Applications of Passive and Active Microwave Sensors ...
467
of flight on a moving platform and attempts to deduce information about a moving surface,
results in very complicated interpretation. Certain limitations exist on the observability of
waves travelling parallel to the flight track. Currently, a first guess using the global WAM
model (community Wave Model, Komen et aI., 1995) is required to obtain wave spectra in
pre-operational work with these data (e.g. Bruning et aI., 1994). Wave statistical information
and wind speed information using a so called 'speckle' algorithm can also be obtained from the
imagettes (Chapron et aI., 1995).
17.2.2 The passive microwave instruments
Microwave radiometers have operated on polar orbiting satellites since the 1960's. This technology is therefore well established. The major systems flown or scheduled for flight soon are
listed in Table 17.2. The Scanning Multichannel Microwave Radiometer flown on Seasat and
Nimbus 7 were considered experimental, while the Special Sensor Microwave/Imagers on DMSP
satellites are operational systems. Even though the DMSP satellites are operated by the U.S.
Defense Department, the data are now available for both research and for civilian operational
uses.
The first SSM/I was launched in July 1987 with a total of 16 systems to be launched in the
series as required. The series should continue well into the next millenium. The applications
discussed below are based on data from these first two microwave radiometer systems, whose
basic characteristics are given in Table 17.3.
Three of the SSM/I frequencies, 19, 37 and 85 GHz, are operated at horizontal and vertical polarization, while the 22 GHz channel measures vertical polarization only (Hollinger et aI., 1990).
The dual polarization and the addition of the 85 GHz channels, providing some information at
15 km resolution, make the SSM/I a particularly versatile instrument.
In general, the microwave radiometers are of low spatial resolution, of the order of 25 to 50
km, but the SSM/I covers a swath of 1400 km. With two functioning SSM/I instruments in
space simultaneously (since fall 1991) several views per day of the same weather system can
be obtained, providing valuable temporal sampling of evolution even in tropical regions. One
valuable aspect of the DMSP satellites is that they have a visible and infrared high resolution
system, the Operational Line-scan System (OLS), operating simultaneously.
17.2.3 The Operational Line-scan System
The Operational Line-scan System (OLS) is a two-channel radiometer flown on the DMSP
satellites with the SSM/I. Its broad spectral bands are 0.4 to 1.1 m for the visible region and
10.2 to 12.8 m for the infrared, with a spatial resolution of 2.8 km for both channels. The OLS
infrared data are calibrated and map linearly to the range of temperatures from 190 to 310
K (Heacock, 1985). The visible data are not calibrated, but may be used to compare relative
brightnesses (Goyette et aI., 1990). Since the DMSP are launched in sun-synchronous orbits
near dawn and dusk and the visible channel therefore observes the earth at low light levels,
with a gain control to obtain maximum contrast in the image.
467
of flight on a moving platform and attempts to deduce information about a moving surface,
results in very complicated interpretation. Certain limitations exist on the observability of
waves travelling parallel to the flight track. Currently, a first guess using the global WAM
model (community Wave Model, Komen et aI., 1995) is required to obtain wave spectra in
pre-operational work with these data (e.g. Bruning et aI., 1994). Wave statistical information
and wind speed information using a so called 'speckle' algorithm can also be obtained from the
imagettes (Chapron et aI., 1995).
17.2.2 The passive microwave instruments
Microwave radiometers have operated on polar orbiting satellites since the 1960's. This technology is therefore well established. The major systems flown or scheduled for flight soon are
listed in Table 17.2. The Scanning Multichannel Microwave Radiometer flown on Seasat and
Nimbus 7 were considered experimental, while the Special Sensor Microwave/Imagers on DMSP
satellites are operational systems. Even though the DMSP satellites are operated by the U.S.
Defense Department, the data are now available for both research and for civilian operational
uses.
The first SSM/I was launched in July 1987 with a total of 16 systems to be launched in the
series as required. The series should continue well into the next millenium. The applications
discussed below are based on data from these first two microwave radiometer systems, whose
basic characteristics are given in Table 17.3.
Three of the SSM/I frequencies, 19, 37 and 85 GHz, are operated at horizontal and vertical polarization, while the 22 GHz channel measures vertical polarization only (Hollinger et aI., 1990).
The dual polarization and the addition of the 85 GHz channels, providing some information at
15 km resolution, make the SSM/I a particularly versatile instrument.
In general, the microwave radiometers are of low spatial resolution, of the order of 25 to 50
km, but the SSM/I covers a swath of 1400 km. With two functioning SSM/I instruments in
space simultaneously (since fall 1991) several views per day of the same weather system can
be obtained, providing valuable temporal sampling of evolution even in tropical regions. One
valuable aspect of the DMSP satellites is that they have a visible and infrared high resolution
system, the Operational Line-scan System (OLS), operating simultaneously.
17.2.3 The Operational Line-scan System
The Operational Line-scan System (OLS) is a two-channel radiometer flown on the DMSP
satellites with the SSM/I. Its broad spectral bands are 0.4 to 1.1 m for the visible region and
10.2 to 12.8 m for the infrared, with a spatial resolution of 2.8 km for both channels. The OLS
infrared data are calibrated and map linearly to the range of temperatures from 190 to 310
K (Heacock, 1985). The visible data are not calibrated, but may be used to compare relative
brightnesses (Goyette et aI., 1990). Since the DMSP are launched in sun-synchronous orbits
near dawn and dusk and the visible channel therefore observes the earth at low light levels,
with a gain control to obtain maximum contrast in the image.