4 Remote Sensing of African Coastal Waters Using Active Microwaves Instrument
81
resolution of 25 m. This SAR image shows the position of the wind front much more
accurately than the Quikscat wind map and reveals fine-scale structures of the front.
Note the wave-like structure of the front (in particular in the northern section), which
suggests that the front generated atmospheric gravity waves (see also next section).
4.3 Synthetic Aperture Radar
A SAR is also an active microwave instrument, but can image the sea surface with
a much higher resolution than a scatterometer. SAR images acquired over the ocean
from satellites have spatial resolutions ranging from few meters to several 100 m
depending on the type of SAR and on the data processing. The high resolution is
achieved by exploiting the phase history of the backscattered signals and then using
a rather complex data processing algorithm to generate the SAR image. The SAR
principle works well when stationary targets are imaged, but when the targets are
moving, the phase history is distorted, which leads to artifacts in the SAR image.
For example, when SAR images a moving train, then, on the SAR image, the train is
displaced from the railroad track (“train- off- the track effect”). Since in the presence
of ocean surface waves the ocean surface is moving, the SAR image provides a
distorted image of the wave field. This is a severe problem, which limits the ability
of SAR to provide reliable information on ocean waves (Alpers et al. 1981).
Originally, the main motivation to fly SARs onboard ocean observing satellites
was to measure wind generated ocean waves and use the data to improve ocean
wave forecast. But even until today, it is still controversial whether assimilation
of SAR-derived ocean wave products has a positive impact on wave prediction
(Abdallah et al. 2010) because of the motion-induced distortions. However, in the
case of swell imaging, the distortion is small when the swell has long wavelength and
small amplitude. In this case, wave information can be retrieved from SAR images.
Contrasting the poor performance of SAR for ocean wave imaging, SAR has turned
out to be a very valuable instrument to image other oceanic phenomena, like internal
waves, underwater bottom topography, oceanic fronts, oceanic eddies, and surface
film coverage. Furthermore, it is also a very valuable instrument to image atmospheric phenomena in the marine boundary layer, like atmospheric gravity waves,
atmospheric fronts, atmospheric eddies/cyclones, and coastal wind fields.
In this paper we present only SAR images acquired by the C-band SARs onboard
the ERS-1, ERS-2, and Envisat satellites. The ERS SAR images have always a
resolution of 25 m and a swath width of 100 km, while the Envisat SAR images,
called ASAR (Advanced SAR) images, have different resolutions: 25 m in the Image
Mode (IMM), 150 m in the Wide Swath Mode (WSM) and 1 km in the Global Mode
(GM). The swath widths are 100 km for the Image Mode and about 400 km for the
Wide Swath Mode and the Global Mode.
81
resolution of 25 m. This SAR image shows the position of the wind front much more
accurately than the Quikscat wind map and reveals fine-scale structures of the front.
Note the wave-like structure of the front (in particular in the northern section), which
suggests that the front generated atmospheric gravity waves (see also next section).
4.3 Synthetic Aperture Radar
A SAR is also an active microwave instrument, but can image the sea surface with
a much higher resolution than a scatterometer. SAR images acquired over the ocean
from satellites have spatial resolutions ranging from few meters to several 100 m
depending on the type of SAR and on the data processing. The high resolution is
achieved by exploiting the phase history of the backscattered signals and then using
a rather complex data processing algorithm to generate the SAR image. The SAR
principle works well when stationary targets are imaged, but when the targets are
moving, the phase history is distorted, which leads to artifacts in the SAR image.
For example, when SAR images a moving train, then, on the SAR image, the train is
displaced from the railroad track (“train- off- the track effect”). Since in the presence
of ocean surface waves the ocean surface is moving, the SAR image provides a
distorted image of the wave field. This is a severe problem, which limits the ability
of SAR to provide reliable information on ocean waves (Alpers et al. 1981).
Originally, the main motivation to fly SARs onboard ocean observing satellites
was to measure wind generated ocean waves and use the data to improve ocean
wave forecast. But even until today, it is still controversial whether assimilation
of SAR-derived ocean wave products has a positive impact on wave prediction
(Abdallah et al. 2010) because of the motion-induced distortions. However, in the
case of swell imaging, the distortion is small when the swell has long wavelength and
small amplitude. In this case, wave information can be retrieved from SAR images.
Contrasting the poor performance of SAR for ocean wave imaging, SAR has turned
out to be a very valuable instrument to image other oceanic phenomena, like internal
waves, underwater bottom topography, oceanic fronts, oceanic eddies, and surface
film coverage. Furthermore, it is also a very valuable instrument to image atmospheric phenomena in the marine boundary layer, like atmospheric gravity waves,
atmospheric fronts, atmospheric eddies/cyclones, and coastal wind fields.
In this paper we present only SAR images acquired by the C-band SARs onboard
the ERS-1, ERS-2, and Envisat satellites. The ERS SAR images have always a
resolution of 25 m and a swath width of 100 km, while the Envisat SAR images,
called ASAR (Advanced SAR) images, have different resolutions: 25 m in the Image
Mode (IMM), 150 m in the Wide Swath Mode (WSM) and 1 km in the Global Mode
(GM). The swath widths are 100 km for the Image Mode and about 400 km for the
Wide Swath Mode and the Global Mode.