SATELLITE MEASUREMENTS
159
whereas the time-variable part of h dyn is comparable in magnitude with the
mean component, of order metres over a few months. Therefore the time
variable part of h dyn , called the sea-surface height anomaly, SSHA, can be
separated from the measured h dyn + h geoid by simply subtracting the time
mean over many orbit cycles. To enable a time-mean to be produced, the
orbit track must be precisely repeated to within a kilometre and the data
must be accumulated from several years of a ten-day cycle. For this reason
it is essential to fly a new altimeter in precisely the same orbit as its
predecessor so that the mean surface topography of the earlier mission can
be used straight away. Then SSHA can be calculated from the first orbit
cycle of the new altimeter, without having to wait another few years to build
up a new mean topography for a different orbit track.
It is important to remember that the SSHA, which is widely used and
assimilated into ocean models, does not contain any information about the
dynamic height of the ocean associated with the mean circulation. Thus in
Figure 6, which is an example of the SSHA from T/P observed during a tenday period in December 2001, the dominant features are mesoscale eddies.
The dynamic topography signatures of the strong ocean currents are not seen
at all, apart from the fact that the eddy-like activity is strongest where the
major currents tend to meander.
Altimeter
Agency
Dates
Height
Orbit
Accuracy
TOPEX/
Poseidon
NASA/
CNES
1992present
1336 km
9.92 day repeat nonsun-synchronous
2-3 cm
Poseidon-2 on
Jason-1
NASA/
CNES
2001present
1336 km
9.92 day repeat nonsun-synchronous
~2 cm
Radar altimeter
(RA) on ERS-1
ESA
19912000
780 km
3 & 35 day repeat
sun-synchronous
~5-6 cm
RA on ERS-2
ESA
19952003
780 km
35 day repeat sunsynchronous
~5-6 cm
RA2 on Envisat ESA
2002present
800 km
35 day repeat sunsynchronous
3 cm
Geosat
U.S. Navy 1986-89
800 km
17.05 day repeat
sun-synchronous
10 cm
reanalysis
Gesosat
Follow-on
U.S. Navy 2000present
880 km
17.05 day repeat
sun-synchronous
~10 cm
Table 1. Recent and current series of satellite altimeters
There are presently three families of altimeters in operation, as listed in
Table 1 with the details of their altitude, orbit repeat and the approximate
accuracy (root mean square) of an averaged SSHA product. The T/P–Jason
family is a joint French/U.S.A. dedicated altimetry mission in a high nonsun-synchronous orbit. In contrast the Geosat and ERS series are on lower
sun-synchronous platforms for which the orbit prediction accuracy would be,
.
on their own, much poorer. However, because these satellites cross over
159
whereas the time-variable part of h dyn is comparable in magnitude with the
mean component, of order metres over a few months. Therefore the time
variable part of h dyn , called the sea-surface height anomaly, SSHA, can be
separated from the measured h dyn + h geoid by simply subtracting the time
mean over many orbit cycles. To enable a time-mean to be produced, the
orbit track must be precisely repeated to within a kilometre and the data
must be accumulated from several years of a ten-day cycle. For this reason
it is essential to fly a new altimeter in precisely the same orbit as its
predecessor so that the mean surface topography of the earlier mission can
be used straight away. Then SSHA can be calculated from the first orbit
cycle of the new altimeter, without having to wait another few years to build
up a new mean topography for a different orbit track.
It is important to remember that the SSHA, which is widely used and
assimilated into ocean models, does not contain any information about the
dynamic height of the ocean associated with the mean circulation. Thus in
Figure 6, which is an example of the SSHA from T/P observed during a tenday period in December 2001, the dominant features are mesoscale eddies.
The dynamic topography signatures of the strong ocean currents are not seen
at all, apart from the fact that the eddy-like activity is strongest where the
major currents tend to meander.
Altimeter
Agency
Dates
Height
Orbit
Accuracy
TOPEX/
Poseidon
NASA/
CNES
1992present
1336 km
9.92 day repeat nonsun-synchronous
2-3 cm
Poseidon-2 on
Jason-1
NASA/
CNES
2001present
1336 km
9.92 day repeat nonsun-synchronous
~2 cm
Radar altimeter
(RA) on ERS-1
ESA
19912000
780 km
3 & 35 day repeat
sun-synchronous
~5-6 cm
RA on ERS-2
ESA
19952003
780 km
35 day repeat sunsynchronous
~5-6 cm
RA2 on Envisat ESA
2002present
800 km
35 day repeat sunsynchronous
3 cm
Geosat
U.S. Navy 1986-89
800 km
17.05 day repeat
sun-synchronous
10 cm
reanalysis
Gesosat
Follow-on
U.S. Navy 2000present
880 km
17.05 day repeat
sun-synchronous
~10 cm
Table 1. Recent and current series of satellite altimeters
There are presently three families of altimeters in operation, as listed in
Table 1 with the details of their altitude, orbit repeat and the approximate
accuracy (root mean square) of an averaged SSHA product. The T/P–Jason
family is a joint French/U.S.A. dedicated altimetry mission in a high nonsun-synchronous orbit. In contrast the Geosat and ERS series are on lower
sun-synchronous platforms for which the orbit prediction accuracy would be,
.
on their own, much poorer. However, because these satellites cross over