Satellite Oceanography for Ocean Forecasting
25
not yet a commitment for long term, operational measurements. Note that, as far as
altimetry is concemed, gravimetric missions (CHAMP, GRACE and GOCE) were
added (see Johanessen et al., 1999). They should allow the estimation of a precise
geoid. As we will see later, without precise knowledge of the geoid , altimetry can
only provide data on the variable part of the ocean dynamic topography.
Finally, among promising future techniques, one should mention the possibility
of measuring sea surface salinity from space. The concept has been demonstrated
from plane measurements and there are projects in the Europe (SMOS) and in the
USA for flying such a mission in the coming years. Although the precision will be
somewhat limited, this will be a very important contribution to the ocean observing
system (see Johanessen et al., 1999).
2.2 Satellite Altimetry
2.2.1 Overview
There are two main reasons for focusing on satellite altimetry. First, it is probably the most important satellite technique for oceanography, particularly for ocean
forecasting. Satellite altimetry has unique capabilities for providing global synoptic
view of the ocean circulation. It provides measurements of sea surface topography
which is an integral of the ocean interior. These measurements are a strong constraint for the 3D ocean circulation estimation. Second, satellite altimetry is a very
mature technique. It started more than 20 years ago with GEOS-3 (1975) and SEASAT (1978) missions. After a seven year gap, these were followed by the US
Navy's GEOSAT mission (1985-1989) and the European Agency ERS-l mission
(1991-1996). Two satellites are now operating: ERS-2, the successor ofERS-l and
the US/French TOPEXlPOSEIDON (TIP) mission. For the next decade, future
missions have already been decided : GEOSAT Follow On was launched in early
1998 and ENVISAT (successor of ERS-1I2) and Jason-l (successor of TIP) missions are scheduled for launch in early 2001.
Satellite altimetry is one of the most complex and challenging techniques in
terms of accuracy. It requires the range between the satellite and the sea surface to
be measured to within a few cm, i.e. assuming a typical satellite orbit altitude of
1000 km a relative accuracy of 10- 8 . There have been major advances in sensor and
processing algorithm performance over the last 20 years. It is important to realize
that these advances were made possible through continuous cooperation between
engineers and scientists. As a result, accuracy has progres sed from several meters
to a few cm only. TIP marked a major improvement in accuracy. Its orbit is known
with an accuracy of about 2 cm rms and the satellite - ocean surface distance is
determined to within 2 cm.
We shall now describe measurement principles, content and errors, and explain
data processing techniques. More details can be found in Stewart (1985), Chelton
(1988), Rummel and Sanso (1993) or Le Traon (1995). We will then look briefly at
25
not yet a commitment for long term, operational measurements. Note that, as far as
altimetry is concemed, gravimetric missions (CHAMP, GRACE and GOCE) were
added (see Johanessen et al., 1999). They should allow the estimation of a precise
geoid. As we will see later, without precise knowledge of the geoid , altimetry can
only provide data on the variable part of the ocean dynamic topography.
Finally, among promising future techniques, one should mention the possibility
of measuring sea surface salinity from space. The concept has been demonstrated
from plane measurements and there are projects in the Europe (SMOS) and in the
USA for flying such a mission in the coming years. Although the precision will be
somewhat limited, this will be a very important contribution to the ocean observing
system (see Johanessen et al., 1999).
2.2 Satellite Altimetry
2.2.1 Overview
There are two main reasons for focusing on satellite altimetry. First, it is probably the most important satellite technique for oceanography, particularly for ocean
forecasting. Satellite altimetry has unique capabilities for providing global synoptic
view of the ocean circulation. It provides measurements of sea surface topography
which is an integral of the ocean interior. These measurements are a strong constraint for the 3D ocean circulation estimation. Second, satellite altimetry is a very
mature technique. It started more than 20 years ago with GEOS-3 (1975) and SEASAT (1978) missions. After a seven year gap, these were followed by the US
Navy's GEOSAT mission (1985-1989) and the European Agency ERS-l mission
(1991-1996). Two satellites are now operating: ERS-2, the successor ofERS-l and
the US/French TOPEXlPOSEIDON (TIP) mission. For the next decade, future
missions have already been decided : GEOSAT Follow On was launched in early
1998 and ENVISAT (successor of ERS-1I2) and Jason-l (successor of TIP) missions are scheduled for launch in early 2001.
Satellite altimetry is one of the most complex and challenging techniques in
terms of accuracy. It requires the range between the satellite and the sea surface to
be measured to within a few cm, i.e. assuming a typical satellite orbit altitude of
1000 km a relative accuracy of 10- 8 . There have been major advances in sensor and
processing algorithm performance over the last 20 years. It is important to realize
that these advances were made possible through continuous cooperation between
engineers and scientists. As a result, accuracy has progres sed from several meters
to a few cm only. TIP marked a major improvement in accuracy. Its orbit is known
with an accuracy of about 2 cm rms and the satellite - ocean surface distance is
determined to within 2 cm.
We shall now describe measurement principles, content and errors, and explain
data processing techniques. More details can be found in Stewart (1985), Chelton
(1988), Rummel and Sanso (1993) or Le Traon (1995). We will then look briefly at