M Roca

The EnviSat RA-2 Instrument Design and Tracking Performance

The EnviSat satellite embarks an innovative radar altimeter, the RA-2 which represents a new generation of radar altimeters compared to previous instruments such as the European Remote Sensing Satellite (ERS) altimeters and The Ocean Topography Experiment/Poseidon. This is due to its integration of many new features. In particular, the RA-2 is the first satellite altimeter to incorporate autonomous resolution selection and a model-free tracker. The behavior of the instrument in orbit is controlled by onboard algorithms which determine the instrument logic, and a list of onboard parameters which control when switches in resolution occur. This logic and the choice of these parameters have a strong influence on the tracking performance. This paper describes the operation of the RA-2 onboard tracker and the resolution selection logic. We provide a detailed description of the algorithm and its configuration parameters after optimization during the EnviSat Commissioning Phase. Finally, we show the tracking performance, in particular compared to the ones of the ERS-2 altimeter, over different types of surfaces.

REAPER: Reprocessing 12 Years of ERS-1 and ERS-2 Altimeters and Microwave Radiometer Data

Twelve years (1991–2003) of ERS-1 and ERS-2 altimetry data have been reprocessed within the European Space Agency (ESA) reprocessing altimeter products for ERS (REAPER) project using an updated, modern set of algorithms and auxiliary models. The reprocessed data set (identified as RP01) has been cross-calibrated against the reprocessed ENVISAT V2.1 data. The format of this reprocessed data set is network common data form (version 3). The new data set shows a clear improvement in data quality beyond that of previous releases. The product validation shows reduction of the mean standard deviation of the sea-surface height differences from 8.1 (previously available product) to 6.7 cm (RP01). This paper presents the details of how the reprocessing was conducted and shows selected results from the validation and quality-assurance processes. The major improvements of the REAPER RP01 data set with respect to the previous ESA ERS radar altimetry (RA) products are due to the use of four ENVISAT RA-2 retrackers, RA calibration improvements, new reprocessed precise orbit solutions, ECMWF ERA-interim model for meteorological corrections, new ionospheric corrections, and new sea state. The intent of this paper is to aid the reader in understanding the benefits of the new data set for their particular use-case.

Transponder calibration of the ENVISAT RA-2 altimeter sigma naught

As far as Ku band is concerned, the backscatter coefficient (sigma naught) of the radar altimeter (RA-2) aboard the ENVISAT satellite has been calibrated using a transponder (TPD) developed at ESA/ESTEC. The transponder has been exploited during the 6 month Commissioning Phase to generate early calibration results. In order to consolidate this calibration results and to check the altimeter during the satellite lifetime, a continuous monitoring was performed by operating the transponder as much as possible. The first calibration campaign dates back to February 24th, 2004. The last campaign occurred on October 5th, 2010. Since then the change of the ENVISAT orbit hampered the prosecution of the activity. This paper aims to review the entire effort for calibrating the RA-2 sigma naught measurements, which lasted for almost seven year. It presents the final outcomes of the activity, providing the users with the correction (bias) to get the calibrated sigma naught and analyzing its stability during almost the entire ENIVISAT lifetime.

The EnviSat RA-2/MWR instrument description, processing chain and data products

Within the ESA Ground Segment, all the Radar Altimeter (RA-2) and Microwave Radiometer (MWR) received data will be systematically processed to Level 1B (geolocated and calibrated engineering parameters) and to Level 2 (geophysical data record). All data will be processed in near real time (availability to users within three hours from data take), then reprocessed as Intermediate GDR (within 3 days) and later in off-line (few weeks after data take). The three version of the Level 2 product have the same layout and provide the typical information of the classical GDR altimeter product. The main difference among them is constituted by the increasing quality of the used orbit. Another main feature of the Level 2 RA-2 product is that every received waveform is processed in parallel by four different algorithms (each one optimised for a particular type of surface: ocean, sea ice, ice 1, ice 2). The Level 2 product (in all its versions) contains the results of the four retracking processes. The Level 1b product NRT and Off-line and the Level 2 NRT products are generated in the ESA station, while the IGDR and the Off-line GDR are produced for ESA by the French Processing and Archiving Centre. Concerning the geophysical corrections, it is important to note that the corresponding MWR measurements are used to correct for the wet tropospheric component, while the dry tropospheric component is corrected making use of the pressure field predicted or reconstructed by ECMWF (European Centre for Medium-Range Weather Forecast).