2 Passive Microwave Remote Sensing of the Ocean
19
Fig. 2.2 Calculation of earth
scene brightness temperatures
using the radiometer counts
and calibration points (cold
mirror and hot absorber)
known temperatures
2-point calibration system continuously compensates for variations in the radiometer gain and noise temperatures. This seemingly simple calibration methodology is
fraught with subtle difficulties. The cold mirror is relatively trouble-free, as long as
lunar contamination is flagged. Occasionally, the cold mirror will not reflect deep
space, but the moon instead. These data must be removed.
The hot absorber has been more problematic as the thermistors often do not adequately measure thermal gradients across the hot absorber. For example, a hot load
correction is required for AMSR-E because of a design flaw in the AMSR-E hot
load. The hot reference load acts as a blackbody emitter and its temperature is measured by precision thermistors. Unfortunately, during the course of an orbit, large
thermal gradients develop within the hot load due to solar heating making it difficult to determine the average effective temperature from the thermistor readings.
The thermistors themselves measure these gradients and may vary by up to 15 K
between themselves at any time for AMSR-E. Several other instruments have had
similar, but smaller, issues. RTM simulations are used to determine an effective hot
load temperature which is a regression of the measured hot load thermistor temperatures. The follow-on instrument, AMSR2 on GCOM-W, has an improved hot
absorber design that should mitigate these issues.
Finally, the main reflector is assumed to be a perfect reflector with an emissivity
of 0.0, but this is not always the case. For example, a bias recognized in the TMI
measurements was attributed to the degradation of the primary antenna. Atomic
oxygen present at TMI’s low altitude (350 km) led to rapid oxidization of the thin,
vapor-deposited aluminum coating on the graphite primary antenna, resulting in
a much higher antenna emissivity than expected. The measured radiation is comprised of the reflected earth scene and antenna emissions. Emissivity of the antenna
was deduced during the calibration procedure to be 3.5%. The antenna emissivity
correction utilizes additional information from instrument thermistors to estimate
the antenna temperature, thereby reducing the effect of the temporal variance. This
emissivity is constant for all the TMI channels. SSMI/S has an emissive antenna
where the emissivity appears to increase as a function of frequency, changing from
0.5 to 3.5%.
19
Fig. 2.2 Calculation of earth
scene brightness temperatures
using the radiometer counts
and calibration points (cold
mirror and hot absorber)
known temperatures
2-point calibration system continuously compensates for variations in the radiometer gain and noise temperatures. This seemingly simple calibration methodology is
fraught with subtle difficulties. The cold mirror is relatively trouble-free, as long as
lunar contamination is flagged. Occasionally, the cold mirror will not reflect deep
space, but the moon instead. These data must be removed.
The hot absorber has been more problematic as the thermistors often do not adequately measure thermal gradients across the hot absorber. For example, a hot load
correction is required for AMSR-E because of a design flaw in the AMSR-E hot
load. The hot reference load acts as a blackbody emitter and its temperature is measured by precision thermistors. Unfortunately, during the course of an orbit, large
thermal gradients develop within the hot load due to solar heating making it difficult to determine the average effective temperature from the thermistor readings.
The thermistors themselves measure these gradients and may vary by up to 15 K
between themselves at any time for AMSR-E. Several other instruments have had
similar, but smaller, issues. RTM simulations are used to determine an effective hot
load temperature which is a regression of the measured hot load thermistor temperatures. The follow-on instrument, AMSR2 on GCOM-W, has an improved hot
absorber design that should mitigate these issues.
Finally, the main reflector is assumed to be a perfect reflector with an emissivity
of 0.0, but this is not always the case. For example, a bias recognized in the TMI
measurements was attributed to the degradation of the primary antenna. Atomic
oxygen present at TMI’s low altitude (350 km) led to rapid oxidization of the thin,
vapor-deposited aluminum coating on the graphite primary antenna, resulting in
a much higher antenna emissivity than expected. The measured radiation is comprised of the reflected earth scene and antenna emissions. Emissivity of the antenna
was deduced during the calibration procedure to be 3.5%. The antenna emissivity
correction utilizes additional information from instrument thermistors to estimate
the antenna temperature, thereby reducing the effect of the temporal variance. This
emissivity is constant for all the TMI channels. SSMI/S has an emissive antenna
where the emissivity appears to increase as a function of frequency, changing from
0.5 to 3.5%.