A study of transient heat conduction and thermal noise in an Earth radiation budget radiometer

The suite of three Clouds and Earth’s Radiative Energy System (CERES) radiometers measure the radiation reflected from and emitted by the Earth from Earth orbit. The instruments are based on a two-mirror reflecting telescope which focuses incident radiation on a thermistor bolometer thermal radiation detector. The CERES radiometers scan back and forth across the Earth and surrounding space as the satellite orbits the Earth. Each scan has a period of about six seconds. This not only results in a transient radiation signal arriving at the detector surface from the scene, but also in temperature transients in the instrument structure. The instrument “zero” is obtained during the “space look” when it views cold space at each end of the scan. Some of the surfaces of the instrument structure are visible from the detector, either directly or through reflections. As a result, the radiation emitted by these surfaces will reach the detector. This form of radiation is called thermal noise and is undesirable. In order to determine whether the thermal noise is significant to cause concern, the transient response and temperature variations of the various components of the instrument must be known. The transient response is determined from observing the temporal variation of the temperature distribution within the instrument structure. Since the instrument orbits the Earth, both the Earth and space make up the environment of the instrument. This means that the temperature distributions for both the space look and Earth scene must be studied. Pseudo time constants were determined from the transient space-look temperatures. The transient thermal noise was then determined from the pseudo time constants and the steady-state space-look and earth-scene temperatures. The thermal noise was shown to vary with magnitude on the order of nanowatts. This means that the thermal noise is not sufficiently large to be of concern.