Spectrally resolved thermal radiances measured from orbit with an accuracy in brightness temperature of constitute a critical observation for climate monitoring. The design of a small, low-cost instrument capable of accuracies of better than , demonstrated on-orbit, is presented and analyzed. It is shown that systematic and random errors inherent in observations from space can be reduced to levels commensurate with the instrumental accuracy of . Monitoring spectrally resolved radiance, accurate to , is feasible, and constitutes a versatile climate Benchmark observation that is needed in the national research strategy.