Description of Mission

Measurement System

(Click on the figure for larger one.)

The TOPEX/Poseidon satellite orbits the Earth, completing one cycle of 127 revolutions around it about every 10 days. (The satellite makes one revolution in 112 minutes.) During one cycle, the satellite collects the sea level height measurements for over 90% of the ice-free oceans of the entire globe at all times using two on-board state-of-the-art altimeters one by NASA for 90 % of the time and the other by CNES for 10 %.

A simple way to describe how TOPEX/Poseidon measures the sea level is that those two on-board radar altimeters measure the altitude of the satellite above the sea surface while three independent satellite tracking systems measure the satellite position, that is, the distance between the satellite and the center of the Earth (geocenter). Then, the altimetric measurements are subtracted from the satellite position to get the height of the ocean above the geocenter, which is the sea level.

Measuring the sea surface height is not as easy as it was explained above. The surface is constantly moving and influenced by some forces to have become what it appears to be. First of all, it is subject to the gravitational and rotational forces of the Earth. Also, it is modified by the wind, tides (both land and ocean tides), currents, and atmospheric pressures. Each force affects the sea surface and causes surface undulation to some degrees. The following table shows the undulation ranges.

Magnitude of Ocean Surface Perturbations

Source Height Perturbation Time Scale
Tides ~ 1m in deep ocean, many meters in shallow ocean Hours
Large-scale currents ~ 1m Months
Mesoscale currents ~ 1m Days
Wind ~ meters near coast Hours
Pressure field ~ centimeters Hours

(Introduction to the Physics and Techniques of remote Sensing, p244)

However, if none of the tidal effects and atmospheric effects (wind and pressures) existed, the sea surface would be relatively at rest and would follow an equipotential surface called "geoid," which would still undulates due to variations in the gravity. The geoid height from the reference ellipsoid is available from the previous oceanographic missions, such as GEOS-3 and Seasat. With a known satellite position relative to the reference ellipsoid (or geocenter) and the measurement from the altimeters, the sea surface height relative to the reference ellipsoid (sea level) can be computed. Also, the ocean topography due to currents, the height difference between the sea surface and the geoid, can be obtained from the determined sea level and the geoid data. Although the geoid models are available from the previous missions, it still must be refined with the data from TOPEX/Poseidon and the ocean topography must be computed again. The figure above should describe the concept visually.

Measurement Accuracy of TOPEX/Poseidon

Measurement Required Accuracy Achieved Accuracy
Altimeter 4.0 cm (1.6 in) 3.2 cm (1.3 in)
Satellite Position 12.8 cm (5.0 in) 2.8 cm (1.1 in)
Sea Surface Height 13.4 cm (5.3 in) 4.3 cm (1.7 in)

Verification Sites

Although TOPEX/Poseidon utilizes the on-board state-of-the-art altimeters with an ability to provide highly reliable data, it is still necessary to ensure that they are working properly and getting really accurate measurements. Therefore, NASA and CNES developed a Joint Verification Plan with which they each established a verification site along the satellite's path. The NASA verification site is located on Texaco's Platform Harvest 11 km south-southwest of Point Arguello and 19.5 km west of Point Conception in California. The CNES verification site is located at Lampione Rock near Lampedusa Island in the Mediterranean Sea. These sites were chosen because they were small enough and far enough from land that the altimeter return signal would not be corrupted by land interference. Available laser coverage, anticipated accuracy of the in situ observations, logistic, and cost were also considered to single out the sites. At each verification site, NASA and CNES conduct in situ (on site) verification, which is to measure the distance between local sea level and the altimeter independently of the satellite altimeter and to compare them with the altimeter measurements. As for the NASA verification site, it is equipped with sea-level monitoring instruments and a Global Positioning System (GPS) receiver as well as a nearby laser site with a GPS receiver. With the nearby laser, the position of the satellite over the verification site in the reference frame of the laser is determined. With the two GPS receivers, the distance between the laser and the verification site is measured. Then, with the GPS receiver on the platform and sea-level monitoring instruments, the vertical distance from the bench mark on the platform to local sea level is measured. Finally, the two vertical distances are added together to obtain the altimeter-to-sea-level distance, which is compared to the altimeter measurements. This analysis is called "closure." The following figure should clarify the concept.

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This page is created by
Masaharu Suzuki
The University of Texas at Austin


Last Modified: Wed Feb 11, 1999