2 Passive Microwave Remote Sensing of the Ocean
15
hot absorber temperature is monitored with thermistors. The frequent calibration
minimizes receiver gain fluctuation contributions to the signal but does not correct
radiometer nonlinearity (if it exists). This well-calibrated instrument’s measurements are used to determine wind speed, water vapor, cloud liquid water, rain rates,
and sea ice concentration over global oceans.
In December 1997, NASA launched the Tropical Rainfall Measuring Mission
(TRMM) carrying the TRMM Microwave Imager (TMI), a PMW radiometer measuring at 10.7, 19.4, 21.3, 37.0, and 85.5 GHz. Similar to SSM/I, all channels
measure both vertical and horizontal polarizations, except the 21.3 GHz which only
measures in the vertical (Kummerow et al., 1998). Designed to measure the tropics
and sample the diurnal cycle, the satellite was launched with an orbital inclination
of 35 ◦ at an altitude of 350 km (later changed to 400 km to extend satellite life). This
equatorial orbit yields coverage from 39N to 39S. The satellite is sun-asynchronous,
processing through the diurnal cycle every 23 days. Again, similar to SSM/I, the
feed horns and main reflector rotate, with a period of 1.9 s, about an axis parallel to
the local spacecraft nadir. The stationary hot reference absorber and cold calibration
reflector are positioned so that they pass between the feed horns and main reflector
once per scan. The temperature of the warm load is monitored by three thermistors
while the cold reflector views the cosmic microwave (MW) background at 2.7 K. At
fairly regular intervals the platform yaws from forward (aft) viewing direction to aft
(forward). Each scan consists of 104 discrete samples spaced by 8 km. In addition
to the geophysical variables measured by SSM/I, TMI is able to measure SST. TMI
suffered calibration problems due to an emissive reflector, for which corrections
were developed and implemented.
NASA’s AQUA satellite carries the JAXA’s Advanced Microwave Scanning
Radiometer – Earth Observing System (AMSR-E). The AQUA satellite was
launched in May 2002 into a polar, sun-synchronous orbit at an altitude of 705 km,
with a LECT of 1:30 AM/PM. AMSR-E has 12 channels corresponding to 6 frequencies: 6.9, 10.7, 18.7, 23.8, 36.5, and 89.0 GHz, all except 23.8 measure both
vertical and horizontal polarizations (Parkinson, 2003). The calibration is completed
similar to SSM/I and TMI using a cold reflector and hot absorber with eight thermistors. The AMSR-E hot absorber has large thermal gradients not well measured
by the thermistors. A correction for this error in the calibration reference point has
been developed and implemented. In addition to the geophysical variables measured
by SSM/I, AMSR-E is able to measure SSTs. Almost global coverage is attainable
in 2 days (Fig. 2.1).
The Naval Research Laboratory (NRL) launched the Coriolis satellite in January
2003. The sun-synchronous orbit is at an altitude of 840 km with a LECT at 6:00
AM/ PM (Gaiser et al., 2004). Coriolis carries the WindSat instrument, a fully
polarimetric PMW radiometer intended to retrieve wind direction in addition to
wind speed. The fully polarimetric channels are at 10.7, 18.7, and 37.0 GHz, but
the instrument also has channels at 6.8 and 23.8 that only measure the vertical and
horizontal polarizations. Calibration is similar to SSM/I with a cold reflector and
hot absorber measured by six thermistors.
15
hot absorber temperature is monitored with thermistors. The frequent calibration
minimizes receiver gain fluctuation contributions to the signal but does not correct
radiometer nonlinearity (if it exists). This well-calibrated instrument’s measurements are used to determine wind speed, water vapor, cloud liquid water, rain rates,
and sea ice concentration over global oceans.
In December 1997, NASA launched the Tropical Rainfall Measuring Mission
(TRMM) carrying the TRMM Microwave Imager (TMI), a PMW radiometer measuring at 10.7, 19.4, 21.3, 37.0, and 85.5 GHz. Similar to SSM/I, all channels
measure both vertical and horizontal polarizations, except the 21.3 GHz which only
measures in the vertical (Kummerow et al., 1998). Designed to measure the tropics
and sample the diurnal cycle, the satellite was launched with an orbital inclination
of 35 ◦ at an altitude of 350 km (later changed to 400 km to extend satellite life). This
equatorial orbit yields coverage from 39N to 39S. The satellite is sun-asynchronous,
processing through the diurnal cycle every 23 days. Again, similar to SSM/I, the
feed horns and main reflector rotate, with a period of 1.9 s, about an axis parallel to
the local spacecraft nadir. The stationary hot reference absorber and cold calibration
reflector are positioned so that they pass between the feed horns and main reflector
once per scan. The temperature of the warm load is monitored by three thermistors
while the cold reflector views the cosmic microwave (MW) background at 2.7 K. At
fairly regular intervals the platform yaws from forward (aft) viewing direction to aft
(forward). Each scan consists of 104 discrete samples spaced by 8 km. In addition
to the geophysical variables measured by SSM/I, TMI is able to measure SST. TMI
suffered calibration problems due to an emissive reflector, for which corrections
were developed and implemented.
NASA’s AQUA satellite carries the JAXA’s Advanced Microwave Scanning
Radiometer – Earth Observing System (AMSR-E). The AQUA satellite was
launched in May 2002 into a polar, sun-synchronous orbit at an altitude of 705 km,
with a LECT of 1:30 AM/PM. AMSR-E has 12 channels corresponding to 6 frequencies: 6.9, 10.7, 18.7, 23.8, 36.5, and 89.0 GHz, all except 23.8 measure both
vertical and horizontal polarizations (Parkinson, 2003). The calibration is completed
similar to SSM/I and TMI using a cold reflector and hot absorber with eight thermistors. The AMSR-E hot absorber has large thermal gradients not well measured
by the thermistors. A correction for this error in the calibration reference point has
been developed and implemented. In addition to the geophysical variables measured
by SSM/I, AMSR-E is able to measure SSTs. Almost global coverage is attainable
in 2 days (Fig. 2.1).
The Naval Research Laboratory (NRL) launched the Coriolis satellite in January
2003. The sun-synchronous orbit is at an altitude of 840 km with a LECT at 6:00
AM/ PM (Gaiser et al., 2004). Coriolis carries the WindSat instrument, a fully
polarimetric PMW radiometer intended to retrieve wind direction in addition to
wind speed. The fully polarimetric channels are at 10.7, 18.7, and 37.0 GHz, but
the instrument also has channels at 6.8 and 23.8 that only measure the vertical and
horizontal polarizations. Calibration is similar to SSM/I with a cold reflector and
hot absorber measured by six thermistors.