A groundbreaking new satellite, the US$1.5 billion NISAR, is poised for launch from India's Satish Dhawan Space Centre near Chennai. This almost 3-tonne satellite, equipped with a 12-meter radar antenna, is designed to detect changes on Earth’s surface with centimetre precision, providing near real-time data irrespective of time of day or weather conditions. Its advanced capabilities promise invaluable information for diverse fields, including agriculture, climate science, and natural disaster response teams.

For decades, Earth-imaging satellites have been indispensable scientific tools, offering critical data for weather forecasting, emergency planning, and tracking long-term environmental changes. However, many of these traditional observation satellites rely on reflected sunlight, limiting their operation to daytime and clear skies. This poses significant challenges in regions with frequent cloud cover, such as tropical areas, or when nighttime imagery is essential.

The NISAR satellite, a collaborative effort between the national space agencies of the United States (NASA) and India (ISRO), overcomes these limitations through the use of synthetic aperture radar (SAR) technology, which also lends the satellite its name (NASA-ISRO SAR). Invented in 1951 for military applications, SAR technology differs fundamentally from passive optical imaging. Instead of relying on reflected sunlight, SAR satellites actively beam a radar signal towards Earth's surface and then detect the reflected signal, akin to using a flash in a dark room to take a photo. This active approach allows SAR satellites to capture images both day and night.

Furthermore, radar signals can penetrate most clouds and smoke unimpeded, enabling SAR satellites to image the Earth's surface even under adverse conditions, which is particularly crucial during natural disasters like floods, bushfires, or volcanic eruptions. Radar signals also have the unique ability to penetrate certain structures, such as thick vegetation, and are effective at detecting the presence of water due to how water influences reflected radar signals. The European Space Agency's recent Biomass mission, for instance, utilized SAR signals' vegetation-penetrating properties to image the 3D structure of forests and accurately measure biomass and carbon storage.

The NISAR mission, which has been in development for over a decade, is one of the most expensive Earth-imaging satellites ever built, and its data will be freely and openly supplied worldwide. It will provide high-resolution images of nearly all land and ice surfaces globally, with a revisit time of twice every 12 days. While similar in scope to the European Space Agency’s Sentinel-1 SAR satellites, NISAR sets itself apart as the first SAR satellite to employ two complementary radar frequencies. This allows for higher resolution imagery compared to Sentinel-1, greater coverage of Antarctica, and radar frequencies that penetrate deeper into vegetation.

The satellite's applications are extensive. It will be crucial for monitoring forest biomass and, through its ability to penetrate vegetation and detect water simultaneously, accurately map flooded vegetation. This is vital for understanding Earth’s wetlands, which are biodiversity hotbeds and significant carbon sinks. NISAR will also detect subtle changes in Earth's surface height, down to centimetres or even millimetres, by analyzing shifts in reflected radar signals. This technique will be used to track the subsidence of dams, map groundwater levels, and monitor land movement and damage resulting from earthquakes, landslides, and volcanic activity, aiding disaster response teams in damage assessment and planning.

Beyond environmental monitoring and disaster management, the NISAR satellite will significantly benefit agricultural applications. It possesses a unique capability to estimate soil moisture levels with high resolution in all weather conditions. This data is invaluable for farmers, helping them determine optimal irrigation times, thereby potentially improving water use efficiency and crop yields. Other key applications of the NISAR mission include tracking the flow of Earth’s ice sheets and glaciers, monitoring coastal erosion, and tracking oil spills. This highly ambitious satellite mission is expected to deliver numerous benefits for both science and society.