Ocean Surface Current Airborne Radar (OSCAR) Demonstrator

Abstract

Monitoring ocean circulation at high resolution in both space and time is of paramount importance for understanding and modelling the ocean-atmosphere climate system, especially in coastal areas. Spaceborne radar altimeters have been used to successfully monitor ocean circulation on a global scale (>30km) in the deep ocean when the geostrophic approximation is generally valid. The ocean structures seen in high-resolution satellite measurements at meso (10-100km) and sub-mesoscale (<10km) are ubiquitous but little is known about their dynamics. During the last two decades, many studies have highlighted the key role played by the ocean submesoscale in air-sea interactions, upper-ocean mixing and ocean vertical transport and the importance of ageostrophic circulation in these processus. Understanding these smaller currents is critical to drive scientific understanding of the exchanges of gas, heat, and momentum between the atmosphere and the ocean, and have important implications for forecasting models and climate projections. A satellite mission concept, called SEASTAR, aims to study ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas by providing simultaneous measurements of current and wind vectors at 1km resolution with accuracy of respectively 10cm/s and 2m/s. OSCAR (Ocean Surface Current Airborne Radar) is the demonstrator for this satellite concept, and is in development at the European Space Agency (ESA) in the frame of its preparatory activities for ocean surface current retrieval with Metasensing as a prime contractor. OSCAR system will be representative of a satellite mission concept, observation parameters and accuracies directly relate to a potential satellite mission. OSCAR is a Ku-band (13.5 GHz) three-look direction SAR system with Along-track Interferometric (ATI) SAR and scatterometric capabilities. It is tailored to the measurement of 2D ocean surface motion and wind retrieval. The OSCAR features an along-track interferometric (ATI) baseline for two fields of view that are squinted 45° fore and aft from the broadside direction. Each of these views provide an interferometric measurement and these two views are angularly separated by 90 degrees, ensuring two independent measurements of the ocean surface move-ment velocity and eventually enabling retrieval of the 2D ocean surface movement vector. In addition, a broadside antenna will ensure measurements in the zero-Doppler direction to improve the retrieval sensitivity to wind direction, which is critical to retrieve an accurate ocean surface current. Indeed the motion sensed by the microwave radar (after correcting for navigation and geometry) has two components: the total ocean surface current – consisting of all currents contributing to actual horizontal transport of water – and an unwanted measurement bias associated with wind-waves (known as wind-wave induced artifact surface velocity — WASV; see Martin et al., 2016). The WASV is un-derstood to be mainly caused by the phase velocity of the surface scatterers responsible for the microwave backscatter (e.g. Bragg waves) and the effect of the orbital motion of longer ocean waves and is at first order function of the wind direction. Fully polarimetric observations are possible and the instrument is designed to be flexible in terms of the operational parameters and configuration, including the ATI baseline length, which is adjustable to be fully repre-sentative of the potential future spaceborne mission.

A dedicated system performance analysis tool has been implemented in order to derive all the instrument and obser-vation specifications to meet the requirements on the ocean surface observables. The key performances, which are driving the instrument are a measurement accuracy of 2D ocean surface motion of 5 cm/s (for velocities below 50cm/s and wind speed > 3m/s). The performance tool is based only on numerical models without assumptions and calculates the reflected power and the Doppler characteristic of each backscattering cell of the illuminated sea surface in dependency on the radar properties, the acquisition geometry, and the flight parameters. The performance of the instrument for the baseline design achieves a swath width larger than 2km in case of wind speed of 3m/s. The noise equivalent sigma zero (NESZ) is between -30dB and -45dB and the radiometric resolution is better than 0.1dB. The velocity accuracy that the system can obtain is fulfilled from look angles lower than 20 degrees up to look angles above 63 degrees, resulting in more than 3km of swath available. A functional test campaign over ocean is planned in summer 2019 in the Iroise Sea to verify the end-to-end functionality and performance of the fully integrated instrument, including internal calibration capabilities, and of the processor by means of on-ground testing and flight tests