Topex/Poseidon mission's objectives

The objectives of the Topex/Poseidon mission, as announced in 1992, were:

• to study ocean circulation on a large scale including its interaction with the atmosphere, to improve our knowledge of the role of the ocean in the global climate,
• to increase our understanding of heat transport in the ocean,
• to model the tides,
• to improve the geophysical study of the marine geoid by analysing altimetry measurements and the accurate location of ground beacons,
• to calculate variation trends in mean sea level on a global scale (an indicator of the greenhouse effect).

The first research proposals for Topex/Poseidon data were therefore mainly focused on large-scale ocean circulation, with another being the modelling of tides offshore (the orbit was deliberately chosen to measure all tidal componants, whichever their period), as well as several geodesy studies.

However, other applications were hoped for:

Although results were expected from Topex/Poseidon on ocean circulation on a global scale, we also hoped for a glimpse of its variability at the eddy scale (mesoscale). In the end, we were able to monitor eddies by combining data from several satellites (first Topex/Poseidon and ERS-1, then ERS-2, then the task was handed over to Jason-1 and Envisat).

Planetary waves (known as Kelvin or Rossby waves), which transport heat and energy from one side of an ocean to another, cause slight variations in sea surface height of the order of about ten centimeters. These are detected by Topex/Poseidon, with a precision of two or three centimeters. They can be seen by looking at a series of maps produced at regular intervals, or more strikingly, in longitude-time diagrams. These diagrams show the change over time and the sea surface height variations on an east-west axis, for a given latitude. Ocean waves appear as a series of straight lines.

In the beginning, Topex/Poseidon was not intended for providing data 'in a hurry'. Its 48-hour processing chain meant that it was possible to demonstrate the potential for operational altimetry (based on known data, even though actual full-scale applications were not possible).

And some unexpected applications arose:

Although we hoped to exceed the three years of 'nominal lifespan' and the five years 'expected' for the mission, thirteen years was a particularly long time for a satellite to survive in such a hostile environment. However, the exceptional duration of the Topex/Poseidon mission is much more than simply anecdotal. It enabled certain phenomena to be observed with homogeneous measurements, in particular the Decadal Oscillations (such as the Pacific Decadal Oscillation and the North Atlantic Oscillation).

Topex/Poseidon (as well as Jason-1) was designed for studying the oceans. However, since 1995, tests have shown that data relating to lakes can also be used. Today, measurements of certain rivers such as the Amazon are even being discussed. It provides a way of monitoring the level of certain rivers and other bodies of water, particularly in areas where there are few hydrographic stations, although due to the satellite's revisit frequency (every ten days), such measurements are unsuitable for any early warning network.

Although we expected to observe, a posteriori, the very strong waves caused by cyclones (assuming the satellite has passed more or less over them), the combination of sea surface height measurements with temperature data to forecast the intensification of tropical storms was certainly not envisaged by the people who designed Topex/Poseidon. Today, these Tropical Cyclone Heat Potential data are 'routinely' integrated into forecast models and should ultimately help us to take efficient preventive measures (see Intensification of Hurricane Katrina).

Certain land measurements from Topex/Poseidon can also be used, in particular because the Topex altimeter transmits at two distinct frequencies. Comparing the two can provide information about the type of land surface.