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The Multispectral Thermal Imager (MTI) is a 15-band satellite-based imaging system. Two of the bands (J, K) are located in the mid-infrared (3–5 ) wavelength region: J, 3.5–4.1 and K, 4.9–5.1 , and three of the bands (L, M, N) are located in the thermal infrared (8–12 ) wavelength region: L, 8.0–8.4 ; M, 8.4–8.8 ; and N, 10.2–10.7 . The absolute radiometric accuracy of the MTI data acquired in bands J-N was assessed over a period of approximately three years using data from the Lake Tahoe, CA/NV, automated validation site. Assessment involved using a radiative transfer model to propagate surface skin temperature measurements made at the time of the MTI overpass to predict the vicarious at-sensor radiance. The vicarious at-sensor radiance was convolved with the MTI system response functions to obtain the vicarious at-sensor MTI radiance in bands J–N. The vicarious radiances were then compared with the instrument measured radiances. In order to avoid any reflected solar contribution in the mid-infrared bands, only nighttime scenes were used in the analysis of bands J and K. Twelve cloud-free scenes were used in the analysis of the data from the mid-infrared bands (J, K), and 23 cloud-free scenes were used in the analysis of the thermal infrared bands (L, M, N). The scenes had skin temperatures ranging between 4.4 and 18.6 . The skin temperature was found to be, on average, cooler than the bulk temperature during the day and cooler than the bulk temperature at night. The smaller skin effect during the day was attributed to solar heating. The mean and standard deviation of the percent differences between the vicarious (predicted) at-sensor radiance convolved to the MTI bandpasses and the MTI measured radiances were , , , , for bands J–N, respectively. The results in- dicate that, with the exception of band L, the instrument measured radiances are warmer than expected.