Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicapable.

Signal coding techniques are available which permit a close approach to the theoretical data-rate limit. Similarly, receiver noise and detection efficiency are approaching reasonable or ultimate limits within the spectral regions of interest except at millimeter and submillimeter wavelengths. Efficient, narrow-band radiative sources of suitable cw power are available in both radio and optical regions of the spectrum except at submillimeter, near-infrared, and visible wavelengths. (Two years ago the entire optical region would have been excluded.) The significant restrictions on system performance are therefore due to external noise sources and atmospheric effects which identify certain favorable regions of the spectrum, and to engineering and technological limitations on aperture gain which determine the achievable levels of performance within these favorable regions.

Galactic noise and atmospheric background tend to define a low noise region between 1 and 10 Gc in good weather, which is narrowed to about 1 to 5 Gc during rain. Above 50 Gc, atmospheric absorption becomes prohibitive to about 12 μ except for a partial window at 94 Gc. Cloud cover extends the blackout through the optical region. Thus, for a ground-based receiver, good performance is limited to the 1-to-10-Gc region and to optical wavelengths. With rain and clouds, the useful frequency band is restricted to 1 to 5 Gc.

Distortions of the incident wavefront caused by atmospheric turbulence limit the effective dimensions, and hence again, of single-element receiver apertures. In the microwave region, gains are limited to about 80 dB; and in the optical region, to about 100 dB. Fabrication tolerances set gain limits on both receiver and transmitter apertures. A given ration of rms deviation in effective path length to aperture diameter yields a gain limit constant with frequency.

In practice, similar tolerance ratios are achievable for smaller diameters so that as aperture dimensions decrease with increasing frequency, an improvement in gain can result. Gain is finally limited at the high-frequency end of the spectrum by need for a beamwidth comparable to expected pointing errors.

The basic limitations on system performance define two spectral regions of interest for a ground-based receiver: one in the microwave region near 3 Gc, another in the optical region near 10 μ. However, since poor weather conditions can effectively black out the entire optical region, the requirement for essentially continuous link operation would require several ground stations at diverse locations to assure good weather for at least one link.

Atmospheric distortions of the wavefront restrict single-aperture receiver diameters in an approximately inverse relationship to frequency. Multi-element apertures are required to provide equivalent performance at higher frequencies. Although for a given performance, over-all receiver aperture area can be reduced in proportion to the increase in gain available from the transmitter aperture, receiver simplicity tends to favor the longer optical wavelengths. The unique availability of an efficient transmitting source at 10.6 μ provides an overriding argument which directs interest to the 10-μ region for an optical communication link employing coherent reception.

For noncoherent reception, receiver sensitivities as limited by thermal or quantum noise are sufficiently below coherent levels to require excessively large aperture areas. Although these “photon buckets” need not be of optical quality, practical limitations on detector size would require fabrication tolerances comparable to those microwave apertures with the additional requirement of a specular surface. Therefore, unless an efficient laser becomes available in the visible region, noncoherent reception is not recommended for wideband communication links.

For a satellite receiver, atmospheric restrictions do not apply. However, the large aperture dimensions appropriate to microwave frequencies, with no appreciable compensating advantages in terms of reduced atmospheric losses and background noise, eliminate consideration of a microwave spacecraft-to-satellite relay link. At the same time, millimeter- and submillimeter-wave regions offer little advantage over the optical region with respect to the state of the art of radiative power sources and detectors. Moreover, the larger apertures required put this region at a comparative disadvantage. The optical region is therefore favored. Gain limitations due to pointing accuracies and the present availability of an efficient transmitting source at 10.6 μ once more direct attention to the 10-μ region.

Three basic system configurations are therefore considered worthy of further investigations. These are:

  • Direct microwave, spacecraft-to-earth communication link in the region of 1 to 5 Gc;
  • Direct optical link at 10.6 μ employing additional ground stations to assure the necessary weather diversity;
  • Satellite relay (one or at most two required) using a 10.6-μ optical link from spacecraft to satellite with a noncritical microwave link for the short link from satellite to earth.

Further research and development are required before detailed analysis can be made to determine the relative costs appropriate to these configurations.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicapable.