Satellite optical link
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Another way to exploit the increased aperture gains at optical wavelengths, while avoiding blackout caused by poor weather conditions, is to use a satellite relay station. The spacecraft-to-satellite link would take advantage of the higher aperture gains available at optical wavelengths while a microwave relay link to earth would circumvent atmospheric losses. Except for a few limited geometries, a single synchronous satellite could provide a continuous link with the spacecraft so that a single satellite receiver and ground station might suffice for the full system.
The cost of a satellite receiver must therefore be weighed against the cost (including maintenance) of three or four large-aperture ground-based microwave receivers, or of a greater number of relatively small-aperture, ground-based optical receivers.
Because of the CO2 laser’s much higher over-all efficiency, 10.6 μ is the only wavelength worth serious consideration at this time for an optical spacecraft-to-satellite link. Fig. 4 shows that performance of a coherent system with one-meter transmitting and receiving apertures is slightly poorer than that of the 3-Gc spacecraft-to-earth microwave link. If effective transmitting and receiving aperture diameters are increased from 1 to 1.2 meters (still within the pointing error limitation assumed), comparable performance is obtained. Except by increased transmitted power, performance can be substantially improved only by an improvement in pointing and tracking accuracy, coupled with either larger aperture dimensions or a higher transmitting frequency.
For comparable performance, the aperture diameter of a noncoherent receiver would be about 30 times that of the coherent receiver and therefore unsuitable for a satellite relay station.
Comparison of systems
Based on a performance goal of 108 bits per second, three specific systems are compared in Table 2. A signal-to-noise ratio of 10 is adequate for an acceptably low error rate with a suitable modulation code. This goal corresponds to a signal-to-noise bandwidth product of 90 dB.
Parameter values are chosen generally within the practical limitations previously described, and with system and spreading losses as discussed before. The increase in performance over present and planned systems as required to meet the goal is distributed among the two aperture areas and the transmitted power. Equivalent noise performance has already been extrapolated close to the fundamental quantum limit or to a reasonable temperature limit. The specific apportionment of these improvement factors is based on estimates of both development and engineering effort and costs required for their achievement.
Two optical systems are shown in the table: a direct communication link between spacecraft and earth, and a link using a satellite relay station. An eventual choice between the two or between microwave and optical links must depend on realistic systems and cost analyses, which will necessarily be influenced by the results of current and future research programs.
Direct microwave link at 3 Gc
A 90-dB information-rate parameter, R0, = (S/N)B, for the direct microwave link operating at a nominal frequency of 3 Gc is achieved by postulating improved performance of receiving antenna, transmitting antenna, and spacecraft transmitter.
The proposed receiver antenna gain of 70 dB requires an aperture equivalent to a circular antenna diameter of 440 ft (55 percent efficiency) at 3 Gc. (At 2.3 Gc the required diameter would increase to about 580 ft.) The difficulties of implementing such an antenna as a single dish lead to consideration of a distributed array. If a 70-percent aperture efficiency is assumed, a rectangular array for effective operation at zenith angles up to 30 deg latitude and 60 deg longitude would have dimensions of about 400 x 700 ft at 3 Gc (or about 500 x 900 ft at 2.3 Gc). These dimensions are within expected atmospheric correlation lengths so that compensation for atmospheric distortions is unnecessary.
Operation is limited to a maximum zenith angle of 60 deg to avoid excessive, inefficient aperture dimensions; however, this would increase the minimum number of receiver sites for continuous angular coverage from three to four. Note that at the reduced zenith angle, atmospheric background noise will be decreased so that an operating frequency closer to 5 Gc may be favored for the microwave link.
A gain of 49 dB for the spacecraft transmitting antenna corresponds to a diameter of about 12 meters. Since weight added to the spacecraft entails a more than proportionate increase in booster weight, a considerably low performance increase normally would be required of the spacecraft antenna. Nevertheless, techniques being developed for the deployment of antennas after the boost phase of flight permit consideration of larger apertures at high performance-to-weight ratios. Thus, an improvement approaching that of the ground antenna is postulated for the spacecraft antenna.
Transmitter power is taken as 1 kW. An increase in transmitter power to this value involves a related increase in spacecraft power supply and heat dissipation capabilities and, therefore, substantially affects over-all vehicle weight and booster requirements. This output power corresponds to a raw power requirement of about 3 kW, which is not incompatible with spacecraft capabilities estimated for 1980.