However, the definition of available resources has changed dramatically over the past
eight to ten years. Specifically, during this period various Earth-observation satellite
sensors have been operationalized and a number of practical aquatic applications have
been developed for these sensors.
Utilization of satellite sensors is becoming less of an option and more of an
essential element of a comprehensive coastal sampling program, one reason being that
satellites allow us to view coastal processes on a repeatable basis at spatial scales not
sampled by any other platform. In so doing, spaceborne (i.e. satellite-based) sensors
have helped us to realize that the aquatic environment is not as homogeneous as once
believed. Additionally they have reminded us yet again that our understanding of
environmental processes is often limited by the techniques available to observe them.
Once deployed, satellite sensors are designed to function for several years. Certain
satellite programs launch replacements once the life of a given satellite has expired.
Aquatic Earth-observation satellites have been launched since the 1970s, with the result
that we are starting to realize time series on the order of a quarter of century. We
caution, however, that some of these series are incomplete, and the inaugural sensors
and calibration programs were not as advanced as those in use today.
In addition to satellites, airborne and shore-based platforms are capable of
providing a synoptic view of coastal waters, albeit not the same view. Although
airborne and shore-based sensors are beyond the scope of this chapter, they should not
be viewed as redundant means of obtaining synoptic aquatic information. Certain
coastal synoptic requirements, which can be fulfilled with existing airborne and shorebased sensors, cannot be providced with existing satellite sensors. Examples of this
include aquatic surveys that require airborne hyperspectral or LIDAR sensors, and
synoptic mapping of surface currents with coastal HF radar technologies.
We recognize two inherent limitations of sensors mounted on satellite, airborne,
and shore-based platforms. First, sensors mounted on these platforms only measure
surficial properties. The sensor’s depth of penetration into the water column varies from
a few microns to a few tens of meters, depending on the particular sensor, platform and
body of water. Second, satellite, airborne and shore-based sensors are separated from
coastal waters by the atmosphere, and can be influenced by or otherwise rendered
unusable by atmospheric effects. The atmosphere contributes about 90% of the signal
recorded by spaceborne ocean color sensors, and a correction must be applied to
account for this contribution (Antoine et al., 2003). Indeed, the presence of clouds,
which represents one aspect of this issue, is arguably the greatest practical limitation of
this type of sensor in coastal waters as clouds typically cover 60% of the tropical ocean
and 75% of the ocean at mid latitudes (Chelton et al., 2001).
3. Elements of Time
3.1 SATELLITE/SENSOR REVISIT TIME
Almost all aquatic environmental satellites are polar orbiting. An inherent feature
of such a satellite is that it will pass over different areas of the planet as it progresses
through its orbit cycle. In addition, the frequency with which it revisits a specific
geographic area varies with latitude. A polar-orbiting satellite can only sample the
aquatic area of interest when the satellite is overhead or thereabout. The length of time
a satellite sensor requires to revisit a specific geographic area is referred to herein as its
revisit time or temporal resolution.
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eight to ten years. Specifically, during this period various Earth-observation satellite
sensors have been operationalized and a number of practical aquatic applications have
been developed for these sensors.
Utilization of satellite sensors is becoming less of an option and more of an
essential element of a comprehensive coastal sampling program, one reason being that
satellites allow us to view coastal processes on a repeatable basis at spatial scales not
sampled by any other platform. In so doing, spaceborne (i.e. satellite-based) sensors
have helped us to realize that the aquatic environment is not as homogeneous as once
believed. Additionally they have reminded us yet again that our understanding of
environmental processes is often limited by the techniques available to observe them.
Once deployed, satellite sensors are designed to function for several years. Certain
satellite programs launch replacements once the life of a given satellite has expired.
Aquatic Earth-observation satellites have been launched since the 1970s, with the result
that we are starting to realize time series on the order of a quarter of century. We
caution, however, that some of these series are incomplete, and the inaugural sensors
and calibration programs were not as advanced as those in use today.
In addition to satellites, airborne and shore-based platforms are capable of
providing a synoptic view of coastal waters, albeit not the same view. Although
airborne and shore-based sensors are beyond the scope of this chapter, they should not
be viewed as redundant means of obtaining synoptic aquatic information. Certain
coastal synoptic requirements, which can be fulfilled with existing airborne and shorebased sensors, cannot be providced with existing satellite sensors. Examples of this
include aquatic surveys that require airborne hyperspectral or LIDAR sensors, and
synoptic mapping of surface currents with coastal HF radar technologies.
We recognize two inherent limitations of sensors mounted on satellite, airborne,
and shore-based platforms. First, sensors mounted on these platforms only measure
surficial properties. The sensor’s depth of penetration into the water column varies from
a few microns to a few tens of meters, depending on the particular sensor, platform and
body of water. Second, satellite, airborne and shore-based sensors are separated from
coastal waters by the atmosphere, and can be influenced by or otherwise rendered
unusable by atmospheric effects. The atmosphere contributes about 90% of the signal
recorded by spaceborne ocean color sensors, and a correction must be applied to
account for this contribution (Antoine et al., 2003). Indeed, the presence of clouds,
which represents one aspect of this issue, is arguably the greatest practical limitation of
this type of sensor in coastal waters as clouds typically cover 60% of the tropical ocean
and 75% of the ocean at mid latitudes (Chelton et al., 2001).
3. Elements of Time
3.1 SATELLITE/SENSOR REVISIT TIME
Almost all aquatic environmental satellites are polar orbiting. An inherent feature
of such a satellite is that it will pass over different areas of the planet as it progresses
through its orbit cycle. In addition, the frequency with which it revisits a specific
geographic area varies with latitude. A polar-orbiting satellite can only sample the
aquatic area of interest when the satellite is overhead or thereabout. The length of time
a satellite sensor requires to revisit a specific geographic area is referred to herein as its
revisit time or temporal resolution.
202
Whitehouse and Hutt