9.3 Remote Sensing Techniques
315
dual-frequency radar altimeter, a three-frequency microwave radiometer and a
precise satellite tracking system (Fu et al., 1994). The performance of the measurement system has been tested extensively and the results indicate that the
root-mean-square accuracy of a single-pass sea level measurement is about 5
cm. In the next sections we will show some application of TOPEX/POSEIDON
data for studying waves, tides and currents.
Other important oceanographic satellites are ERS-l and ERS-2, launched by
the European Space Agency (ESA). Satellite ERS-l (launched in July 1991)
occupies a sun-synchronous orbit of altitude 785 km, and has a 35-day repeat
cycle. It is equipped with a high resolution imaging system, known as Synthetic
Aperture Radar (SAR). It is based on the emission of short microwave pulses
at an incidence angle of 20° to 25°. The typical swath scanned by SAR is of
the order of 100 km, with a resolution of 20 m x 20 m. Thus, the SAR is able
to detect and characterize surface and internal wave fields.
The ERS-l satellite is also equipped with an Along Track Scanning Radiometer (ATSR), specifically designed to provide precise measurements of sea surface
temperature (SST) for application in climate research, with an accuracy of better than 0.3 K. For measurements, the ATSR uses the same wave lengths as the
Advanced Very High Resolution Radiometer (AVHRR) which is the operational
instrument on the NOAA meteorological satellites.
In July 1997, the Orbview satellite was launched with the Sea-viewing Wide
Field-of-view Sensor (SeaWiFS) on board. This system is designed primarily
for the study of ocean biochemistry and is a joint venture between NASA and
private industry. The data can be used for the study of such phenomena as
ocean primary production and phytoplankton processes; cycles of carbon, sulfur
and nitrogen; and ocean influences on global climate, including heat storage in
the upper ocean and marine aerosol formation. Moreover, the data will be of
great importance for commercial fishing, navigation and weather prediction.
In next section we briefly summarize the contributions of these dedicated
satellites to observe and measure various ocean water features.
9.3.2 Surface Waves Observed by Satellites
When studying surface waves, two typical satellite sensors are usually used.
The application of the satellite altimeter is obvious, as a radar altimeter is a
nadir looking instrument. The measurement of the travel time of the reflected
microwave pulse yields the position of the sea surface relative to the orbit of
the satellite. The distortion of the mean shape of the return pulse provides an
estimate of the sea variance, 0"(, and hence of significant wave height, H = 40"(.
To estimate the mean pulse shape, several hundred pulses need to be averaged,
resulting in one significant wave height estimate about every 7 km along the
satellite track.
The comparison of GEOSAT estimates of significant wave height, H s , against
values from buoys in NOAA's National Data Buoy Center network, showed
that GEOSAT underestimated the significant wave height by 13% (Carter et
315
dual-frequency radar altimeter, a three-frequency microwave radiometer and a
precise satellite tracking system (Fu et al., 1994). The performance of the measurement system has been tested extensively and the results indicate that the
root-mean-square accuracy of a single-pass sea level measurement is about 5
cm. In the next sections we will show some application of TOPEX/POSEIDON
data for studying waves, tides and currents.
Other important oceanographic satellites are ERS-l and ERS-2, launched by
the European Space Agency (ESA). Satellite ERS-l (launched in July 1991)
occupies a sun-synchronous orbit of altitude 785 km, and has a 35-day repeat
cycle. It is equipped with a high resolution imaging system, known as Synthetic
Aperture Radar (SAR). It is based on the emission of short microwave pulses
at an incidence angle of 20° to 25°. The typical swath scanned by SAR is of
the order of 100 km, with a resolution of 20 m x 20 m. Thus, the SAR is able
to detect and characterize surface and internal wave fields.
The ERS-l satellite is also equipped with an Along Track Scanning Radiometer (ATSR), specifically designed to provide precise measurements of sea surface
temperature (SST) for application in climate research, with an accuracy of better than 0.3 K. For measurements, the ATSR uses the same wave lengths as the
Advanced Very High Resolution Radiometer (AVHRR) which is the operational
instrument on the NOAA meteorological satellites.
In July 1997, the Orbview satellite was launched with the Sea-viewing Wide
Field-of-view Sensor (SeaWiFS) on board. This system is designed primarily
for the study of ocean biochemistry and is a joint venture between NASA and
private industry. The data can be used for the study of such phenomena as
ocean primary production and phytoplankton processes; cycles of carbon, sulfur
and nitrogen; and ocean influences on global climate, including heat storage in
the upper ocean and marine aerosol formation. Moreover, the data will be of
great importance for commercial fishing, navigation and weather prediction.
In next section we briefly summarize the contributions of these dedicated
satellites to observe and measure various ocean water features.
9.3.2 Surface Waves Observed by Satellites
When studying surface waves, two typical satellite sensors are usually used.
The application of the satellite altimeter is obvious, as a radar altimeter is a
nadir looking instrument. The measurement of the travel time of the reflected
microwave pulse yields the position of the sea surface relative to the orbit of
the satellite. The distortion of the mean shape of the return pulse provides an
estimate of the sea variance, 0"(, and hence of significant wave height, H = 40"(.
To estimate the mean pulse shape, several hundred pulses need to be averaged,
resulting in one significant wave height estimate about every 7 km along the
satellite track.
The comparison of GEOSAT estimates of significant wave height, H s , against
values from buoys in NOAA's National Data Buoy Center network, showed
that GEOSAT underestimated the significant wave height by 13% (Carter et