76
W. Alpers
3.8–2.5 cm), and Ku-band (12–18 GHz, 2.5–1.7 cm). These strange names for the
frequency bands are leftovers of World War 2, when microwave technology was
developed in different countries and when frequency bands were top-secret.
There exist two kinds of microwave instruments used for remote sensing: passive
and active instruments. Passive instruments, called microwave radiometers, measure
the emitted microwave radiation from an object at the Earth surface, while active
instruments emit microwave pulses and receive them after backscattering from the
object. The active microwave instruments are called radars, an abbreviation for Radio
Detection and Ranging. Three different types of radars are employed from space for
remote sensing of the ocean: (1) scatterometers for measuring near-surface wind
vectors over the ocean on a global scale; (2) Synthetic Aperture Radars (SARs)
for measuring ocean surface waves, meso-scale phenomena in the ocean and in
the marine boundary layer, near-surface ocean winds, and surface currents; and (3)
radar altimeters for measuring geostrophic ocean currents, wave heights, and near
near-surface wind speeds (not wind vectors).
The microwaves do not penetrate into the water body and thus microwave instruments can only provide information on the ocean and the marine boundary layer via
sea surface characteristics. In the case of active microwave instruments, all information is extracted from the sea surface roughness and from the time a radar pulse
needs to travel the distance radar- sea surface and back. In the case of passive microwave instruments (radiometers), all information is extracted from the microwave
radiation emitted from the sea surface at different frequencies and polarizations. The
emitted radiation (or the brightness temperature) contains information on sea surface
roughness (and thus the near-surface wind speed), Sea Surface Temperature (SST)
and Sea Surface Salinity (SSS).
Remote sensing of the ocean became a large boost from the American Seasat mission in 1978 (Evans et al. 2005). This satellite carried a scatterometer, a SAR, a radar
altimeter, and a microwave radiometer. Although this mission only lasted for about
three months, it provided the oceanographic community with a wealth of data that motivated the European Space Agency (ESA) and several other space agencies to launch
satellites carrying similar active microwave sensors, among them (1) scatterometers
onboard the European ERS-1, ERS-2 and MetOp satellites, the Japanese ADEOS
satellite, theAmerican Quikscat satellite, the Indian OceanSat-2 satellite, and the Chinese HY-2A satellite; (2) SARs onboard the European ERS-1, ERS-2, and Envisat
satellites, the Russian Almaz-1 satellite, the Canadian Radarsat-1 and Radarsat-2
satellites, the Japanese JERS-1 and ALOS satellites, the German TerraSAR-X and
TanDEM X satellites, the Italian COSMO-Skymed satellites, the Indian RISAT-1,
and Risat-2 satellites, and the Chinese HJ-1C satellite; and (3) radar altimeters onboard the European ERS-1, ERS-2, Envisat, the French-American Topex-Poseidon,
Jason-1 and Jason-2 satellites, and the Indian-French SARAL/AltiKa satellite.
In the following, we shall present only scatterometer and SAR data that were
acquired over coastal areas of the African continent. Radar altimeter data are less
suited for investigations of coastal areas: they unfold their full value only when
assimilated in general ocean circulation models and wind/wave forecast models.
W. Alpers
3.8–2.5 cm), and Ku-band (12–18 GHz, 2.5–1.7 cm). These strange names for the
frequency bands are leftovers of World War 2, when microwave technology was
developed in different countries and when frequency bands were top-secret.
There exist two kinds of microwave instruments used for remote sensing: passive
and active instruments. Passive instruments, called microwave radiometers, measure
the emitted microwave radiation from an object at the Earth surface, while active
instruments emit microwave pulses and receive them after backscattering from the
object. The active microwave instruments are called radars, an abbreviation for Radio
Detection and Ranging. Three different types of radars are employed from space for
remote sensing of the ocean: (1) scatterometers for measuring near-surface wind
vectors over the ocean on a global scale; (2) Synthetic Aperture Radars (SARs)
for measuring ocean surface waves, meso-scale phenomena in the ocean and in
the marine boundary layer, near-surface ocean winds, and surface currents; and (3)
radar altimeters for measuring geostrophic ocean currents, wave heights, and near
near-surface wind speeds (not wind vectors).
The microwaves do not penetrate into the water body and thus microwave instruments can only provide information on the ocean and the marine boundary layer via
sea surface characteristics. In the case of active microwave instruments, all information is extracted from the sea surface roughness and from the time a radar pulse
needs to travel the distance radar- sea surface and back. In the case of passive microwave instruments (radiometers), all information is extracted from the microwave
radiation emitted from the sea surface at different frequencies and polarizations. The
emitted radiation (or the brightness temperature) contains information on sea surface
roughness (and thus the near-surface wind speed), Sea Surface Temperature (SST)
and Sea Surface Salinity (SSS).
Remote sensing of the ocean became a large boost from the American Seasat mission in 1978 (Evans et al. 2005). This satellite carried a scatterometer, a SAR, a radar
altimeter, and a microwave radiometer. Although this mission only lasted for about
three months, it provided the oceanographic community with a wealth of data that motivated the European Space Agency (ESA) and several other space agencies to launch
satellites carrying similar active microwave sensors, among them (1) scatterometers
onboard the European ERS-1, ERS-2 and MetOp satellites, the Japanese ADEOS
satellite, theAmerican Quikscat satellite, the Indian OceanSat-2 satellite, and the Chinese HY-2A satellite; (2) SARs onboard the European ERS-1, ERS-2, and Envisat
satellites, the Russian Almaz-1 satellite, the Canadian Radarsat-1 and Radarsat-2
satellites, the Japanese JERS-1 and ALOS satellites, the German TerraSAR-X and
TanDEM X satellites, the Italian COSMO-Skymed satellites, the Indian RISAT-1,
and Risat-2 satellites, and the Chinese HJ-1C satellite; and (3) radar altimeters onboard the European ERS-1, ERS-2, Envisat, the French-American Topex-Poseidon,
Jason-1 and Jason-2 satellites, and the Indian-French SARAL/AltiKa satellite.
In the following, we shall present only scatterometer and SAR data that were
acquired over coastal areas of the African continent. Radar altimeter data are less
suited for investigations of coastal areas: they unfold their full value only when
assimilated in general ocean circulation models and wind/wave forecast models.