In conclusion of the algorithms maturity assessment and the observation system review, these are the main conclusions and perspectives, to efficiently prepare the CRISTAL mission:

  • Above all, there is a clear need for improving our knowledge and undertanding of the dual Ku/Ka band behaviour over ice surfaces. In particular:
    • to retrieve the altimeter range at snow/ice interface over sea-ice and ice sheets, and subsequently to remove variable biases induced by volume scattering
    • to improve snow depth retrieval over sea-ice floes exploiting simultaneous Ku and Ka measurements
    • => These improvements can be assessed by comparing and analysing the measurements provided by the current altimetry constellation (Sentinel-3 ; CryoSat-2 ; AltiKa ; ICESat2) and simulations (see WP3 outcomes)
  • A physical retracker accounting for Mean Square Slope (linked to the roughness of the surface) is mandatory to provide continuity of sea topography measurements from open ocean to leads into the sea-ice. Several retracking algorithms are already available, research and development are still highly recommended for improving their performances and validating their outcomes.
  • Research and development must also pursue on the Fully-Focused algorithm, providing a dramatic reduction of the along-track footprint, highly valuable over sea-ice (and inland water).
  • Over the ice sheets, benefits of the Ka-band have been poorly documented, and only few publications can be found in the literature about SARAL/AltiKa performances [Remy et al., 2014 ; Suryawanshi et al. ,2019]. 7 years of AltiKa data (2013 – 2020) over ice sheets have to be studied in detail, along with Ku/Ka comparisons.
  • Current concerns relative to the Open Loop Tracking Command (OLTC) over ice sheets (in particular for Sentinel-3) have to be overcome.
  • Open-burst (interleaved) chronogram would be highly desirable over inland waters to largely benefit of the Fully Focused-SAR processing without ambiguities.
  • Similar performances expected between Sentinel-3 and CRISTAL over open-ocean considering the similarity of altimeter configurations. Studies have to be made to anticipate how the dual Ku/Ka bands can be exploited.

As a conclusion to this work, several modifications were made on the Mission Requirement Document (version 2.0). These modifications were discussed by the Mission Advisory Group. A dozen of requirements were finally modified to be more consistent with CRISTAL capabilities and objectives. This led to a new version of the MRD (V3.0).