Thermal radiometers are radiometers with the capability of measuring the spectrum of infrared emission. As such, they are characterized by a relatively high spectral resolution (normally better than 1 cm-1 in wave number units). Modern Spectrometers on board satellites have a spectral resolution better than 0.7 cm -1 in order to properly resolve CO2 lines used for the retrieval of the atmospheric temperature profile. Based on the optical layout they are further classified in grating spectrometers and Fourier Transform Spectrometers or FTIR.