An improved dynamic electrothermal model of the ERBE total, nonscanning channels has been formulated and implemented as a computer program. This model, which is a modification of an earlier model, is used to simulate two types of solar observation: those obtained through the solar port during solar calibration, and those obtained during the satellite pitchover maneuver in which the sun is observed by the radiometer while this latter is in its Earth-viewing configuration. New results of both simulations are compared with actual flight data. These results show an improved agreement between the simulated and observed radiometer response over previous simulations. The improvement in these severe cases justifies the modification to the model and establishes its accuracy. Thermal noise has been studied also, using a separate model, to evaluate its contribution to the radiative energy absorbed by the active cavity. This study has revealed that scattering of the collimated solar radiation contributes, on average, 0.071 mW during solar calibration, and 0.207 mW during the pitchover maneuver. On the other hand, the maximum amounts of diffuse power due to emission from the field-of-view (FOV) limiter and the aperture plate are, respectively, 0.120 and 0.011 mW, which amount to 0.270 and 0.011 percent of the peak power that enters the cavity (≈45 mW). Finally, the cavity self-contamination contributes only 0.034 mW, or 0.071 percent of the peak power absorbed by the active cavity radiometer. This study confirms the assumption that, due to the geometry of the radiometer assembly and the optical properties of its components, thermal noise is well within the range of previous estimates.