18
C.L. Gentemann et al.
geolocation is different than a correction for erroneous satellite pointing information (roll/pitch/yaw), which is a correction for the mounting of the instrument
on the satellite. Pointing is usually off by about 0.1 ◦ from the satellite specified
roll/pitch/yaw. The geolocation correction uses ascending minus descending TA to
ensure that islands do not “move”. The geolocation analysis has been performed
by a number of groups, NRL and Remote Sensing Systems (RSS) both contributed
to SSM/I, TMI was completed by Goddard, and other instruments as specified in
Table 2.2.
Corrections from this point onward are determined by comparisons between
the satellite TA measurements and TAs simulated using a radiative transfer model
(RTM). Using collocated environmental information, RTM simulated TBs are determined. These TBs are then transformed into TAs using the instrument, channel
specific antenna patterns.
After the pointing is corrected, the spacecraft reported roll/pitch/yaw are then
examined for errors using comparisons of the observed minus RTM TAs. Spacecraft
pointing is determined by a number of different methods, the preferred being a star
tracker. Another method is a horizon balancing sensor. For SSM/I no pointing information was given, so it was assumed to be correct. TMI has a dynamic pointing
correction that changes within an orbit because the horizon sensor used prior to
the orbit boost is not as accurate as a star tracker. After boost, the horizon sensor
was disabled and pointing was determined from two on-board gyroscopes, also not
as accurate as a star tracker. AMSR-E had no pointing problems, as the AQUA
had a star tracker. The AMSR on ADEOS-II needed a dynamic correction, while
WindSAT needed a simple fixed correction to the roll/pitch/yaw.
Once instrument mounting errors and satellite attitude errors have been corrected
for, an along-scan correction is completed. It is very important to complete the first
two corrections first because TA is dependent on incidence angle. Not correcting
for pointing errors would result in an erroneous cross-scan biasing. As the mirror
rotates, at the edge of the earth scene the view will begin to contain obstructions
such as the satellite itself or part of the cold mirror. Additionally, during the scan,
the antenna side-lobe pattern may result in contributions from different parts of the
spacecraft. Therefore, the difference between the TA and RTM simulated TAs are
again used to examine the data for along-scan biases. This correction is needed for
every instrument.
The antenna pattern correction (APC) is then completed. Pre-launch, an APC is
determined, consisting of the spill over and cross-polarization values. After launch,
the spill over and cross-polarization values are adjusted so that the measured TAs
match the simulated TAs. This correction is needed for all instruments. Next, a correction for the hot load thermal gradients and antenna emissivity are developed.
These are only needed for specific instruments. The determination of TB from
counts for PMW radiometers is completed using two known temperatures to infer
the scene temperature. For each scan, the feedhorns view a mirror that reflects cold
space, a known 2.7 K, a hot absorber, measured by several thermistors, and Earth
scenes. Assuming a linear response, the Earth scene temperatures are then determined by fitting a slope to the two known measurements as shown in Fig. 2.2. This
C.L. Gentemann et al.
geolocation is different than a correction for erroneous satellite pointing information (roll/pitch/yaw), which is a correction for the mounting of the instrument
on the satellite. Pointing is usually off by about 0.1 ◦ from the satellite specified
roll/pitch/yaw. The geolocation correction uses ascending minus descending TA to
ensure that islands do not “move”. The geolocation analysis has been performed
by a number of groups, NRL and Remote Sensing Systems (RSS) both contributed
to SSM/I, TMI was completed by Goddard, and other instruments as specified in
Table 2.2.
Corrections from this point onward are determined by comparisons between
the satellite TA measurements and TAs simulated using a radiative transfer model
(RTM). Using collocated environmental information, RTM simulated TBs are determined. These TBs are then transformed into TAs using the instrument, channel
specific antenna patterns.
After the pointing is corrected, the spacecraft reported roll/pitch/yaw are then
examined for errors using comparisons of the observed minus RTM TAs. Spacecraft
pointing is determined by a number of different methods, the preferred being a star
tracker. Another method is a horizon balancing sensor. For SSM/I no pointing information was given, so it was assumed to be correct. TMI has a dynamic pointing
correction that changes within an orbit because the horizon sensor used prior to
the orbit boost is not as accurate as a star tracker. After boost, the horizon sensor
was disabled and pointing was determined from two on-board gyroscopes, also not
as accurate as a star tracker. AMSR-E had no pointing problems, as the AQUA
had a star tracker. The AMSR on ADEOS-II needed a dynamic correction, while
WindSAT needed a simple fixed correction to the roll/pitch/yaw.
Once instrument mounting errors and satellite attitude errors have been corrected
for, an along-scan correction is completed. It is very important to complete the first
two corrections first because TA is dependent on incidence angle. Not correcting
for pointing errors would result in an erroneous cross-scan biasing. As the mirror
rotates, at the edge of the earth scene the view will begin to contain obstructions
such as the satellite itself or part of the cold mirror. Additionally, during the scan,
the antenna side-lobe pattern may result in contributions from different parts of the
spacecraft. Therefore, the difference between the TA and RTM simulated TAs are
again used to examine the data for along-scan biases. This correction is needed for
every instrument.
The antenna pattern correction (APC) is then completed. Pre-launch, an APC is
determined, consisting of the spill over and cross-polarization values. After launch,
the spill over and cross-polarization values are adjusted so that the measured TAs
match the simulated TAs. This correction is needed for all instruments. Next, a correction for the hot load thermal gradients and antenna emissivity are developed.
These are only needed for specific instruments. The determination of TB from
counts for PMW radiometers is completed using two known temperatures to infer
the scene temperature. For each scan, the feedhorns view a mirror that reflects cold
space, a known 2.7 K, a hot absorber, measured by several thermistors, and Earth
scenes. Assuming a linear response, the Earth scene temperatures are then determined by fitting a slope to the two known measurements as shown in Fig. 2.2. This