How Satellite Altimetry Works

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As Figure 4.1 on the previous page illustrates, satellite altimetry measurements are obtained by a system of instruments carried on a satellite orbiting the Earth. The instruments include an altimeter and antenna, which measure sea surface height; a radiometer, which measures atmospheric disturbances, and a GPS system for precisely determining the satellite’s location. The altimeter transmits rapid (1700/second) pulses of microwave energy towards the Earth, which reflect back to the satellite. The average round-trip time of these pulses is accurately measured to determine the exact distance between the satellite and the sea surface (range).  Water vapor measurements are also made as the level of water vapor affects the rate of transmission of the pulses, and a correction must be made to obtain the final range, which is accurate to 2 cm. This range must be referenced to the reference ellipsoid, which is an approximation of the Earth’s surface (the sphere flattened at the poles discussed above). The GPS receiver onboard and ground-based radio receivers track the satellite’s exact location. Using these data, sea surface height can be accurately measured. In addition, the ocean surface topography (the highs and lows depicted on the images) are obtained through calculations. This information is key to understanding the ocean’s surface as a dynamic and complex terrain and to determining changes over time.

The Jason satellites have revealed critically important information that was not available prior to the mid 1990s. As the technology develops and more data are added to the database, our understanding of the changing ocean increases. Among the many scientific goals of the Jason and other altimetry satellite systems currently in use, are to extend the time series of ocean topography measurements begun in 1992 and to monitor the changes in global mean sea level and its relationship to global climate change. Since the mid 1990s, there has been an explosive growth in ocean and climate studies, and multiple altimetry satellites have provided longer and more accurate measurements and have led to better spatial and temporal coverage and resolution. These accurate and detailed measurements, in turn, inform predictive science on sea level change.

In addition, important information on ocean circulation and the relationships between heat transport and other variables such as nutrients and salt content are obtained, as well as measurements of wave height. These data can be used in modeling that inform our understanding of tides, weather, and other dynamic phenomena at work on our planet. This technology continues to add the knowledge and understanding of our ocean.

Recommended Reading

More detail on the Jason mission can be found on Jason-2: Using Satellite Altimetry to Monitor the Ocean  and NASA Visualization Explorer: Earth's Rising Seas.

The uneven nature of the surface of the ocean is expressed in the maps below. These images were compiled from satellite altimetry data to show anomalies in sea levels and temperature. These types of data are used in sea level predictions. The complex science involved in tracking sea levels is evolving rapidly as it answers a pressing need to provide accurate predictions in a rapidly changing world.

Sea-level variations from Sentinel-3A. Higher along southern coasts and equator
Figure 4.2. Sea level anomaly data mapped from European Sentinel satellite data. Data show differences in sea-surface height compared to the mean. Cool colors represent lower sea levels and warm colors represent higher sea levels.
Credit: ESA: Space in Images. Copyright: contains modified Copernicus Sentinel data (2016), processed by ESA and CNES, CC BY-SA 3.0 IGO