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The curve for a satellite receiving station given in Fig. 4 shows maxima near of 3 x 109 and 3 x 1013 cps corresponding to wavelengths of approximately 10 cm and 10 μ, respectively. These are directly related to the maximum frequencies for which constant aperture areas were taken for both the transmitter and receiver. The reduced performance on the low-frequency side of the microwave peak results from a direct limitation on aperture dimension; the cutoff on the high-frequency side of this peak results both from a restriction on antenna size due to fabrication tolerances (down to dimensions where optical fabrication techniques apply) and from the higher receiver noise temperatures appropriate to an uncooled satellite receiver in the millimeter-wave region of the spectrum. The cutoff on the high-frequency side of the infrared peak is caused by the limitation assumed for pointing and tracking accuracy of the space vehicle transmitting aperture.
Similar maxima are indicated for a ground-based receiver for clear weather. Performance at microwave frequencies, however, is increasing to about 10 Gc, where atmospheric distortions tend to limit aperture size. This assumes the possibility of a single-element receiving antenna with an effective diameter of 100 meters and a fabrication tolerance of about σ/D = 2 x 10-5 or, alternatively, the employment of an equivalent antenna array. Where information rates of 106 bits per sec are considered, an array large enough to provide this effective aperture at low elevation angles will require compensation for variation not only of the carrier phase but also of the signal delay across the aperture.
The performance peak in the infrared is lowered and shifted slightly to longer wavelengths by atmospheric distortion; transmission in the submillimeter and far infrared region is effectively cancelled by atmospheric absorption. Smaller subsidiary peaks appear, however, at the 35- and 94-Gc “windows.”
The expected performance of two noncoherent optical systems is included. Because the detection mechanisms for these systems vary, it is not convenient to show a general functional dependence of performance on frequency.
When the restrictions imposed by clouds and rain are considered, the familiar single-performance maximum appears in the microwave region at about 3 Gc. Undue significance should not be given to the exact position of the peak as shown, since it is determined more by the somewhat arbitrary limits chosen for the transmitter and receiver antenna dimensions than by the more fundamental restrictions of antenna noise temperature and atmospheric transmission.
The curves of Fig. 4 indicate the relative performance of deep-space communication links operating in the various regions of the frequency spectrum. It is evident that a major hindrance to the improvement promised at higher frequencies is presented by the attenuation and distortion of the atmosphere.
Space missions likely to require the wide signal bandwidths are those involving real-time transmission of data and hence an essentially continuous capability. (Such a requirement can be expected for a fly-by mission, a landing mission, or a mission requiring voice communication.) At microwave frequencies, continuous coverage can be achieved under all weather conditions, and a direct spacecraft-to-earth link is appropriate. However, at optical frequencies, clouds may completely disable a ground site. Thus, if a continuous link is to be established between a spacecraft and earth within an acceptable atmospheric loss, alternative transmission configurations must be considered for wavelengths shorter than microwave.
There seems little reason to consider seriously a millimeter-wave system except possibly for a space vehicle-to-satellite link if a breakthrough in receiver technology should occur. For a ground receiver, performance at 30 Gc comparable to that at 3 Gc would require an equivalent antenna area with means to correlate atmospheric phase distortions over the aperture, or an order of magnitude increase in other system parameters. At 94 Gc, ground receiver performance is down by another two orders of magnitude from 30 Gc.
Potential microwave and optical systems are discussed briefly below and appropriate parameters are then given for the most promising cases.