Exploring SAR interferometer calibration solutions for ocean surface current errors for the Wavemill space mission

National Oceanographic Centre with Starlab Ltd and Airbus D&S Ltd

Project Funded in the CEOI-ST 7th Call for EO Technologies.

Fast Track Project

The Wavemill concept for measuring ocean surface currents globally, at unprecedented spatial resolution using a novel SAR approach, makes it a potential Earth Explorer 9 Call candidate mission. It is also a CEOI indicative mission. The potential of this approach has been shown using the Wavemill airborne demonstrator, but that work pointed up the need for accurate calibration of the interferometic phase from which the ocean surface current measurements are obtained. Here we propose to investigate the calibration problem theoretically and suggest a solution that can be implemented technologically both for a future Wavemill space mission and for the existing airborne demonstrator. As part of the study new antenna hardware will be developed that can be used with the  Wavemill  airborne  demonstrator  to  enable  dual-sided  viewing  and  therefore  emulate  the calibration approach of the flight instrument. This study on the calibration aspects of the Wavemill instrument represents a new and ground-breaking area of work: namely, the calibration of squinted interferometric SAR systems.

Project Status and Achievements

This CEOI-ST Fast Track project developed system and instrument phase calibration solutions for Wavemill. Its measurement principle relies on single-pass squinted along-track SAR interferometry to deliver high-resolution maps of total ocean surface current vectors with collocated wind vector and spectral wave data. The stringent 5 cm/s science requirement on current accuracy demands rigorous assessment of the contribution of phase calibration errors to the geophysical error budget. The project led to significant advances in estimating unwanted wave-induced velocities and ocean surface velocity accuracy and the impact of errors in platform attitude and interferometric baseline knowledge. It provided clear recommendations for the system and mission definition, including the fact that dual-sided viewing is not an absolute requisite to achieve the necessary phase calibration accuracy.

Major benefits:

  • New world-leading scientific capability at NOC to analyse and interpret airborne Wavemill data, to model and correct airborne and spaceborne ATI SAR and SAR data for unwanted wave-induced velocities, to support the design of future systems and plan future airborne campaigns.
  • Refined instrument and mission concept for the Wavemill mission, particularly regarding the calibration elements, resulting in a more mature concept.
  • Improved understanding of the calibration requirements and identification of the parameters that impact calibration for an along-track interferometric instrument. New implementations and techniques to ensure performance remains within required levels. Several results are applicable to both along-track and across-track interferometry and are therefore transferable.
  • Improved software tools for performance analysis and Wavemill data simulation.
  • Roadmap for Wavemill to identify critical priorities, including hardware and software developments, validation of the geophysical inversion and new airborne campaigns.

Key outputs and achievements

  • Scientific presentations and publications
  • Documented interferometric phase calibration strategies and instrument and system definition.
  • These were of mutual benefit to the ESA Ocean Surface Current Mission study.
  • Wavemill technical and scientific roadmap

Improved positioning for future opportunities

  • High-visibility within ESA of existing UK technological and scientific capability for the development of the Wavemill mission, leading to increased leveraging of ESA funding from later competitive ESA Invitations To Tender e.g. ESA OSCAR airborne demonstrator.
  • The Wavemill Along-Track Interferometric mission design has been matured through this CEOI study; tackling the key question of calibration. This positions the consortium well should such a mission be required.
  • Consolidated UK presence at international science meetings (remote sensing, oceanography and operational forecasting) to present the capability and high level of maturity of the Wavemill concept and therefore increase the credibility of the mission as a candidate to EE9.
  • High-visibility within the science community to consolidate the science objectives and requirements, to gather international support for the mission and improve its chances of selection in EE9.