Atmospheric transmission measurements at HF and DF laser wavelengths using a heterodyne radiometer

Heterodyne radiometric systems operating in the wavelength ranges 2.5-3 micrometers and 3.5-4 micrometers have been constructed and have been used to make atmospheric transmission measurements at selected HF and DF laser lines. The spectral resolution of the system is 0.002/cm, and the measured radiometer NEP was 5 x 10 to the minus 15th power watts for an output time constant of 30 sec. The measured transmissions at HF lines appear less than that predicted by theoretical considerations, while for the DF lines, the measured and calculated transmissions are consistent.