150
IAN ROBINSON
few metres of the water column, with the potential to carry information
about those contents of sea water such as chlorophyll, dissolved organic
material and suspended particulates that affect the colour of sea water. Solar
radiation in the near infra-red (wavelengths above 700 nm) is rapidly
absorbed in water and so is not reflected out. Consequently any NIR
radiation detected by an ocean-viewing radiometer is evidence of
atmospheric scattering or surface reflection, and can be used to correct the
visible part of the spectrum for those effects.
There are several narrow windows at wavelengths between about 3.5 Pm
and 13 Pm that are exploited by infrared (IR) radiometers. This is the
thermal IR part of the spectrum in which most of the detected radiation has
been emitted by surfaces according to their temperature. In ocean remote
sensing it is used for measuring sea surface temperature (SST). Like ocean
colour, the presence of cloud interrupts the use of this waveband.
At much longer wavelengths, greater than a few millimetres, the
atmosphere becomes almost completely transparent. This is referred to as
the microwave spectral region. Between those parts of the microwave
frequency spectrum allocated by international regulation to radio and TV
broadcasts, telecommunications, mobile telephony and so on, a few narrow
bands are reserved for remote sensing, within the broad regions indicated in
Figure 1. Different bands are used for microwave radiometry and radars.
Microwave radiometers are passive sensors, simply measuring the naturally
ambient radiation that is emitted by the ocean, atmosphere and land surfaces.
Radars are active microwave devices which emit pulses and measure the
echoes from the sea surface, in order to gain information about some aspect
of the surface.
There is a variety of different types of radar, which can be distinguished
by the direction in which they point, the length and modulation of the
emitted microwave pulse, and the way the echo from the sea surface is
analysed. Radars can be classed as either viewing straight down at the nadir
point below the platform, or viewing obliquely to encounter the surface at an
incidence angle between 15º and 60º. The nadir sensors measure the surface
height or slope and are called altimeters. Those viewing obliquely measure
the surface roughness at length scales comparable to the radar wavelength.
This is represented by a property of the radar interaction with the material
and the geometry of the surface called V 0 , the normalised radar backscatter
cross-section.
2.2
Generic processing tasks for analysing ocean remote
sensing data
Figure 2 illustrates schematically what is involved in measuring
properties of the ocean using a sensor that is typically hundreds or thousands
of kilometres from the sea surface. An electromagnetic signal of a particular
IAN ROBINSON
few metres of the water column, with the potential to carry information
about those contents of sea water such as chlorophyll, dissolved organic
material and suspended particulates that affect the colour of sea water. Solar
radiation in the near infra-red (wavelengths above 700 nm) is rapidly
absorbed in water and so is not reflected out. Consequently any NIR
radiation detected by an ocean-viewing radiometer is evidence of
atmospheric scattering or surface reflection, and can be used to correct the
visible part of the spectrum for those effects.
There are several narrow windows at wavelengths between about 3.5 Pm
and 13 Pm that are exploited by infrared (IR) radiometers. This is the
thermal IR part of the spectrum in which most of the detected radiation has
been emitted by surfaces according to their temperature. In ocean remote
sensing it is used for measuring sea surface temperature (SST). Like ocean
colour, the presence of cloud interrupts the use of this waveband.
At much longer wavelengths, greater than a few millimetres, the
atmosphere becomes almost completely transparent. This is referred to as
the microwave spectral region. Between those parts of the microwave
frequency spectrum allocated by international regulation to radio and TV
broadcasts, telecommunications, mobile telephony and so on, a few narrow
bands are reserved for remote sensing, within the broad regions indicated in
Figure 1. Different bands are used for microwave radiometry and radars.
Microwave radiometers are passive sensors, simply measuring the naturally
ambient radiation that is emitted by the ocean, atmosphere and land surfaces.
Radars are active microwave devices which emit pulses and measure the
echoes from the sea surface, in order to gain information about some aspect
of the surface.
There is a variety of different types of radar, which can be distinguished
by the direction in which they point, the length and modulation of the
emitted microwave pulse, and the way the echo from the sea surface is
analysed. Radars can be classed as either viewing straight down at the nadir
point below the platform, or viewing obliquely to encounter the surface at an
incidence angle between 15º and 60º. The nadir sensors measure the surface
height or slope and are called altimeters. Those viewing obliquely measure
the surface roughness at length scales comparable to the radar wavelength.
This is represented by a property of the radar interaction with the material
and the geometry of the surface called V 0 , the normalised radar backscatter
cross-section.
2.2
Generic processing tasks for analysing ocean remote
sensing data
Figure 2 illustrates schematically what is involved in measuring
properties of the ocean using a sensor that is typically hundreds or thousands
of kilometres from the sea surface. An electromagnetic signal of a particular