OVERVIEW
Since 1992 we have been tracking global ocean surface topography with
TOPEX/Poseidon, a joint US-French space mission from an orbit 1336 km
above the ocean surface. The spacecraft's radar altimeters
measure the
precise distance between the satellite and sea surface by measuring the
round-trip travel time of microwave pulses bounced from the spacecraft to the sea surface and back to the spacecraft.
The altitude of the
satellite is determined by a sophisticated estimation procedure based on an
orbit determination measurement system both onboard the satellite and on
the ground all over the world. The details of the shape of the returned
radar pulses give information on wind speed and the wave height.
TOPEX/Poseidon monitored large-scale features like Rossby and
Kelvin waves, tracked
the great El Niño of
1997-1998 and the subsequent La Niña,
and has begun to explore possible long-term changes such as the Pacific
Decadal Oscillation, of which El Niño and La Niña are but a part.
Additionally,
the high accuracy of the TOPEX/Poseidon measurement has made satellite
altimetry an efficient method for monitoring the variation of global mean
sea level in relation to global climate change. TOPEX/Poseidon will be
joined in 2001, and later replaced after the conclusion of a Tandem
Mission, by Jason-1, which will continue to build the database. The two
GRACE (the Gravity Recovery and Climate Experiment) spacecrafts, scheduled
for launch after Jason-1, will refine the geoid measurements,
increasing the utility of all previous altimetry data. Ocean Surface Topography Mission (OSTM), scheduled
for launch in June 2008, will take ocean surface topography measurement into an
operational mode for continued climate forecasting research as well as
scientific and industrial applications.
There is also a PDF version of the brochure Changing Climates: the Ocean Connection (2.1 MB)
with information on TOPEX/Poseidon and Jason-1, as well as other Earth Observing missions.
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