Earth's geoid as seen by GOCE

Therefore, sea levels in areas of warm water are higher than sea levels in areas of cold water. A rise in sea level of an average of 3 mm/yr was observed between 1993 and 2008. This rise, however, was not the same everywhere. In some regions it was much higher than average, while in others it was much lower.
 
Sea level is measured by means of a technique called altimetry, which uses satellite images. An altimeter is an active radar sensor. It works in the microwave range of the electromagnetic spectrum. This kind of sensor emits regular pulses and records the travel time, the magnitude, and the shape of each return signal after reflection from the surface.

If the height of the satellite (H) is known relative to a reference level (usually a standard regular ellipsoid-shaped surface), then the height of the sea above the reference level (h) is determined by subtracting the distance (R) measured by the altimeter: h=H-R. The surface topography (h) changes over time and place. Some physical factors contribute to it, such as the distribution of gravity over the Earth, represented by a geoid (h_geoid). A geoid is a representation of the Earth’s surface in the absence of winds, currents and Earth’s rotation. The geoid has a potato-like shape.  
 
Other factors that affect surface topography include the instantaneous tidal height of the sea surface, the motion of the sea or ocean dynamic topography, and the local response of the ocean to the atmospheric pressure distribution over the ocean.

Some of the most important altimeter missions include ESA’s ERS-2 and Envisat, and CNES/EUMETSAT/NASA’s Jason series and Geosat Follow-on (GFO). Repetitive altimeter measures of sea level achieve an accuracy of about 1–2 cm. Anomalously high heat content causes abnormally high sea surface height. This positive anomaly is shown in red in the series of data provided (see right navigation bar). The negative anomalies appear in blue, indicating that sea level is lower than usual.
 
 

Satellite Origin Altitude Revisit time ERS-1 (1991) ESA 800 km 35 days ERS-2 (1995)       ENVISAT (2002) RA-2       TOPEX/POSEIDON (1992) France/USA 1330 km 10 days JASON-1 (2001)       GFO(1998) USA 880 km 17 days

 


LEOWorks exercise

Make an animation with the series of data provided by the altimetry sensor.

There are three series of data, one from 1997-1998, one from 2005-2006, and one from 2007-2008. Compose three animations with the three sets and study them. Describe what happens in every period and make comparisions.

In the LEOWorks image processing programme, open Tools/Image Animation, which brings up the Select Files for Animation window. Choose the proper animation speed. You can see the number of the slide by activating the Active Slider.