Throughout history, sailors have measured winds, waves, currents, and tides. Only occasionally would they recognize long-term trends that raised or lowered sea levels. As technology advanced, oceanographers placed precise measuring devices on ships and buoys. This provided a collection of data, which allowed at least a partial look at the changing conditions of the world's oceans.
In recent years scientists have discovered that they can obtain a continuous, comprehensive look at the surface of the world's oceans by using a satellite in space equipped with an instrument called an altimeter.
Building on three earlier earth-orbiting missions TOPEX/Poseidon -- a joint mission between the United States and France -- is part of a global oceanographic effort to acquire a comprehensive look at the world's oceans. TOPEX/Poseidon's contribution involves continuous observation of the surface currents of the ocean.