Satellite altimetry
Introduction
Satellite altimetry is a remote sensing technique used for measuring the altitude above the planetary reference ellipsoid of a point on the Earth's surface. The technique involves the use of radar altimeters on board satellites to measure the time delay between the transmission of a pulse and its return after reflecting off the surface. This information, combined with precise satellite location data, enables the determination of the height of the Earth's surface.
History
The concept of satellite altimetry was first proposed in the 1960s, with the launch of the first altimetry satellite, Seasat, by NASA in 1978. This was followed by the Geosat mission in 1985 and the European Remote-Sensing Satellite (ERS) in 1991. These early missions provided valuable data, but their coverage was limited and the accuracy of their measurements was not sufficient for many applications. The advent of the TOPEX/Poseidon satellite in 1992 marked a significant advancement in the field of satellite altimetry, providing global coverage and improved accuracy.
Principles of Operation
The basic principle of operation of a satellite altimeter involves the transmission of a radar pulse towards the Earth's surface and the measurement of the time it takes for the pulse to return to the satellite after reflection. This round-trip time, when multiplied by the speed of light, gives the total distance travelled by the pulse. Subtracting the known altitude of the satellite above the reference ellipsoid from this distance gives the height of the surface above the reference ellipsoid.
Altimetry Data Processing
The raw data received by the altimeter is subject to several sources of error, including atmospheric delay, sea state bias, and instrument errors. These errors are corrected through a series of data processing steps, which include:
- Orbit Determination: This involves the precise determination of the satellite's position and velocity at the time of the altimeter measurement. This is typically achieved through the use of precise orbit determination techniques, which make use of data from tracking stations and/or Global Navigation Satellite Systems (GNSS).
- Atmospheric Correction: The radar pulse transmitted by the satellite is delayed as it passes through the Earth's atmosphere. This delay is corrected for by using models of the atmospheric pressure and humidity, and data from meteorological reanalysis.
- Sea State Bias Correction: The sea state, or the height and roughness of the ocean waves, can affect the altimeter's measurements. This is corrected for by using models of the sea state and data from wave height sensors.
- Instrument Correction: This involves the correction of errors introduced by the altimeter instrument itself, such as timing errors and electronic noise.
Applications
Satellite altimetry has a wide range of applications, including:
- Oceanography: Altimetry data is used to measure sea level rise, ocean circulation, and wave height. These measurements are critical for understanding the Earth's climate system and predicting future climate change.
- Glaciology: Altimetry data is used to measure the height and volume change of glaciers and ice sheets. This information is important for understanding the impacts of climate change on the Earth's cryosphere.
- Hydrology: Altimetry data is used to measure the height of rivers and lakes, providing valuable information for flood forecasting and water resource management.
- Geodesy: Altimetry data is used to determine the Earth's geoid and to study tectonic activity.
Future Directions
The field of satellite altimetry continues to evolve, with new missions planned and improvements in data processing techniques. Future missions are expected to provide even more accurate and higher resolution data, enabling new applications and improved understanding of the Earth's system.