2 Passive Microwave Remote Sensing of the Ocean
17
and NASA’s Global Precipitation Mission (GPM) will carry the GPM Microwave
Imager (GMI). For all these instruments, the planned calibration is similar to SSM/I
using a cold reflector and hot absorber.
GCOM-W is to be launched in February 2012 into NASA’s A-Train satellite
formation in a sun-synchronous orbit with an altitude of 700 km and a LECT of
1:30 AM/PM. AMSR2 is similar to AMSR-E but has an improved hot absorber and
an additional channel at 7.3 GHz to minimize Radio Frequency Interference (RFI).
With a launch date set for February 2012, it is hoped that the AQUA AMSR-E
remains healthy until then to allow for satellite inter-calibration.
Two other future instruments, the European Space Agency’s Soil Moisture and
Ocean Salinity (SMOS) Microwave Imaging Radiometer using Aperture Synthesis
(MIRAS) and the Satélite de Aplicaciones Científicas-D (SAC-D) Aquarius are
intended to measure ocean salinity and only have a single channel at 1.4 GHz.
SMOS launched in November 2009 into a sun-synchronous orbit at 800 km with
an LECT of 6:00 AM/PM. Aquarius is scheduled to be launched in May 2010 into
a sun-synchronous orbit at 650 km with a LECT of 6:00 AM/PM. Both of these
instruments are designed to provide measurements of ocean salinity.
2.3 Calibration
To create a climate quality, inter-calibrated dataset of PMW geophysical retrievals,
it is necessary to start the process using radiometer counts and work towards calibrated geophysical retrievals. Table 2.2 describes the steps to produce a calibrated
brightness temperature (TB). First, it is necessary to reverse engineer the antenna
temperatures (TAs) or TBs back to radiometer counts. Often there are small provider
added corrections or adjustments put into the TA or TBs which are sometimes
undocumented. For example, SSMI/S had five TB version changes in the first 2 years
of data. Therefore, the first step is to reverse these steps and remove any corrections.
Starting from radiometer counts, the first two steps in the calibration procedure are
crucial to accurately determining other errors.
To ensure that any subsequent collocations or comparisons that are performed
are correct, it is necessary to do a geolocation analysis. The correction to the
Table 2.2 Calibration steps for PMW radiometers
Geolocation
analysis
Attitude
adjustment
Along-scan
correction
Absolute
calibration
Hot load
correction
Antenna
emissivity
SSM/I
NRL/RSS
No
Yes
APC
No a
0
TMI
Goddard
Dynamic
Yes
APC
No
3.5%
AMSRE
RSS
Fixed
Yes
APC
Yes
0
AMSRA
RSS
Dynamic
Yes
APC
Yes
0
WindSat
NRL/RSS
Fixed
Yes
APC
Yes
0
SSMIS
RSS
No
Yes
APC
Yes
0.5–3.5%
a Errors due to hot load are removed when doing the zonal TB inter-calibration
17
and NASA’s Global Precipitation Mission (GPM) will carry the GPM Microwave
Imager (GMI). For all these instruments, the planned calibration is similar to SSM/I
using a cold reflector and hot absorber.
GCOM-W is to be launched in February 2012 into NASA’s A-Train satellite
formation in a sun-synchronous orbit with an altitude of 700 km and a LECT of
1:30 AM/PM. AMSR2 is similar to AMSR-E but has an improved hot absorber and
an additional channel at 7.3 GHz to minimize Radio Frequency Interference (RFI).
With a launch date set for February 2012, it is hoped that the AQUA AMSR-E
remains healthy until then to allow for satellite inter-calibration.
Two other future instruments, the European Space Agency’s Soil Moisture and
Ocean Salinity (SMOS) Microwave Imaging Radiometer using Aperture Synthesis
(MIRAS) and the Satélite de Aplicaciones Científicas-D (SAC-D) Aquarius are
intended to measure ocean salinity and only have a single channel at 1.4 GHz.
SMOS launched in November 2009 into a sun-synchronous orbit at 800 km with
an LECT of 6:00 AM/PM. Aquarius is scheduled to be launched in May 2010 into
a sun-synchronous orbit at 650 km with a LECT of 6:00 AM/PM. Both of these
instruments are designed to provide measurements of ocean salinity.
2.3 Calibration
To create a climate quality, inter-calibrated dataset of PMW geophysical retrievals,
it is necessary to start the process using radiometer counts and work towards calibrated geophysical retrievals. Table 2.2 describes the steps to produce a calibrated
brightness temperature (TB). First, it is necessary to reverse engineer the antenna
temperatures (TAs) or TBs back to radiometer counts. Often there are small provider
added corrections or adjustments put into the TA or TBs which are sometimes
undocumented. For example, SSMI/S had five TB version changes in the first 2 years
of data. Therefore, the first step is to reverse these steps and remove any corrections.
Starting from radiometer counts, the first two steps in the calibration procedure are
crucial to accurately determining other errors.
To ensure that any subsequent collocations or comparisons that are performed
are correct, it is necessary to do a geolocation analysis. The correction to the
Table 2.2 Calibration steps for PMW radiometers
Geolocation
analysis
Attitude
adjustment
Along-scan
correction
Absolute
calibration
Hot load
correction
Antenna
emissivity
SSM/I
NRL/RSS
No
Yes
APC
No a
0
TMI
Goddard
Dynamic
Yes
APC
No
3.5%
AMSRE
RSS
Fixed
Yes
APC
Yes
0
AMSRA
RSS
Dynamic
Yes
APC
Yes
0
WindSat
NRL/RSS
Fixed
Yes
APC
Yes
0
SSMIS
RSS
No
Yes
APC
Yes
0.5–3.5%
a Errors due to hot load are removed when doing the zonal TB inter-calibration