TOPEX/POSEIDON Crossover Points

The oceans play a fundamental role in maintaining a life-sustaining
climate on Earth by transporting an enormous amount of heat through
their large scale circulation systems. Understanding the role that the
ocean circulation plays in climate change is the main goal of the
TOPEX/POSEIDON mission, a cooperative project between the United
States' National Aeronautics and Space Administration (NASA) and the
French Space Agency, Centre National d'Etudes Spatiales (CNES).
TOPEX/POSEIDON uses advanced satellite radar altimetry, a microwave
radiometer and very precise tracking systems (DORIS, laser
retroreflector and an experimental GPS receiver) to make very precise
and accurate observations of sea level -altimeter accuracy better than
2 centimeters, orbit accuracy better than 3 centimeters- for three
(nominally) to five (expected) years. The orbit, satellite bus and
payload instruments are optimized to measure sea level, to map
basin-wide variations of currents, and to monitor the effects of the
currents such as the Gulf Stream, on global climate change. Altimeter
measurements also allow scientists to study ocean tides and waves,
marine geophysics and winds. Launched by Ariane on August 10, 1992,
TOPEX/POSEIDON is a core element of the international World Ocean
Circulation Experiment (WOCE) and Tropical Ocean and Global Atmosphere
(TOGA) measurement programs. Analysis of the satellite altimetry data,
together with WOCE and TOGA ocean measurements, yields major advances
in our quantitative understanding of the ocean circulation and its
climate. TOPEX/POSEIDON lays the foundation for long term ocean
monitoring from space.

The TOPEX/POSEIDON satellite observes the Earth from 66 North to 66
South latitude at an altitude of 1330 kilometers. It allows to observe
90% of the ice-free oceans and all major straits. The orbit is not
sun-synchronous and is prograde. It takes nearly 2 hours to circle the
Earth (revolution period). The repeat-cycle is about 10 days, i.e. the
satellite passes vertically over the same location in a 1-kilometer
bandwith every 10 days (or 127 revolutions). The distance between
successive tracks is on the order of 315 kilometers at the Equator.

The TOPEX/POSEIDON crossover point files are computed from the AVISO
Merged Geophysical Data Records (GDR-Ms) of the TOPEX/POSEIDON
mission. This product contains both ascending and descending arc
information for each crossover point The crossover point files are
distributed on CD-ROMs (ISO 9660). Each CD-ROM contains two ten-day
repeat-cycles of data : Merged GDRs, crossover files and CNES and NASA
orbit epehemerides. They are swath data and no images. It is
emphasized that this product is considered research data because of
the form and content of the data - over a hundred parameters including
flags for each crossover point. Analysis softwares are the
responsibility of the user. Calculation of sea surface height from
the altimeter range and environmental corrections (e.g. dry and wet
tropospheric corrections, tidal effects, electromagnetic bias) is the
responsibility of the user. CD-ROMs are being produced and are
available as the data are processed. They are disseminated
systematically to TOPEX/POSEIDON Principal Investigators and
Co-Investigators. User handbooks - one for the CD-ROM, one for the
data products - are distributed with the first CD-ROM. For more
information, you can connect to the AVISO World Wide Web server or
contact either one of the investigators, the AVISO data center or the
AVISO technical contact.