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2.12 Data Handling Cookbook

2.12.1 Hints And Algorithms For Product Use

The evaluation of the quality of the EnviSat Altimetry geophysical data products and the cross calibration on ERS-2 and other flying altimeters (e.g. TOPEX, Jason, GFO,...) has been performed during the Commissioning Phase within a team of scientists drawn up from the pre-launch announcement of opportunity: the RA-2/ MWR Cross-Calibration and Validation Team (CCVT).

The product validation approach has been to verify, with real data, the consistency of the product package (document, media, format and actual data set), to quantify the inherent validity and accuracy of the range, altitude, wave height, wind speed measurements. etc. and geophysical corrections.

Here follow the main results of the work performed by the CCVT in terms of altimetric parameters performance, errors, proposals for future algorithmic improvements, etc.

  • SWH:

    RA-2 measures low wave heights much better than ERS, due to the presence of two additional digital filters located on the waveform leading edge to improve the slope estimation. The zero-clipping has been reduced (although not completely eliminated) in EnviSat, wrt ERS-2, due to a better SWH retrieval algorithm.

    EnviSat SWH compares well with ERS-2, buoys and models: Envisat SWH are 21 cm (or 3%) higher than ERS-2, there is a 10 cm bias with buoys, a 16 cm bias with WAM SWH model and 21 cm rms/WAM.

  • Sigma0/wind speed:

    RA2 wind speed rms is only 1.4 m/s, compared to ECMWF fields.

    Ku-band sigma0 was artificially aligned with ERS-2 to satisfy modified Witter and Chelton wind model.

    The estimated value of the sigma0 bias, coming from the absolute calibration activities, is around 0.95 dB (RA-2 higher), pretty much in line with the results of the RA2 passive calibration (0.91 dB). The passive calibration activities have provided also with a (preliminary) value of the interfrequency bias, set currently to 1.05 dB.

    This value will be further tuned with larger sets of data.

    The actual values of Ku sigma0 coming from the absolute calibration will be applied to the data only when a new wind model will be developed.

  • Range:

    The range noise, estimated on short segments over low waveheight areas of the ocean, is 1.8 cm and 5.2 cm (at 1 Hz) for Ku-band and S-band, respectively.

    From the cross-calibration with ERS-2, a bias of around -40 cm was found (but this value could be still sensitive to preliminary sea state bias model).

    An improved noise spectrum, with a higher frequency signal (than in ERS) is present: 14 Km, and less when 20 Hz data is used.

    Using the output of the ocean retracker and the geophysical corrections in the product, the sea surface height rms at cross-overs was estimated by CLS, Newcastle Univ. and MSSL, at respectively, 8.22, 7.0 and 8.5 cm, with a further reduction to 6.25 cm when an orbit error correction was applied by CLS.

  • Time-tag bias:

    The pseudo time-tag was estimated minimising cross-overs over ocean. No significant slope was detected in the sea surface height as a function of the altitude rate, therefore confirming that there is no significant time tag error.

  • Mispointing:

    The square estimated mispointing is less than 0.01 deg2. The mispointing value in the L2 products (around 0.026 deg2) is erroneous.

    The algorithm in the operational processors is being investigated.

  • Geophysical corrections:

    The dual frequency ionospheric correction was evaluated against JPL GIM model (depending on the SSB which is not yet final on the limited data set available) with good agreement between both. The DORIS model, instead, showed an unrealistically limited dynamic range compared to the GIM and dual-freq corrections.

    The DORIS iono has been in the meantime improved.

    All other geophysical corrections have been validated.

  • MWR calibration and validation:

    The Level 2 wet tropo correction is in very good agreement with the ERS-2 MWR one (relative bias < 5 mm) and with the radio-soundings.

    MWR 36 GHz channel may require anyway some stabilisation due to aging and thermal cycling of the components. The current drift from the beginning of the mission is of 0.25 counts/K.

    A new MWR side lobes correction, to be implemented in a near future in the operational L1b processor, will improve the MWR corrections in the affected areas (coastal zones).

    A new neural network algorithm has been recently implemented for the computation of the MWR corrections.

    Continuous monitoring of the 36 GHz channel gain drift and local oscillator behaviour is recommended, to assess possible stabilisation.

  • Orbit error:

    Orbit heights from CNES, NCL, DUT and ESOC agree to within 2-3 cm rms. Radial orbit error on the GDRs is estimated < 4 cm rms.

    The orbit propagated into the NRT FDGDR/FDMAR products does not yet use the DORIS Navigator, so it is very poor: 20 to 30 m error in the radial component with jumps of several meters between orbit state vectors.

    The orbit upgrade is currently under investigation and will be implemented in a near future.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry