grouping includes NOAA’s AVHRR sensor, which is flown on a constellation of
satellites and is the most widely used marine spaceborne sensor. The extensive
operational usage of the AVHRR satellite program reflects a number of factors. NOAA
has a constellation of these sensors in service at any given time, resulting in an effective
revisit time of about six hours at mid-latitudes; it readily shows thermal structure in
surface waters, such as that associated with certain eddies, fronts and terrestrial runoff;
it images Earth’s surface constantly (i.e. does not require user intervention); its data can
be downloaded directly by the user with a relatively inexpensive L-band receiver; it is
broadcast free-of-charge; and there are readily available processing and cloud-masking
algorithms. In short, it responds very well to user requirements. The substantive
drawback to this and other sensors operating in the thermal IR band is the influence of
atmospheric effects upon performance. For example, as is the case for sensors listed in
Table 1, sensors listed in Table 2 are inhibited by the presence of clouds.
4.3 ALTIMETERS AND SYNTHETIC APERTURE RADARS
Sensors presented in Table 3 operate in the microwave band of the electromagnetic
spectrum. They are not restricted by clouds, atmospheric particles or the absence of
sunlight (i.e. operate day and night). Note that the applications listed in Table 3 differ
somewhat from those listed in the previous tables.
With the exception of altimeters, all of the satellite sensors identified in this
chapter are restricted to providing information pertaining to surface waters. Altimeters
are capable of providing information pertaining to the entire water column and
therefore provide insight into the three-dimensional structure of the ocean. This results
in altimeters being of considerable interest to oceanographers. The spatial and temporal
scales at which these sensors operate, however, result in them having much less direct
benefit to coastal managers. However, altimeter data are being incorporated into
operational ocean models, which can result in these data being of indirect benefit to
coastal managers.
Altimeters provide information pertaining to ocean circulation, significant wave
height and wind speed (Chelton et al., 2001). This information is extracted from the
range, shape and power of the sensor’s returning signal, respectively. Ocean circulation
information from altimeters is restricted to meso and basin scales, where mesoscale
variability is defined as being on the order of 50-100 km and 10-100 days at midlatitudes.
Presently, altimeters are restricted to providing information related to the time
variant component of ocean circulation. This means that they are not able to provide
information on absolute currents, only those components that vary with time. This is
due to the present level of imprecision in our ability to measure the marine geoid, which
is used as a reference surface for altimetric measurements. This situation is changing. In
March 2002, the German/US GRACE satellite was launched with the objective of
measuring this geophysical property with greater precision than previously possible.
All of the sensors identified in this chapter are able to detect aspects of ice in marine
waters. However, the synthetic aperture radar sensors listed in Table 3 have emerged as
the sensors of choice for operational ice monitoring. SAR sensors have been
demonstrated in coastal flooding operations, and as discussed previously, they can detect
surface oil slicks. Unfortunately, the limited temporal resolution (revisit time) of singlesatellite SAR sensors appears to be constraining the extent to which they are used for
operational purposes (i.e. they are platform limited).
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satellites and is the most widely used marine spaceborne sensor. The extensive
operational usage of the AVHRR satellite program reflects a number of factors. NOAA
has a constellation of these sensors in service at any given time, resulting in an effective
revisit time of about six hours at mid-latitudes; it readily shows thermal structure in
surface waters, such as that associated with certain eddies, fronts and terrestrial runoff;
it images Earth’s surface constantly (i.e. does not require user intervention); its data can
be downloaded directly by the user with a relatively inexpensive L-band receiver; it is
broadcast free-of-charge; and there are readily available processing and cloud-masking
algorithms. In short, it responds very well to user requirements. The substantive
drawback to this and other sensors operating in the thermal IR band is the influence of
atmospheric effects upon performance. For example, as is the case for sensors listed in
Table 1, sensors listed in Table 2 are inhibited by the presence of clouds.
4.3 ALTIMETERS AND SYNTHETIC APERTURE RADARS
Sensors presented in Table 3 operate in the microwave band of the electromagnetic
spectrum. They are not restricted by clouds, atmospheric particles or the absence of
sunlight (i.e. operate day and night). Note that the applications listed in Table 3 differ
somewhat from those listed in the previous tables.
With the exception of altimeters, all of the satellite sensors identified in this
chapter are restricted to providing information pertaining to surface waters. Altimeters
are capable of providing information pertaining to the entire water column and
therefore provide insight into the three-dimensional structure of the ocean. This results
in altimeters being of considerable interest to oceanographers. The spatial and temporal
scales at which these sensors operate, however, result in them having much less direct
benefit to coastal managers. However, altimeter data are being incorporated into
operational ocean models, which can result in these data being of indirect benefit to
coastal managers.
Altimeters provide information pertaining to ocean circulation, significant wave
height and wind speed (Chelton et al., 2001). This information is extracted from the
range, shape and power of the sensor’s returning signal, respectively. Ocean circulation
information from altimeters is restricted to meso and basin scales, where mesoscale
variability is defined as being on the order of 50-100 km and 10-100 days at midlatitudes.
Presently, altimeters are restricted to providing information related to the time
variant component of ocean circulation. This means that they are not able to provide
information on absolute currents, only those components that vary with time. This is
due to the present level of imprecision in our ability to measure the marine geoid, which
is used as a reference surface for altimetric measurements. This situation is changing. In
March 2002, the German/US GRACE satellite was launched with the objective of
measuring this geophysical property with greater precision than previously possible.
All of the sensors identified in this chapter are able to detect aspects of ice in marine
waters. However, the synthetic aperture radar sensors listed in Table 3 have emerged as
the sensors of choice for operational ice monitoring. SAR sensors have been
demonstrated in coastal flooding operations, and as discussed previously, they can detect
surface oil slicks. Unfortunately, the limited temporal resolution (revisit time) of singlesatellite SAR sensors appears to be constraining the extent to which they are used for
operational purposes (i.e. they are platform limited).
210
Whitehouse and Hutt