SATELLITE MEASUREMENTS
155
not quite reach the poles) the orbit plane is constrained to precess at a rate of
once per year relative to the stars. This locks the overpasses to the position
of the sun and means that every orbit always crosses the Equator at the same
local solar time. For most ocean observing sensors this is very convenient,
since it ensures that the longitudinal position of the sun does not change
from one sample to the next, even though the solar latitude inevitably
changes with the annual cycle. However, for altimetry the sun-synchronous
orbit is to be avoided since it aliases the solar semidiurnal tidal constituent.
2.4
Strengths and weaknesses of ocean remote sensing
The global, spatially detailed and regularly repeated views of the oceans
that have been obtained from satellites for more than a decade have made
them an important part of the design of operational ocean monitoring
systems. It is therefore worth summarising the benefits that satellite ocean
data bring as well as noting their limitations.
The importance of satellite data to oceanography can be highlighted by
the way they have opened up the study of global ocean phenomena. We can
now ask questions about large scale processes which could not properly be
addressed scientifically until remote sensing methods allowed us to make
observations of ocean scale phenomena which test and stretch the theoretical
models. A good example of this is the study of oceanic Rossby waves
(Challenor et al., 2004). To some extent 21st Century Oceanography has
become dependent on satellite observations. All branches of ocean science
now expect to use satellite image data and interest in the subject is no longer
limited only to specialist “satellite oceanographers”. Another powerful
impact has come from the immediacy of satellite data. Observations from
all around the world are now being made available within hours, minutes in
some cases, of their acquisition by the sensor, and this has reinforced their
importance for use in operational ocean monitoring and forecasting
At the same time we must not overlook the fundamental limitations of
satellite ocean remote sensing methods. They can observe only some of the
ocean’s properties and variables. They measure the ocean only at or near the
surface although it can be argued that, of all the parts of the ocean, the
surface is the most critical place to be able to measure. Most critically,
ocean measurements may be corrupted by the atmosphere and some methods
cannot see through clouds at all. Moreover, measurements cannot be made
“to order” but only when the satellite is in the right place. Finally it must not
be overlooked that all measurements from satellites require calibration and /
or validation using in situ data. While it might be carelessly thought that
satellites can remove the need for measurements at sea, the reverse is in fact
the case. The full benefit of the wider and higher perspective achieved from
satellite data will only be realised when combined with an integrated array of
in situ sensors interfacing with operational ocean models.
155
not quite reach the poles) the orbit plane is constrained to precess at a rate of
once per year relative to the stars. This locks the overpasses to the position
of the sun and means that every orbit always crosses the Equator at the same
local solar time. For most ocean observing sensors this is very convenient,
since it ensures that the longitudinal position of the sun does not change
from one sample to the next, even though the solar latitude inevitably
changes with the annual cycle. However, for altimetry the sun-synchronous
orbit is to be avoided since it aliases the solar semidiurnal tidal constituent.
2.4
Strengths and weaknesses of ocean remote sensing
The global, spatially detailed and regularly repeated views of the oceans
that have been obtained from satellites for more than a decade have made
them an important part of the design of operational ocean monitoring
systems. It is therefore worth summarising the benefits that satellite ocean
data bring as well as noting their limitations.
The importance of satellite data to oceanography can be highlighted by
the way they have opened up the study of global ocean phenomena. We can
now ask questions about large scale processes which could not properly be
addressed scientifically until remote sensing methods allowed us to make
observations of ocean scale phenomena which test and stretch the theoretical
models. A good example of this is the study of oceanic Rossby waves
(Challenor et al., 2004). To some extent 21st Century Oceanography has
become dependent on satellite observations. All branches of ocean science
now expect to use satellite image data and interest in the subject is no longer
limited only to specialist “satellite oceanographers”. Another powerful
impact has come from the immediacy of satellite data. Observations from
all around the world are now being made available within hours, minutes in
some cases, of their acquisition by the sensor, and this has reinforced their
importance for use in operational ocean monitoring and forecasting
At the same time we must not overlook the fundamental limitations of
satellite ocean remote sensing methods. They can observe only some of the
ocean’s properties and variables. They measure the ocean only at or near the
surface although it can be argued that, of all the parts of the ocean, the
surface is the most critical place to be able to measure. Most critically,
ocean measurements may be corrupted by the atmosphere and some methods
cannot see through clouds at all. Moreover, measurements cannot be made
“to order” but only when the satellite is in the right place. Finally it must not
be overlooked that all measurements from satellites require calibration and /
or validation using in situ data. While it might be carelessly thought that
satellites can remove the need for measurements at sea, the reverse is in fact
the case. The full benefit of the wider and higher perspective achieved from
satellite data will only be realised when combined with an integrated array of
in situ sensors interfacing with operational ocean models.