SATELLITE MEASUREMENTS
161
acquisition. While the data errors will be greater, the input will be timely. If
the model variables are properly matched to the altimeter data it may even
be possible to use the ageostrophic information contained in the nonaveraged along track record.
Close to the Equator the SSHA cannot be interpreted directly in terms of
surface currents since here f is very small and the geostrophic Eq. (3) cannot
be applied.
In the relatively near future it is hoped that the lack of knowledge about
the Geoid can be remedied. What is needed is a means of measuring h geoid
without using altimetry, and this is provided by the measurement of the
gravity field above the Earth from satellites. Both the presently operating
Gravity Recovery And Climate Experiment (GRACE) and the Gravity and
Ocean Circulation Explorer (GOCE) mission which is due for launch by
2007 measure elements of the gravity field from which it is possible to
recreate the sea-level Geoid. At the required accuracy of about 1 cm the
GRACE can achieve this only at a length scale longer than at least 1000 km,
but it is expected that the GOCE can do so once-for-all down to a length
scale of about 100 km. This will allow the steady state ocean currents to be
derived from archived altimetric data and greatly improve the capacity to
utilise altimetric data in near-real time.
3.1.4
The impact of altimeters on oceanography
Despite the present limitations of altimetry to measuring just the timevariable part of surface currents in the open ocean only, this achievement by
itself has made a tremendous difference to Oceanography in the 21
st
Century. It has opened up the whole approach to operational ocean
forecasting based on numerical modelling, since without the capacity of
altimetry to monitor the mesoscale turbulence of the ocean at time scales of
days to weeks there would be little hope of ensuring that ocean models
remain consistent with the real ocean. The ability to measure changes in the
absolute height of the sea to an accuracy of 2-3 cm (a measurement
uncertainty of just 2 parts in 10
8
) is technologically quite breathtaking, yet
instrument engineers and data analysts do not believe the limits to further
improvement have been reached.
The measurement of the geoid
independently of altimetry will liberate more information from the altimeter
record within a few years from now. Meanwhile, there remains the
challenge to develop methods of assimilating SSHA into ocean models
which fully recognizes the character of the altimeter data and maximises the
utilisation of the available information.
161
acquisition. While the data errors will be greater, the input will be timely. If
the model variables are properly matched to the altimeter data it may even
be possible to use the ageostrophic information contained in the nonaveraged along track record.
Close to the Equator the SSHA cannot be interpreted directly in terms of
surface currents since here f is very small and the geostrophic Eq. (3) cannot
be applied.
In the relatively near future it is hoped that the lack of knowledge about
the Geoid can be remedied. What is needed is a means of measuring h geoid
without using altimetry, and this is provided by the measurement of the
gravity field above the Earth from satellites. Both the presently operating
Gravity Recovery And Climate Experiment (GRACE) and the Gravity and
Ocean Circulation Explorer (GOCE) mission which is due for launch by
2007 measure elements of the gravity field from which it is possible to
recreate the sea-level Geoid. At the required accuracy of about 1 cm the
GRACE can achieve this only at a length scale longer than at least 1000 km,
but it is expected that the GOCE can do so once-for-all down to a length
scale of about 100 km. This will allow the steady state ocean currents to be
derived from archived altimetric data and greatly improve the capacity to
utilise altimetric data in near-real time.
3.1.4
The impact of altimeters on oceanography
Despite the present limitations of altimetry to measuring just the timevariable part of surface currents in the open ocean only, this achievement by
itself has made a tremendous difference to Oceanography in the 21
st
Century. It has opened up the whole approach to operational ocean
forecasting based on numerical modelling, since without the capacity of
altimetry to monitor the mesoscale turbulence of the ocean at time scales of
days to weeks there would be little hope of ensuring that ocean models
remain consistent with the real ocean. The ability to measure changes in the
absolute height of the sea to an accuracy of 2-3 cm (a measurement
uncertainty of just 2 parts in 10
8
) is technologically quite breathtaking, yet
instrument engineers and data analysts do not believe the limits to further
improvement have been reached.
The measurement of the geoid
independently of altimetry will liberate more information from the altimeter
record within a few years from now. Meanwhile, there remains the
challenge to develop methods of assimilating SSHA into ocean models
which fully recognizes the character of the altimeter data and maximises the
utilisation of the available information.