Direct microwave link
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Direct microwave link
Near 3 Gc, atmospheric losses and restrictions on potential performance are small. At the same time, much larger apertures can reasonably be contemplated for ground-based than for satellite-borne receivers. Thus, a direct spacecraft-to-earth link is the obvious choice for a microwave system.
Suitable components for such a system have been under progressive development and only modest advancement in the state of the art should be required to permit information rates on the order of 106 bits per sec. Note that for the extrapolated antenna gains considered, a 10-W space vehicle transmitter would be sufficient. Indeed, the penalty paid for a microwave system is in the size of transmitting and receiving antennas needed to achieve the desired performance at moderate transmitted-power levels.
Direct optical link
To provide an essentially continuous direct optical link between a spacecraft and earth, multiple ground stations evidently would be necessary (i.e., several times the number needed to provide the requisite angular coverage). The number of additional stations required will depend on the statistical weather conditions at the specific sites available. The availability of enough good sites could, in principle, result in an over-all cost saving because of the smaller installation cost of an optical receiver.
Coherent 10-μ system. For coherent reception, the CO2 laser wavelength at 10.6 μ lies very near the performance peak of Fig. 4. Coupled with the high efficiency of the CO2 laser, this wavelength is an obvious choice for a coherent optical system. It is evident at this wavelength that multi-element apertures may be required in very high data rate systems. However, at 10.6 μ the (S/N)B product for each element is more than sufficient to phase-lock the local oscillator to follow the low-frequency signal-phase distortions imparted by the atmosphere. The elementary signals then can be correlated at the heterodyne difference frequency to provide a useful signal-to-noise ratio at the information bandwidth.
A total effective aperture diameter of about 2 meters, consisting of say 33 elements, would equal the performance of the 3-Gc system considered (100-meter receiving antenna diameter). The number of elements could be reduced somewhat by using larger, but less efficient, elemental apertures. However, implementation of such a system would not be trivial and substantial increases in other parameter values would need to be considered for high data-rate links.
Noncoherent 10.6-μ system. Noncoherent detection of 10.6 μ is thermal-noise limited. The sensitivity of the detector in this mode is about three orders of magnitude less than in the coherent mode. (Because the limiting noise is independent of the signal, the actual degradation factor depends on the square root of the signal-to-noise ratio, which is taken here to be 10.) In as much as atmospheric phase correlation lengths, do not restrict the elemental aperture area, a single large “photon bucket” may be used. But practical fabrication tolerances and detector dimensions will set an upper limit on noncoherent aperture diameters.
If a σ/D ratio comparable to that postulated for the 100-meter microwave antenna could be achieved for an optical reflector, diameters as large as 50 meters, sufficient to match the performance of the microwave link, might be feasible. (Because the limiting noise level is determined to a large extent by stray capacitance at the detector output, division of the aperture into elements with separate detectors could degrade performance severely.) In spite of the reduced complexity, the much greater aperture area required for noncoherent as opposed to coherent reception at 10. 6 μ makes this system unattractive.
Noncoherent 0.5-μ system. The communication link performance indicated in Fig. 4 discourages wide-band coherent detection in the visible region of the spectrum for a ground-based system in view of the short-phase correlation distances.
However, the situation is somewhat improved for noncoherent detection. For a system operating at approximately 0.5 μ a photomultiplier detector can provide essentially noise-free quantum detection. As compared with coherent detection at 10.6 μ, detection sensitivity will be down by a factor of two due to the higher quantum noise limit for noncoherent detection; another factor of about 2.5 is needed to account for the lower quantum efficiency of the photoemissive detector surface; a further loss factor of 20 is due to the increase in quantum noise level with frequency.
Sky background illumination will cause additional degradation depending on receiver field of view and aperture size. For an optimized system this might be as low as 3 dB. The total sensitivity indicated is about two orders of magnitude worse than that of a coherent 10.6-μ receiver but an order of magnitude better than a noncoherent 10.6-μ system.
However, a 0.5-μ system suffers from a major disadvantage: present lasers in this region of the spectrum are only 0.1 percent efficient. Barring discovery of a new laser material, over-all system performance would be over an order of magnitude below the low level anticipated for a noncoherent CO2 (10.6-μ) system. While direct solar pumping of the laser might give a small improvement in over-all efficiency, the improvement would not overcome the basic deficiency of the laser.
Higher laser efficiencies are available in the near and intermediate infrared at 0.84 and 2.1 μ. But over-all system performance at these wavelengths is currently lower than at 0.5 percent when refrigeration requirements and poorer detection sensitivities are taken into account.