Abstract's details

Sentinel-6-MF Poseidon-4: Main results from the first year and half of mission from the S6PP LRM and HRM Chain

Salvatore Dinardo (CLS, France)


Emeline Cadier (CLS, FRANCE); Thomas Moreau (CLS, FRANCE); Claire Maraldi (CNES, FRANCE); Francois Boy (CNES, FRANCE); Adrien Guerou (CLS, FRANCE); Nicolas PICOT (CNES, FRANCE)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Sentinel-6 Validation Team (S6VT) feedbacks

Presentation type: Type Poster

Sentinel-6 is an Earth Observation satellite constellation part of the EU Copernicus Programme developed by ESA, operated by EUMETSAT in collaboration with NASA, NOAA and CNES. The first satellite of the Sentinel-6 constellation (Sentinel-6 Michael Freilich, S-6 MF in short) has been launched on 21 November 2020.
The S6-MF satellite embarks as main scientific payload the sensor Poseidon-4 (POS4) which is a dual frequency redundant radar altimeter. It represents a significant breakthrough with respect to its predecessors Jason-class altimeters thanks to its digital architecture based on an on-board digital matched-filtering.
In the frame of the exploitation of the S6 MF altimetry mission, CNES has contracted CLS for the development of the Sentinel-6 Processing Prototype (S6PP). S6PP is a multi-chain (LRM, UF-SAR, FF-SAR, Pulse-Pair, Transponder) processor in which the novel algorithms developed in the CNES/CLS R&D activities are implemented and validated in support to the different thematic applications and in view of promoting them for a possible implementation in operational ground segment.
The present work covers in particular the main results over open ocean for the main altimetric geophysical measurements the LRM (Low Resolution Mode) and High Resolution Mode (HRM, also known as UF-SAR) chains of S6PP in terms of precision, accuracy and spectral content in Ku band.
Given the POS4 sensitivity to in orbit temperature variations, the instrument drift and the requirement to measure the GMSL (Global Mean Sea Level) in seamless continuity with Jason-3, the mandate for S6PP was to process the S6-MF with minimum possible level of approximations along the processing chain.
Hence, for this purpose, novel algorithms have been developed and implemented inside the S6PP LRM chain, as:
o LRM Physics-Based Waveform (frequency-domain) Model [REF6] with possibility to set in input the ocean skewness coefficient (as 0.1 per Jason heritage)
o Numerical LRM Retracking based on In-Flight PTR or on Theoretical PTR (sinc**2)
o Possibility to retrack the LRM waveform built on board (LRM I2Q2) or the one built on ground from HRM RAW/RMC L1A data products

and also inside the S6PP HRM chain, as:
o Beam-Forming carried-out by Chirp-Zeta Transform (CZT) in the approximate beam-steering configuration in order to correct for the range-walk effect [REF1] with only a limited increment of CPU time (10%)
o Possibility to use different multi-look number in building the waveform in order to be less impacted by the ocean surface motion effect as orbital wave velocity [REF2]
o Posting Rate of the SAR waveforms at the standard 20 Hz or higher [REF3, REF4, REF5]
o SAR Physics-Based (frequency-domain) Waveform Model [REF6] with possibility to set in input the ocean skewness coefficient (as 0.1 per Jason heritage)
o Numerical SAR Retracking based on In-Flight PTR or on Theoretical PTR
o Delay-Doppler Map ambiguities natively modelled in the SAR waveform Model [RE7]

The input data products are from STC latency for the HRM chain and NTC for the LRM chain; however, the orbits have been updated for both chains using the JPL ones.
Furthermore, it was decided in LRM mode to retrack the LRM waveform built on board (LRM I2Q2) at full PRF.
The ionospheric and wet tropospheric correction will be the one from Jason-3 in order to have a homogeneous set of geophysical corrections for the whole mission time.
The entire first year of S-6 MF data (from beginning of the Tandem Phase) have been processed in the afore-said baselines for the LRM and HRM data flavors and the GMSL in both the modes will be derived and compared to Jason-3 in order to verify the continuity between the two missions in LRM and HRM.
Finally, the LRM data will be processed with and without the In-Flight PTR in order to assess the impact of the In-Flight PTR shape evolution in term of sea level stability, considering the strong aging reported by S6 MF POS4 altimeter.
The In-Flight PTR will be the one built from the ECHO-CAL calibration data.
Instead, the HRM data will be processed with a beam-forming based on CZT (with a range walk correction) and with a beam-forming based on Fast Fourier Transform (i.e. no range walk) in order to assess the impact of the range walk effect on the S6-MF sea level stability. The impact of the ocean topography skewness in HRM will be also addressed and even estimated for the first time from the HRM data in order to confirm the heritage average value of 0.1
The reference dataset for the spectral analysis and stability validation will be Jason-3 dataset.

[REF1] https://doi.org/10.1016/j.asr.2019.11.032
[REF2] https://doi.org/10.3390/jmse8060447
[REF3] https://www.aviso.altimetry.fr/fileadmin/documents/OSTST/2013/posters/Dinardo_coastal.pdf
[REF4] https://doi.org/10.1016/j.asr.2020.03.014
[REF5] https://doi.org/10.1016/j.asr.2020.09.037
[REF6] https://doi.org/10.1016/j.asr.2017.11.039

Poster show times:

Room Start Date End Date
Mezzanine Tue, Nov 01 2022,17:15 Tue, Nov 01 2022,18:15
Mezzanine Thu, Nov 03 2022,14:00 Thu, Nov 03 2022,15:45
Salvatore Dinardo