1 kW Signals At 10 A.U. From Earth (At Saturn)

9006-018

This graph compares the efficacy of three continuous wave (CW) beacon systems, one in the microwave band, one in the far-infrared and one in the visible.  The receiving dish or telescope is identical in size to that of the respective transmitter, and the carrier-to-noise ratio (CNR) has been normalized to 1 kW transmitter power and a 1 Hz intermediate frequency (IF) detection bandwidth.

For communications within the solar system, the worse case signal-to-noise ratio (SNR) is obtained when the sun is directly behind the transmitter, as seen from the receiver.  This worse case SNR is determined by the planckian radiation of the sun at all wavelengths.  On a watt for watt basis, the resulting CNRs and SNRs are very similar.

The real benefit of employing free-space laser communications for deep-space probes is that since the antenna size is so small, the probes can be very compact, lower cost, cost less to launch, and be more reliable.

The Columbus Optical SETI Observatory
Copyright (c), 1990