46
monitoring the global oceans and providing oceanographers
with repeated synoptic coverage.
The next section of the chapter introduces the topic of
satellite remote sensing of ocean color. It discusses briefly
the developments in ocean color remote sensing over the last
few decades, the challenges and processes involved and its
applications.
Space-Borne Remote Sensing
Therese Keck
Why Do We Use Satellite Measurements?
Remote sensing is a technique describing properties of an
object without having physical contact. Human eyes are sensible to the solar electro-magnetic spectrum from 400 to
700 nm ranging from violet to red (visible spectrum, VIS).
Similar to the cones in our eyes, which detect different “colors”, water color measurement instruments are designed
mostly within the optical spectrum in the visible and nearinfra- red from 380 to 800 nm. Beyond these borders, water is
strongly absorbing and the instruments receive no signal
anymore. A monochromatic measurement may contain
information about specific properties and a combination of
certain bands can result, for instance, in an RGB image.
Most of the instruments measure in a passive way by receiving reflected and back-scattered light from the water.
Generally, one of the most common questions in satellite
remote sensing is “Why do we spend so much effort in converting electro-magnetic signals sensed with expensive and
complex instruments which are far away in space”? Indeed,
in situ and field measurements directly offer properties of the
observed matter (e.g., algae content, temperature). Similar
results from remote sensing require planning and operation
of expensive sensors and their platforms as well as sophisticated algorithms to retrieve physical “products” (e.g., chlorophyll a concentration, water vapor content, temperature)
from the satellite sensor signals. Nevertheless, the advantage
is a relatively high and continuous spatial and temporal coverage of the entire globe.
For example, in Lake Erie (Fig. 4) at the border of Canada
and the United States, large algae blooms appear every summer that can vary quickly in spatial and temporal dimension
(Rowe et al. 2016). Harmful algae blooms (HABs) have a
strong impact on the environment and are toxic to animals
and humans. Satellite remote sensing enables us to investigate such events without being at the location or taking in situ
samples. Therefore, measurements from even hardly or seldom reachable areas such as the open ocean or at high latitudes can be provided. Analyzing satellite sensor images,
information about the spatial extent, location, and chlorophyll
concentration are retrieved alongside other parameters. These
data can be used to create climatologies and warnings.
Additionally, it is possible to detect a pattern’s temporal
and spatial variability because satellites revisit the same geographic area every few days (e.g., the polar-orbiting satellites
Terra and Aqua have a revisiting time of 1–2 days) or scan
the area every few minutes (e.g., the geostationary Meteosat
Second Generation MSG-10; EUMETSAT 2017). We are
able to observe the atmosphere, the Earth’s surface, and the
waters with space-borne remote sensing since more than
50 years on a daily to weekly base in a reasonable spatial
resolution ranging from a few meters to several kilometers
covering the entire earth. However, there is also remote sensing on Earth conducted in the field (e.g., on ships or at the
Aeronautic Robotic Network (AERONET) stations) or in the
air with instruments mounted on planes.
The knowledge of short- and long-term variability in the
oceans and their constituents measured by remote sensing
techniques serves as an important resource in oceanographic
science. Since the 1960s, space-borne remote sensing supports human needs. “Satellite product users” (e.g., governmental administrations, environmental agencies, or scientific
institutions) use “satellite products” to monitor freshwater
Fig. 4 The western Lake Erie at the border of Canada and U.S. is
known for extreme algae blooms. The OLCI RGB image shows a large
bloom from 15 September 2017. (OLCI data provided from Copernicus/
Eumetsat, RGB image produced with the freely available software
SNAP (http://step.esa.int/main/toolboxes/snap/))
V. Mascarenhas and T. Keck
monitoring the global oceans and providing oceanographers
with repeated synoptic coverage.
The next section of the chapter introduces the topic of
satellite remote sensing of ocean color. It discusses briefly
the developments in ocean color remote sensing over the last
few decades, the challenges and processes involved and its
applications.
Space-Borne Remote Sensing
Therese Keck
Why Do We Use Satellite Measurements?
Remote sensing is a technique describing properties of an
object without having physical contact. Human eyes are sensible to the solar electro-magnetic spectrum from 400 to
700 nm ranging from violet to red (visible spectrum, VIS).
Similar to the cones in our eyes, which detect different “colors”, water color measurement instruments are designed
mostly within the optical spectrum in the visible and nearinfra- red from 380 to 800 nm. Beyond these borders, water is
strongly absorbing and the instruments receive no signal
anymore. A monochromatic measurement may contain
information about specific properties and a combination of
certain bands can result, for instance, in an RGB image.
Most of the instruments measure in a passive way by receiving reflected and back-scattered light from the water.
Generally, one of the most common questions in satellite
remote sensing is “Why do we spend so much effort in converting electro-magnetic signals sensed with expensive and
complex instruments which are far away in space”? Indeed,
in situ and field measurements directly offer properties of the
observed matter (e.g., algae content, temperature). Similar
results from remote sensing require planning and operation
of expensive sensors and their platforms as well as sophisticated algorithms to retrieve physical “products” (e.g., chlorophyll a concentration, water vapor content, temperature)
from the satellite sensor signals. Nevertheless, the advantage
is a relatively high and continuous spatial and temporal coverage of the entire globe.
For example, in Lake Erie (Fig. 4) at the border of Canada
and the United States, large algae blooms appear every summer that can vary quickly in spatial and temporal dimension
(Rowe et al. 2016). Harmful algae blooms (HABs) have a
strong impact on the environment and are toxic to animals
and humans. Satellite remote sensing enables us to investigate such events without being at the location or taking in situ
samples. Therefore, measurements from even hardly or seldom reachable areas such as the open ocean or at high latitudes can be provided. Analyzing satellite sensor images,
information about the spatial extent, location, and chlorophyll
concentration are retrieved alongside other parameters. These
data can be used to create climatologies and warnings.
Additionally, it is possible to detect a pattern’s temporal
and spatial variability because satellites revisit the same geographic area every few days (e.g., the polar-orbiting satellites
Terra and Aqua have a revisiting time of 1–2 days) or scan
the area every few minutes (e.g., the geostationary Meteosat
Second Generation MSG-10; EUMETSAT 2017). We are
able to observe the atmosphere, the Earth’s surface, and the
waters with space-borne remote sensing since more than
50 years on a daily to weekly base in a reasonable spatial
resolution ranging from a few meters to several kilometers
covering the entire earth. However, there is also remote sensing on Earth conducted in the field (e.g., on ships or at the
Aeronautic Robotic Network (AERONET) stations) or in the
air with instruments mounted on planes.
The knowledge of short- and long-term variability in the
oceans and their constituents measured by remote sensing
techniques serves as an important resource in oceanographic
science. Since the 1960s, space-borne remote sensing supports human needs. “Satellite product users” (e.g., governmental administrations, environmental agencies, or scientific
institutions) use “satellite products” to monitor freshwater
Fig. 4 The western Lake Erie at the border of Canada and U.S. is
known for extreme algae blooms. The OLCI RGB image shows a large
bloom from 15 September 2017. (OLCI data provided from Copernicus/
Eumetsat, RGB image produced with the freely available software
SNAP (http://step.esa.int/main/toolboxes/snap/))
V. Mascarenhas and T. Keck