Copernicus Altimetry Services (COPAS)

About

Altimetry products are important for operational oceanography, weather forecasting, marine meteorology, and climate and environment monitoring. The key geophysical variables relating to the ocean are sea-surface height anomaly (SSHA), significant wave height (SWH) and surface wind speed. The SSHA is essential for monitoring sea level rise and climate evolution, whilst SWH and wind speed are used in operational oceanography and are assimilated into ocean models.

EUMETSAT provides high-quality altimetry products from the Copernicus Sentinel-3, Sentinel-6 and Jason-3 missions. In the future EUMETSAT will also operate the global ocean products for the High-Priority Candidate Mission (HPCM), Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL). These operational mission satellites contribute to EUMETSAT's long-term measurement of global sea-levels for climate monitoring and supply altimetry data for marine meteorological forecasts.

To continue supporting these application areas and their users, and to fully meet evolving standards and user needs, continuous performance monitoring of the altimetry operational products is necessary. This supports the provision of operational services. Additionally, routine improvements to the altimetry products are required to meet application needs. Evolutions and improvements are prepared for subsequent implementation into EUMETSAT’s suite of operational products.

COPAS Service is implemented by a consortium of experts in the ocean, coastal and sea ice domain, led by Collecte Localisation Satellites (CLS):

Objectives

The Copernicus altimetry service (COPAS) has the following objectives:

• Activity 1: Mission Performance of Sentinel-3 altimetry marine data products and routine monitoring of Sentinel-3 altimetry marine and Sentinel-6 altimetry data products.
• Activity 2: Study to improve the Sentinel-3 Polar Ocean retrievals (WP#2.1)
• Activity 3: Sentinel-6 Numerical Retracker (WP#2.3, 2.4)
• Activity 4: Sentinel-3 and Sentinel-6 Range Walk (WP#2.5)
• Activity 5: Sentinel-3 SRAL Autocal (WP#2.9)
• Activity 6: CRISTAL Scientific Studies (WP#2.6)

Overview

Activity 1: Mission Performance and Routine Monitoring of Altimetry Products from Sentinel-3 and Sentinel-6 Missions

This service includes the implementation and operation of a Mission Performance Service to support the Sentinel-3 and Sentinel-6 altimetry products generated by EUMETSAT’s Ground Segment. Specifically, it encompasses:

• NRT Service for Sentinel-3 and Sentinel-6:
  • Quality control of Sentinel-3 and Sentinel-6 Level-1 and Level-2 NRT products generated at EUMETSAT.
  • Systematic monitoring of key parameters impacting the data quality of the NRT products using automatic tools.
  • Generation and dissemination of daily reports and initial investigation of product anomalies.
• STC/NTC Service for Sentinel-3:
  • Quality control of STC data and long-term monitoring and validation (NTC data) of Marine Sentinel-3 Level-1 and Level-2
  • Monitoring of the tracking mode of Sentinel-3.
  • Monitoring of the internal calibration of Sentinel-3 and its use in the Marine science processing (Level 1 CAL-1, CAL-2, LTMs).
  • Monitoring and validation of the STC and NTC Level 2 Marine products using external and independent datasets (e.g. in-situ, numerical model or satellite data).
  • Generation of Daily Reports for the STC product.
  • Generation of Cyclic Reports for the NTC products.
  • Investigations of products anomalies.

Activity 2: Scientific Study to Improve the Sentinel-3 Polar Ocean retrievals (WP2.1a and WP2.1b)

To meet the needs of the Copernicus Marine Environment Monitoring Services (CMEMS) for polar ocean data, the Sentinel-3 Marine data processed at EUMETSAT requires enhancements in sea surface height retrievals over sea-ice leads, ensuring continuity with open ocean data. EUMETSAT has defined a roadmap for these improvements and has identified specific scientific studies to support this effort. This activity is divided into two work-packages, WP2.1a and WP2.1b:

• 1a: Conduct a scientific study on spectral weighting windowing in the along-track direction for Delay-Doppler processing.
• 1b: Conduct a scientific study focused on waveform classification using neural networks.

EUMETSAT will use the results of these studies, along with in-house activities outlined in the evolutions roadmap, for the implementation and validation of the evolutions in the SRAL Level 1 and Level 2 Processors within the Sentinel-3 Ground Segment Payload Data Processing (PDP).

For WP 2.1a, the spectral weighting windowing ensures the suppression of side-lobe leakage over specular targets (such as leads between sea-ice floes) while minimizing degradation of the ocean sea level wavenumber spectrum and measurement random noise. This weighting window is particularly relevant for polar ocean altimetry as it rejects off-nadir spurious returns from the side lobes of the along-track Point Target Response, which can impair an accurate retracking of the nadir leading edge of the waveform. This study aims to define an ideal spectral windowing for global ocean altimetry.

