Products Along-track Experimental Sentinel 6MF products (from CNES R&D processor)
Type of dataset: monomission along-track products
DOI: 10.24400/527896/a01-2022.015 - more metadata
The altimeter data processing
The CNES Sentinel-6 Processing Prototype (S6PP) is a multi-chain processor [1, 2, 3] in which innovative algorithms are implemented and their performance evaluated in view of improving the altimeter data quality of the Sentinel-6MF mission. Among the recent developments made in the S6PP, some have been endorsed for future implementation in the Payload Data Acquisition and Processing (PDAP) operational ground segment of EUMETSAT, as detailed in the following table. The present S6PP data release aims at preparing users for these future PDAP evolutions [2].
| S6PP LR | PDAP LR | S6PP HR | PDAP HR |
|---|---|---|---|---|
| Retracking algorithms | Numerical retracking [1, 4] | - MLE4 - Numerical retracking [4] in Q4 2022 | Numerical retracking [4] | - Samosa - Numerical retracking [4] in Q2 2023 (TBC) |
| Calibration | ECHOCAL | ECHOCAL | ECHOCAL | ECHOCAL |
| Range walk correction processing | NA | NA | chirp-z transform (CZT) [1, 5] | CZT [1, 5] in Q2 2023 (TBC) |
| Vertical wave velocity compensation | NA | NA | HR Look Up Table (LUT) from NOAA [6] | HR LUT from NOAA [6] in Q4 2023 (TBC) |
| Ocean topography skewness coefficient | 0.1 | 0.1 | 0.1 | (TBD) |
| C-band (used for ionospheric correction) | Numerical retracking [4] | MLE3 | NA | NA |
S6PP performances over open-ocean
Thanks to numerical retracking (LR and HR) and the range walk correction (HR), instrumental drifts are taken into account in both LR and HR processings. There is no more instrument impact on Global Mean Sea Level (GMSL) estimations. Moreover, instrument orbital variations are taken into account by using the 10 Hz ECHOCAL Point Target Response (PTR).
Sentinel-6MF HR data are strongly impacted by ocean vertical velocity (OVV) effect, leading to a strong overestimation of HR Significant Wave Height (SWH): up to 60 cm bias between LR and HR SWH. Thanks to the LUT developed by NOAA [6], the OVV impact on HR SWH can be corrected, as shown on Figure 1. Original S6PP HR data and data corrected from OVV are included in the present dataset. Corresponding Sea Stata Bias (SSB) and Sea Surface Height Anomaly (SSHA) are also provided.
Thanks to numerical retracking, S6PP LR SSHA is more in line with J3 than PDAP (Figure 2, grey curves). Remaining SWH small dependency is due to pulse-to-pulse correlation effects. A first estimate of this effect on SSH data has been estimated and is provided in this S6PP dataset.
In HR, SWH dependency is reduced with respect to PDAP (Figure 2, red curves), thanks to both numerical retracking and skewness aligned with LR and J3. Once the OVV correction is applied (though the SSB), the SWH dependency is again reduced and even suppressed for SWH > 2.5 m (Figure 2, orange curve).
Product
These products use the same netcdf format as standard PDAP L2 altimetry products: same grouping and variable names are used (for more details:
https://eumetsatspace.atlassian.net/wiki/spaces/SEN6/pages/1644036189/S6+altimetry+products+level-2). The content has been adapted: some variables have been suppressed (such as instrumental corrections) and others have been added.
Below is the list of additional variables:
- In LR products:
- data_01/ku/p2p_corr_ssh: Correction to be applied on Ku-band LR Sea Surface Height to correct for the pulse-to-pulse correlation effect.
- In HR products:
- data_01/ku/swh_ocean_ovv: Ku-band SWH corrected from ocean vertical velocity effect
- data_01/ku/sea_state_bias_ovv : SSB computed using above SWH
- data_01/ku/ocean_geo_corrections_ovv: computed using above SSB
- data_01/ku/ssha_ovv: computed using above geophysical corrections
- data_20/ku/swh_ocean_ovv: Ku-band SWH corrected from ocean vertical velocity effect (at 20 Hz)
Radiometer parameters such as the wet tropospheric correction have been retrieved from PDAP L2 data. Geophysical models have been updated using inhouse CLS tools.
Metadata
Description: Sentinel-6MF altimetry products generated by the CNES/CLS processing prototype (S6PP) including l1/l2 algorithms that are planned to be implemented within the S6 operational chain.
Contents: Along-track level-2 estimations at 20-Hz and 1-Hz rate in LR and HR mode. The geophysical corrections applied to the altimeter range are also included.
Use: open ocean (in LR/HR mode) and coastal (in HR mode) applications
Geographic coverage: Global coverage corresponding to Sentinel-6MF spatial sampling
Condition of access: Users should keep in mind that these products are experimental products. They are not produced on an operational basis. They are freely distributed without any warranty of any kind.
Citation: When using the S6PP products, please cite: "Those products were processed by CLS and distributed by AVISO+ (https://www.aviso.altimetry.fr) with support from CNES (https://doi.org/10.24400/527896/a01-2022.015)”
Format: NetCDF
Copyright: 2021-2022 CNES/CLS
References
[1] Dinardo et al., "Sentinel-6 MF Poseidon-4 Radar Altimeter: Main Scientific Results from S6PP LRM and UF-SAR chains in the first year of the mission", in review
[2] Maraldi C. et al., “Sentinel-6 Processing prototype Data Release”, OSTST 2022 poster,
[3] Dinardo et al., “Sentinel-6MF Poseidon-4 main results from the first year of mission from the S6PP LRM and UF-SAR chain”, OSTST 2022 poster http://doi.org/10.24400/527896/a03-2022.3375
[4] Buchhaupt C., Fenoglio-Marc L., Dinardo S., Scharroo R., Becker M.: A fast convolution-based waveform model for conventional and unfocused SAR altimetry. Advances in Space Research, Vol. 62, Issue 6, 2018, Pages 1445-1463, ISSN 0273-1177. https://doi.org/10.1016/j.asr.2017.11.039
[5] Guccione P.: Beam Sharpening of Delay/Doppler Altimeter Data Through Chirp Zeta Transform. In IEEE Transactions on Geoscience and Remote Sensing, vol. 46, no. 9, pp. 2517-2526, Sept. 2008, https://doi.org/10.1109/TGRS.2008.918863
[6] Egido A., Buchhaupt C., Boy F., Maraldi C.: Correcting for the Discrepancies in the S6-MF HR SWH measurements. 3rd Sentinel6 Validation Team Meeting, October 2021. Available on-line at the URL https://cdn.eventsforce.net/files/ef-xnn67yq56ylu/website/29/2.4-agencies_validation_noaa_vwm_final_short_version.pdf
 | to get access Aviso products and select the product "Along-track Experimental Sentinel 6MF products (CNES R&D processor)" |
| Satellite | Dataset | Distribution media | Frequency | Delivery delay | Data period | Data used | File weight |
|---|---|---|---|---|---|---|---|
Experimental | HR | Please refer to MY AVISO+ | - | one shot | Apr 2021 - Apr 2022 | One pass per file | 20MB per pass file |
| LR | 12MB per pass file |
