10 Design Examples 10.1 Near-Term Earth to Space When designing a program to systematically demonstrate the technologies necessary to achieve a long-term, expensive goal, a variety of different factors must be taken into account. Each step should be achievable with a minimum of new or untested equipment, should be as cheap as possible while still demonstrating the necessary technologies, and should point the way to the next step in the program. To design such a program, two essential questions must first be answered. First of all, what technologies need to be demonstrated? Secondly, where do we start? As has been described in earlier sections, there are several technologies which need to be demonstrated at the present time. Point to point microwave power transmission and reception has already been demonstrated on Earth several times. Solar collection technologies in space have been demonstrated on the tens of kilowatts scale, and early demonstrations of assembly in space will be demonstrated in the next few years with the assembly of Space Station Freedom. Things which have not been demonstrated include microwave power transmission over large distances, microwave transmission at high power levels, reception of microwave power in space, and transmission effects of beaming microwaves through the atmosphere at high power levels. The first steps toward demonstrating several of these essential technologies were taken in 1977 with the lonosphere/Microwave Beam Interaction Study undertaken using the facilities available at the Arecibo Observatory in Puerto Rico. In the 1977 study, however, power was beamed into the ionosphere, not through it. [Duncan, 77] The above considerations, when taken together, suggest a possible demonstration which could be conducted within the next few years. By using the Arecibo facilities or one of the large military phased-array radars to beam power to an orbiting receiver, several technologies essential to the continued progress of the space solar power concept could be tested at minimum expense and on a rapid schedule. Such a test would demonstrate both the atmospheric penetration necessary for space to Earth power beaming and the rectenna technologies necessary for space to space beaming, as well as providing an example of microwave beaming at high power levels. See section 10.3.5 for a detailed discussion of microwave beaming effects on the atmosphere. 10.1.1 Facilities As a first iteration, let's take a look at what could be done with the Arecibo facilities. Arecibo has two large radar systems suitable for transmission demonstrations and a third system which can be used for ionospheric heating experiments but is not really suitable for power transmission. The first of these radars, which transmits at 430 MHz, has a peak power of about 2 MW, a 6% duty cycle, and an antenna with 61.5 db of gain. The second radar, which may be of most interest for power beaming, transmits at a frequency of 2.38 GHz with a power level of 400 kW and a continuous duty cycle. It's antenna has a gain of 71 db. The 2.38 GHz radar at Arecibo is probably the most powerful continuous wave system on Earth at the present time. [Sulzer, 92] The facility itself centers around a dish 300 meters in diameter, again the largest in the world. Due to its large size, and the fact that it takes up an entire small valley as shown in Figure 10.1.1, the dish itself cannot be moved. Pointing is achieved by moving around the transmitting antenna which hangs over the dish suspended by a network of cables. This imposes two effective limitations on the use of the facility. The first of these is that it can point at most 20° from zenith. This is a fairly restrictive limit on the area of the sky which can be covered, but it should be noted that Arecibo itself lies at a lattitude of 18.3°, and so some equatorial orbits could be covered. The transmitters focus at infinity only, but this should not be a significant problem. The major problem with the facility is that it cannot track quickly; it can follow planets but not satellites. All of these numbers would seem to make Arecibo an ideal facility for the type of demonstration envisioned except for the problem with tracking. Figure 10.1.2 shows the tracking rate necessary for satellites at various orbital heights. For a satellite moving at about 8 km/sec in a 1000 km orbit, tracking on the order of 0.57sec is necessary. Two ideas were considered for ways of getting around Arecibo's tracking limitations.
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