For WP 2.1b, EUMETSAT will use the study results in the operational SRAL Level 1 Processor to categorize SAR waveform shapes into several typological classes. Since diverse geometrical shapes of waveforms are expected in polar ocean regions due to different surface backscattering conditions, distinguishing these shapes is crucial. For example, identifying leads among ice-floes ensures continuity between open-ocean and polar ocean altimetry data.

Activity 3: Sentinel-6 Numerical Retracker (WP2.3 and WP2.4)

The purpose of this activity is to specify a numerical retracker for the low resolution (WP2.3) and high resolution (WP2.4) measurements of Sentinel-6. This retracker will be implemented in the Altimeter Level 2 Processor within the Sentinel-6 Payload Data Processing (PDP). The derivation of the numerical ocean retracking power waveform model will be based on the approach proposed by Buchhaupt et al. (https://doi.org/10.1016/j.asr.2017.11.039). This approach solves the convolutional model equation in the frequency-domain and subsequently expresses it in the time-domain via an IFFT operation, commonly referred as “frequency-domain approach”.

Activity 4: Sentinel-3 and Sentinel-6 Range Walk (WP2.5)

The purpose of this activity is to provide the specification description of Delay-Doppler Beam Forming algorithm on Sentinel-3 and Sentinel-6 HR burst data (comprising 64 pulses) to focus a synthetic fan of 64 Doppler Beams. The Beam-Forming is performed in both the approximated and exact beam-steering configurations, including the range-walk correction. In the first case, the correction is embedded in a Chirp-Zeta Transform (CZT) process, and in the second case, it is implemented via a series of 64 Fast Fourier Transform (FFT) operations.

Range-walk refers to the variation in the range between the sensor and the generic scatterer (also referred to as “surface-sample location”) within the illumination (or coherent integration) time.

The CZT algorithm allows for range-walked beam-forming without significant approximations, while being computationally efficient.

Activity 5: Sentinel-3 SRAL Autocal and SWH LUT (WP2.9)

EUMETSAT is working to enhance the measurement stability of marine data in both power and range to make the data more useful for climatic applications and to better align scientific algorithms across Copernicus marine altimetry missions. Additionally, EUMETSAT is focused on evolving the contents of Level 2 products by including more detailed information about product quality, particular validity flags.

The purpose of Work Package WP2.9 is to conduct two scientific studies aimed at improving Sentinel-3 data products:

1. Study 1: Improve the stability of Sentinel-3 marine data through a new autocalibration strategy. Autocal calibration refers to the CAL-1 SAR calibration mode of the SRAL altimeter, which involves calibrating the programmable radio-frequency attenuation steps of the altimeter. Accurate and stable calibration is crucial for climate applications, where high stability in range and power is required. This task will address limitations of the current SRAL autocal scheme and explore potential improvements for implementation in operations.
2. Study 2: Develop a significant wave height (SWH) look-up table (LUT) to support enhanced Level 2 product evolutions. This LUT will enable the filtering of both nominal and degraded SWH data. Currently, the Sentinel-3 Level 2 product includes quality flags for sea level, wind, and waves. However, the quality flags for SWH lack a discriminator based on SWH root mean square (RMS), as outlined in the literature (Queffeulou, 2016). This study aims to improve these quality flags to help users better distinguish between reliable and spurious SWH data.

EUMETSAT may use the results of these studies, along with in-house activities defined in the evolutions roadmap, to implement and validate enhancements in the SRAL Level 1 and Level 2 Operational Processors within the Sentinel-3 Ground Segment Payload Data Processing (PDP).

Activity 6: CRISTAL Scientific Studies (WP 2.6)

The Copernicus Expansion Missions (CEM) include new missions prioritised by the European Commission (EC) for implementation in the coming years. A high priority for the EU is monitoring the rapid changes occurring in the Arctic environment and Polar Regions. The Copernicus Polar Ice and Snow Topography Mission (CRISTAL) is one of these priority missions, alongside CIMR and ROSE-L.

CRISTAL’s secondary objective is “to contribute to the observation of the global ocean topography as a continuum up to the polar seas [....] and to support applications related to coastal [….] waters”. This makes CRISTAL highly relevant to EUMETSAT’s marine topography user community. The mission is expected provide new insights into ocean topography and expand EUMETSAT’s topography product portfolio.

EUMETSAT has been entrusted with operating the marine data centre for the CRISTAL mission. This study aims to support EUMETSAT’s Remote Sensing Products Division Altimetry team in defining the technical specifications for the CRISTAL marine Ground Segment Product Generations Functions. These specifications are needed to produce Test Data Sets for the verification and validation of the CRISTAL operational ground segment.

The study will focus on:

• Establishing a state-of-the-art framework for the CRISTAL marine data centre highlighting the challenges of the CRISTAL mission.
• Defining the specifications for Level-1 and Level-2 product generation at the CRISTAL marine data centre.
• Defining the formats for Level-1 and Level-2 products at the CRISTAL marine data centre.