3.2 DATA RECEPTION AND PROCESSING TIME
Another element of time we wish to address is the amount of time required
to receive and process satellite data and to subsequently deliver the derived
environmental information to the user. The act of processing satellite data into
useful environmental information is a critical function, the subject of which is of
encyclopedic proportion. We make no attempt to address this subject, but simply make
the point that ultimately the user requires environmental information, not unprocessed
satellite data, and therefore there is a time delay between the satellite passing over the
area of interest and the user receiving the resulting environmental information.
Depending upon the system employed and given application, this may vary from
minutes to days or weeks.
If satellite data can be obtained by the user within seconds or a few minutes of the
satellite passing overhead, we say that the data can be obtained in real time. If it can be
obtained by the user within two hours of overpass, we define it as being obtained in
near-real time. This is subjective terminology and there appears to be no convention
within the environmental satellite community on this matter.
This time delay limits certain applications but not others. If one is using multispectral satellite sensors to map mangroves for inventory purposes, for example, this is
not likely to be an issue as the satellite imagery could arrive days to weeks after the
satellite overpass without losing its value. If, on the other hand, you are mapping the
same area in support of a developing toxic algal bloom, or national security or
emergency operations, it could be an issue. Although not all research applications
require real or near-real time satellite data, surveillance and emergency response
applications almost always have this requirement.
3.3 DATA ORDERING TIME
The final element of time we wish to address is how far in advance one needs to
request satellite data in order to have the satellite sensor image the user’s area of
interest. For most aquatic satellite systems, this is not a relevant issue as the sensor
images Earth’s surface continuously and therefore the area of interest is imaged without
user intervention. NOAA’s AVHRR sensor falls within this category as does NASA’s
multispectral Modis sensor, the private-sector SeaWiFS sensor, the various altimeters,
scatterometers, etc. For certain systems, however, such as synthetic aperture radar
(SAR) sensors, the satellite/sensor must be programmed to image a given area at a
given time, and perhaps in a given mode. As a result, advance notice must be given by
the user. This tends to be a critical factor for applications where the user has little
warning of a need for the data, such as operational applications pertaining to emergency
response, or science applications involving episodic events, such as heavy rains.
4. Sensors and Their Applications
This section identifies polar-orbiting Earth-observation satellite sensors of
relevance to the aquatic environment. It also cross references these sensors with their
aquatic applications, lists their revisit or effective revisit time, their spatial resolution
and whether or not resulting environmental information is obtainable in near-real time.
Spatial resolution refers to the smallest physical unit discernable by the sensor (Kramer,
2002).
205
Observing Coastal Waters with Spaceborne Sensors
Another element of time we wish to address is the amount of time required
to receive and process satellite data and to subsequently deliver the derived
environmental information to the user. The act of processing satellite data into
useful environmental information is a critical function, the subject of which is of
encyclopedic proportion. We make no attempt to address this subject, but simply make
the point that ultimately the user requires environmental information, not unprocessed
satellite data, and therefore there is a time delay between the satellite passing over the
area of interest and the user receiving the resulting environmental information.
Depending upon the system employed and given application, this may vary from
minutes to days or weeks.
If satellite data can be obtained by the user within seconds or a few minutes of the
satellite passing overhead, we say that the data can be obtained in real time. If it can be
obtained by the user within two hours of overpass, we define it as being obtained in
near-real time. This is subjective terminology and there appears to be no convention
within the environmental satellite community on this matter.
This time delay limits certain applications but not others. If one is using multispectral satellite sensors to map mangroves for inventory purposes, for example, this is
not likely to be an issue as the satellite imagery could arrive days to weeks after the
satellite overpass without losing its value. If, on the other hand, you are mapping the
same area in support of a developing toxic algal bloom, or national security or
emergency operations, it could be an issue. Although not all research applications
require real or near-real time satellite data, surveillance and emergency response
applications almost always have this requirement.
3.3 DATA ORDERING TIME
The final element of time we wish to address is how far in advance one needs to
request satellite data in order to have the satellite sensor image the user’s area of
interest. For most aquatic satellite systems, this is not a relevant issue as the sensor
images Earth’s surface continuously and therefore the area of interest is imaged without
user intervention. NOAA’s AVHRR sensor falls within this category as does NASA’s
multispectral Modis sensor, the private-sector SeaWiFS sensor, the various altimeters,
scatterometers, etc. For certain systems, however, such as synthetic aperture radar
(SAR) sensors, the satellite/sensor must be programmed to image a given area at a
given time, and perhaps in a given mode. As a result, advance notice must be given by
the user. This tends to be a critical factor for applications where the user has little
warning of a need for the data, such as operational applications pertaining to emergency
response, or science applications involving episodic events, such as heavy rains.
4. Sensors and Their Applications
This section identifies polar-orbiting Earth-observation satellite sensors of
relevance to the aquatic environment. It also cross references these sensors with their
aquatic applications, lists their revisit or effective revisit time, their spatial resolution
and whether or not resulting environmental information is obtainable in near-real time.
Spatial resolution refers to the smallest physical unit discernable by the sensor (Kramer,
2002).
205
Observing Coastal Waters with Spaceborne Sensors