6.3 Summary of SAR Imaging Satellites
Remote sensing of sea ice can be performed in visible, infrared (IR) ranges, using
sensors such as MODIS (Moderate Resolution Imaging Spectroradiometer), and
AVHRR (Advanced Very High Resolution Radiometer) and SeaWiFS (Sea Wide
Field of view Spectrometer). However, these sensors are limited by their coarse
spatial resolution and the susceptibility to the influence of weather conditions.
In contrast, the SAR sensors onboard satellites have been widely used for sea ice
monitoring, primarily because of the following reasons. First, SAR sensors are
active sensors, which are capable of sending radiations to the earth surface, and
receiving the backscattered signals. Consequently, SAR sensors do not rely on the
sun illumination and are able to work 24 h a day. This characteristic is especially
important for the polar regions that have long dark period in winter. Second, SAR
sensors work at microwave ranges, and therefore are capable of penetrating the
cloud. This feature also makes SAR sensors more suitable than the optical sensors
for monitoring the polar regions, where cloud is easily formed in the melting season
due to open water and the cold atmosphere. Third, SAR sensors are generally
characterized by wide coverage with medium spatial resolution. And this combination is suitable for delineating sea ice without compromising too much the
monitoring efficiency.
Various SAR sensors have been used for ocean monitoring, as summarized in
Table 6.1. Seasat launched in 1978 is almost the earliest SAR sensor that is
designed for ocean monitoring. ERS-1 and -2, launched by the European Space
Agency (ESA) in 1991 and 1995 are also specifically designed for ocean surveillance. RADARSAT-1, launched by Canadian Space Agency (CSA) in 1995, has the
optimal combination of spatial resolution and coverage that is suitable for detecting
ocean features over large area. For example, with a spatial resolution of 50 m
(100 m) and swath width of 300 km (500 km), the ScanSAR Narrow (Wide) beam
mode in RADARSAT-1 is capable of providing detail information of large sea ice
area. The ENVISAT, launched by ESA in 2002, carries an advanced SAR (ASAR)
sensor, which is capable of providing the Precision and ScanSAR modes, with
spatial resolution of 30 m and 150 m, and swath width of 100 km and 400 km
respectively. In 2007, two X-band SAR satellites, the TerraSAR-X and COSMOSkyMed were sent into space. RADARSAT-2 was launched by CSA in the same
year, and it has widely been used for sea ice monitoring (Ochilov and Clausi 2012).
It is worth to mention that Canada has initiated a new mission, called the
RADARSAT Constellation Mission (RCM) in 2005 and intends to launch three
RCM satellites in 2018. Using the three satellites, RCM will be able to provide
complete coverage of Canada’s land and ocean with daily revisit, as well as daily
access to 95 % of the Earth’s surface.
116
L. Xu and J. Li
Remote sensing of sea ice can be performed in visible, infrared (IR) ranges, using
sensors such as MODIS (Moderate Resolution Imaging Spectroradiometer), and
AVHRR (Advanced Very High Resolution Radiometer) and SeaWiFS (Sea Wide
Field of view Spectrometer). However, these sensors are limited by their coarse
spatial resolution and the susceptibility to the influence of weather conditions.
In contrast, the SAR sensors onboard satellites have been widely used for sea ice
monitoring, primarily because of the following reasons. First, SAR sensors are
active sensors, which are capable of sending radiations to the earth surface, and
receiving the backscattered signals. Consequently, SAR sensors do not rely on the
sun illumination and are able to work 24 h a day. This characteristic is especially
important for the polar regions that have long dark period in winter. Second, SAR
sensors work at microwave ranges, and therefore are capable of penetrating the
cloud. This feature also makes SAR sensors more suitable than the optical sensors
for monitoring the polar regions, where cloud is easily formed in the melting season
due to open water and the cold atmosphere. Third, SAR sensors are generally
characterized by wide coverage with medium spatial resolution. And this combination is suitable for delineating sea ice without compromising too much the
monitoring efficiency.
Various SAR sensors have been used for ocean monitoring, as summarized in
Table 6.1. Seasat launched in 1978 is almost the earliest SAR sensor that is
designed for ocean monitoring. ERS-1 and -2, launched by the European Space
Agency (ESA) in 1991 and 1995 are also specifically designed for ocean surveillance. RADARSAT-1, launched by Canadian Space Agency (CSA) in 1995, has the
optimal combination of spatial resolution and coverage that is suitable for detecting
ocean features over large area. For example, with a spatial resolution of 50 m
(100 m) and swath width of 300 km (500 km), the ScanSAR Narrow (Wide) beam
mode in RADARSAT-1 is capable of providing detail information of large sea ice
area. The ENVISAT, launched by ESA in 2002, carries an advanced SAR (ASAR)
sensor, which is capable of providing the Precision and ScanSAR modes, with
spatial resolution of 30 m and 150 m, and swath width of 100 km and 400 km
respectively. In 2007, two X-band SAR satellites, the TerraSAR-X and COSMOSkyMed were sent into space. RADARSAT-2 was launched by CSA in the same
year, and it has widely been used for sea ice monitoring (Ochilov and Clausi 2012).
It is worth to mention that Canada has initiated a new mission, called the
RADARSAT Constellation Mission (RCM) in 2005 and intends to launch three
RCM satellites in 2018. Using the three satellites, RCM will be able to provide
complete coverage of Canada’s land and ocean with daily revisit, as well as daily
access to 95 % of the Earth’s surface.
116
L. Xu and J. Li