A breakthrough of radar remote sensing of the ocean: the  Surface Water and Ocean Topography (SWOT) Mission

Remote sensing of Earth’s surface water is crucial to the study of climate change and its impact to society.  Radar remote sensing is particularly important because it penetrates cloud cover, providing observations under all weather conditions.  Forty years ago, Seasat, the first satellite designed for studying the ocean from space, laid the foundation of radar remote sensing of the ocean with radar altimeter, scatterometer, and synthetic aperture radar (SAR). The first two have become the pillars of a global observing system that has revolutionized oceanography. Precise measurement of sea surface height by radar altimetry has provided a modern record of global sea level change and the state of ocean circulation, but its spatial resolution is limited by the large radar footprint (~20 km) and measurement noise, making it difficult to study small-scale, rapidly changing ocean processes, especially near coasts. While SAR provides high-resolution images of many features of the ocean and land waters, it is difficult to derive quantitative information to study the underlying dynamics. Using the phase differences of consecutive SAR observations (a technique called radar interferometry) has allowed determination of the slow movement of ice sheets since the early 1990s. In the early 2000s, a mission was conducted onboard the Space Shuttle to map the earth’s land topography. The concept of applying radar interferometry onboard a satellite for oceanography and land hydrology was developed in the 2000s. Twenty years later the global mission called Surface Water and Ocean Topography (SWOT) was launched in December 2022. We will present early results from SWOT with a focus on the ocean. The fundamental advancement of SWOT is the capability of observing the elevation of the ocean surface and with the resolution of SAR.  The spatial resolution of the resolved ocean dynamics more than an order of magnitude better than in conventional altimetry, enabling the study of small-scale ocean eddies and fronts that are essential to the ocean’s heat and carbon uptake from the atmosphere. The increased resolution will also advance the study of near shore processes to assess the coastal impact of sea level rise and severe weather.