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Status Report on AMSR-E On-Orbit Calibration; May 2003

Calibration

AMSR-E's calibration system has a cold mirror that provides a clear view of deep space (a known temperature of 2.7 K) and a hot reference load that acts as a blackbody emitter; its temperature is measured by eight precision thermistors. After launch, large thermal gradients due to solar heating developed within the hot load, making it difficult to determine from the thermistor readings the average effective temperature, or the temperature the radiometer sees. The hot load temperature is not uniform or constant, and empirical calibration methods must be employed.
Prior to 07 January 2005, RSS used coincident Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and SSM/I satellite data over oceans to estimate the effective hot load temperature. A radiative transfer model used these data to compute the intensity of radiation entering the feedhorns. The model took into account different view geometries and channel differences between AMSR-E, SSM/I, and TMI. This process essentially provided earth-target calibration points, which were combined with the cold mirror temperature to compute a two-point linear extrapolation that yielded the hot load effective temperature for 10.7 to 89 GHz.

Since 07 january 2005, to estimate the effective hot load temperature, Remote Sensing Systems (RSS) assumes that the effective temperature, Teff, is independent of channel polarization and then uses climatic cloud and water vapor data, along with daily SST and NCEP winds, to define Teff as a function of the observed polarization differences


The radiometer calibration accuracy budget, exclusive of antenna pattern correction effects, is composed of three major contributors: warm load reference error, cold load reference error, radiometer electronics non-linearities and errors. Each error component is discussed in the remaining paragraphs of this section. Accounting for all errors, the total sensor bias error is 0.66 K at 100 K and changes with temperature to 0.68 K at 250 K.

Warm load reference error: The major part of this error comes from the following four components: a) the accuracy of the platinum resistance thermistors (PRTs), measured by the manufacturer, on the order of ±0.1 K b) the temperature gradient over the load area (SSM/I gradient reached values as high as ±0.4 K per the entire load width, or 0.032 K/cm); c) load - feedhorn coupling errors due to the design of the system, and d) reflections out of the feedhorn due to receiver electronics. An estimate of the warm load reference error, when RSS'ing the aforementioned components, is ±0.5 K.

Cold load (cold sky reflector) reference error: The error in the cold reference measurement is mainly produced by the error in coupling between the cold sky reflector and the feedhorn. This is estimated to be ±0.5 K. Other factors affecting the cold reference error are the reflections out of the feedhorn due to receiver electronics and the resistive losses of the cold sky reflector itself. An estimate of this error can be as high as ±0.62 K.

Radiometer electronics non-linearities and errors: The main factor that drives the electronics non-linearity is the imperfect operation of the square law detector. This non-linearity results in an error that can be estimated during the thermal vacuum calibration testing. (On SSM/I this error was ±0.4 K). A source of error due to the receiver electronics is the gain drift due to the temperature variation over one orbit. This error depends on the design of the receiver and overall design of the sensor. This drift can be as much as ±0.24 K for a temperature variation of less than 10°C over one orbit.
 
 
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