Atmospheric transmission measurements at HF and DF laser wavelengths

Atmospheric transmission at selected HF and DF laser lines were measured by a simple technique in which a heterodyned radiometer measured atmospheric attenuation along the earth-to-space path, with high spectral resolution. Atmospheric transmission was determined by measuring the intensity of an extraterrestrial radiation source (the sun) at various zenith angles. Radiation from the source was then mixed with the local oscillator signal in a HgCdTe photocell; the IF signal at the difference frequency was then amplified by a low-noise amplifier, correlated with the chopper reference signal, and read out on a stripchart. The measurements are compared to predicted data, with good agreement.