA French Processing and Archiving Center(F-PAC) activities. This paper presents the main results in terms of Envisat data quality:verification of data availability and validity, monitoring of the most relevant altimeter(ocean1 retracking) and radiometer parameters, assessment of the Envisat altimeter systemperformances. This includes a cross-calibration analysis of Envisat data with Jason-1, ERS-2 and T/P. Envisat data show good general quality. A good orbit quality and a low level ofnoise allow Envisat to reach the high level of accuracy of other precise missions such as T/Pand Jason-1. Some issues raised in this paper, as the gravity induced orbit errors, will besolved in the next version of GDR products. Some others, as the Envisat Mean Sea Level inthe first year, still need further investigation. Full article