Analyses of data collected by the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) and precipitation radar (PR) show that the radiometric responses to rainfall profiles are sensitive to spatial inhomogeneities within the sensor field of view (FOV). The uncertainty, the so-called beam-filling error, is associated with the variability in horizontal and vertical rainfall structures within a sensor FOV coupled to the non-linear relationship between brightness temperature (Tb) and rain rate (R). This study classifies the beam-filling error as an inherent error because the sensor itself has the non-unique radiometric signatures associated with the rainfall inhomogeneity. The specific forward models hardly overcome the error inherent in the sensor. It is also shown that lower resolution retrievals are less sensitive to the inherent error than the retrievals at higher resolutions due to the less non-linearity in the Tb-R relationship at lower resolutions.