Mapping Sea Level from Space Precision Orbit Determination and Satellite Altimetry

Since 1992, a series of satellite missions, beginning with TOPEX/Poseidon (T/P) and followed by Jason-1 and the Ocean Surface Topography Mission on Jason-2 (OSTM/Jason-2), have combined precision orbit determination (POD), a sophisticated method to determine precise height of spacecraft above the center of the Earth, and satellite altimetry to make precise measurements of sea surface height (SSH) and to map ocean surface topography. These missions' unprecedented continuous 18-year-long record of SSH has revolutionized oceanography. With support provided by the National Aeronautics and Space Administration (NASA), the National Oceanographic and Atmospheric Administration (NOAA), and European partners (the French space agency, also known as the Centre National d'Etudes Spatiales (CNES), and the European Organisation for the Exploitation of Meteorological Satellites (Eumetsat)), these altimetry missions continue to help us understand the effects of the changing ocean on climate and provide significant benefits to society. Their measurements are being used to map SSH, geostrophic velocity, significant wave height, and wind speed over the global oceans. Orbiting at a height of 1,336 km above Earth's surface, the satellites measure the SSH every 6 km along the ground track, with an accuracy of 3-4 cm, covering the global oceans every 10 days. These highly accurate measurements would not be possible without the ability to determine the satellite's exact position relative to the center of the Earth. This is achieved by using POD. Three of the five instruments on board the spacecraft provide critical satellite tracking information for POD. The NASA Laser Retroreflector Array (LRA) uses satellite laser ranging. The CNES Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system uses Doppler radio data and a high-performance global positioning system (GPS) receiver that provides range, precise carrier phase, and timing signals. POD combines satellite tracking information with accurate models of the forces acting on the satellite (e.g., gravity, aerodynamic drag) that govern the satellite motion. This process provides the very-high-precision satellite orbital heights that, together with satellite altimetry, allow accurate estimation of SSH. Data from these missions have proved to be a key to understanding Earth's delicate climate balance and are a critical component of global climate studies. They provide insight on short-term climate events, such as El Nino and La Nina, as well as longer-term climate events, such as the Pacific Decadal Oscillation (PDO). Altimeter data products are currently used by hundreds of researchers and operational users over the globe to monitor ocean circulation and improve our understanding of the role of the changing ocean in climate and weather. The missions' measurement of rising sea level, a direct result of Earth's warming climate, are especially important for coastal communities and decision makers and might help save lives and property. The legacy of satellite altimetry created by TIP; Jason-1, and OSTM/Jason-2 and the important data record they have collected are being continued. To ensure continuity with these missions, a group of nations and their science organizations plan to launch Jason-3 in 2013, Jason-CS/4 by 2017, and a next-generation Surface Water and Ocean Topography (SWOT) mission by end of the decade.