System measurements of a CO2 laser communications link

Measurements were made under simulated operating conditions to establish the capability of a CO2 laser transmitting heterodyne receiver link to acquire weak laser transmitter beams and maintain spatial tracking during communications. For the case of a synchronous satellite-to-earth station link, analysis and laboratory measurements indicate that the infrared heterodyne receiver subsystem is capable of performing the acquisition, acquisition confirm, and spatial tracking functions at IF signal-to-noise ratio of -17 dB. For the case of a low-altitude satellite-to-earth station link, the laser transmit signal is Doppler-shifted due to the difference in relative velocity of the two stations. The resultant Doppler frequency shift varies with time and must be spectrally acquired and tracked.