4 Remote Sensing of African Coastal Waters Using Active Microwaves Instrument
77
4.2 Scatterometer
A scatterometer is an all-weather instrument which is used from satellites to measure
two-dimensional near-surface wind fields (wind vectors). It measures the wind vectors indirectly via the short-scale sea surface roughness. Short-scale roughness means
in this context waves having wavelengths of the order of centimeters to decimeters.
A scatterometer emits radar pulses from several antennas or from a rotating antenna
and records the backscattered radar power or normalized radar cross section (NRCS)
from the wind-roughened sea surface. The higher the wind speed, the rougher the
sea surface, and the higher is the backscattered radar power or the normalized radar
cross section (NRCS). However, the backscattered radar power depends on not only
on wind speed, but also on incidence angle and wind direction relative to the antenna look direction (i.e., on azimuth angle). Thus, in order to measure wind speed
and direction with a scatterometer, the backscattered radar power from a resolution
cell must be measured from different azimuth directions. This is achieved by using
several antennas or a rotating antenna.
The scatterometer onboard the American Seasat satellite, called SASS (launched
in 1978), and the one on the Japanese satellite ADEOS, called NSCAT (launched
in 1996) had 6 antennas, three antennas looking to the right and three to the left of
the satellite track. The C-band scatterometer Active Microwave Instrument (AMI)
onboard the European satellites ERS-1 and ERS-2, which were launched in 1991 and
1995, respectively, had three antennas pointing only to the one side of the satellite
track (to the right side). On the other hand, the Ku-band (13.4 GHz) scatterometer
SeaWinds, later dubbed QuikSCAT, onboard the American Quikscat satellite, which
was launched in June 1996 and whose mission ended in November 2009, used a
rotating antenna with two spot beams that swept in a circular pattern. The instrument
collected data in a continuous, 1800-kilometer-wide swath covering 90 % of Earth’s
surface in one day. Standard Quikscat wind fields have a resolution of 25 km (12.5 km
pixel spacing) and are available on-line
1 until 19 November 2009.
At present, two scatterometers are flying in space, the C-band ASCAT (Advanced
Scatterometer) onboard the European MetOp satellite (launched in 2006) and the
Ku-band OSCAT (Oceansat-2 Scatterometer) onboard the Indian Oceansat-2 satellite
(launched in 2009). WhileASCAT uses the multiple antenna principle (three antennas
looking to each side of the satellite track), OSCAT uses, like SeaWinds, the rotating
antenna principle. ASCAT wind fields
2 have a resolution of 25 km (12.5 km pixel
spacing) and are measured on two sides of the satellite track on two 500 km wide
swathes (strips). OSAT wind fields
3 have a resolution of 50 km and are measured on
a 1420 km wide swath.
1 For details, see the websites: http://www.remss.com/qscat/qscat_browse.html, http://www.knmi.
nl/scatterometer/qscat_prod/.
2 See http://www.knmi.nl/scatterometer/ascat_ear_25_prod/ascat_app.cgi.
3 See http://www.knmi.nl/scatterometer/oscat_50_prod/oscat_app.cgi.
77
4.2 Scatterometer
A scatterometer is an all-weather instrument which is used from satellites to measure
two-dimensional near-surface wind fields (wind vectors). It measures the wind vectors indirectly via the short-scale sea surface roughness. Short-scale roughness means
in this context waves having wavelengths of the order of centimeters to decimeters.
A scatterometer emits radar pulses from several antennas or from a rotating antenna
and records the backscattered radar power or normalized radar cross section (NRCS)
from the wind-roughened sea surface. The higher the wind speed, the rougher the
sea surface, and the higher is the backscattered radar power or the normalized radar
cross section (NRCS). However, the backscattered radar power depends on not only
on wind speed, but also on incidence angle and wind direction relative to the antenna look direction (i.e., on azimuth angle). Thus, in order to measure wind speed
and direction with a scatterometer, the backscattered radar power from a resolution
cell must be measured from different azimuth directions. This is achieved by using
several antennas or a rotating antenna.
The scatterometer onboard the American Seasat satellite, called SASS (launched
in 1978), and the one on the Japanese satellite ADEOS, called NSCAT (launched
in 1996) had 6 antennas, three antennas looking to the right and three to the left of
the satellite track. The C-band scatterometer Active Microwave Instrument (AMI)
onboard the European satellites ERS-1 and ERS-2, which were launched in 1991 and
1995, respectively, had three antennas pointing only to the one side of the satellite
track (to the right side). On the other hand, the Ku-band (13.4 GHz) scatterometer
SeaWinds, later dubbed QuikSCAT, onboard the American Quikscat satellite, which
was launched in June 1996 and whose mission ended in November 2009, used a
rotating antenna with two spot beams that swept in a circular pattern. The instrument
collected data in a continuous, 1800-kilometer-wide swath covering 90 % of Earth’s
surface in one day. Standard Quikscat wind fields have a resolution of 25 km (12.5 km
pixel spacing) and are available on-line
1 until 19 November 2009.
At present, two scatterometers are flying in space, the C-band ASCAT (Advanced
Scatterometer) onboard the European MetOp satellite (launched in 2006) and the
Ku-band OSCAT (Oceansat-2 Scatterometer) onboard the Indian Oceansat-2 satellite
(launched in 2009). WhileASCAT uses the multiple antenna principle (three antennas
looking to each side of the satellite track), OSCAT uses, like SeaWinds, the rotating
antenna principle. ASCAT wind fields
2 have a resolution of 25 km (12.5 km pixel
spacing) and are measured on two sides of the satellite track on two 500 km wide
swathes (strips). OSAT wind fields
3 have a resolution of 50 km and are measured on
a 1420 km wide swath.
1 For details, see the websites: http://www.remss.com/qscat/qscat_browse.html, http://www.knmi.
nl/scatterometer/qscat_prod/.
2 See http://www.knmi.nl/scatterometer/ascat_ear_25_prod/ascat_app.cgi.
3 See http://www.knmi.nl/scatterometer/oscat_50_prod/oscat_app.cgi.