Incidence of seven degrees or less between the microwave beam and the earth's magnetic field lines (Ref 8). The theoretical results indicate that a microwave power density of 20-25 mW/cm2 is the threshold for achieving thermal runaway in the D and E regions and thermal self-focusing in the F-region (with the added requirement of 7° or less as the angle between the power beam and the magnetic field lines). Experimentally, a series of tests were conducted at the Arecibo Observatory, Puerto Rico, in June 1977, under the direction of Dr. W. Gordon of Rice University to heat the ionosphere and to look for possible communications effects. Heating frequencies of 10 MHz, 430 MHz, and 2380 MHz were used to simulate SPS heating effects. Using the existing facilities at Arecibo the heat input levels were below the equivalent SPS levels and the heated volume was smaller. For these conditions and employing a diagnostic radar properly located to detect any field-alined striations, no thermal runaways in the D and E regions nor thermal self-focusing instabilities in the F region were observed. In addition no communication effects were detected. A series of tests have been proposed using higher powers to equal or exceed the equivalent SPS heating levels in order to simulate the conditions more closely of the solar power satellite. These experiments could involve communications and dia- nostic tests at several facilities over the next several years, with the objectives of determining the extent of the ionosphere/microwave beam interactions and any effects on communications/navigation systems. The question of phase perturbations induced upon the uplink pilot beam by the heated ionosphere 1s also under study now. Previously, there had been evidence of scintillations produced by a normal ionosphere on microwave signals between geosynchronous satellites and the ground (Ref 11). However, these scintillations usually occur at ground stations in the tropics, in the early evening, and at the times of equinox. For the SPS pilot beam system, the region of interest in the heated ionosphere is a 10 meter diameter spot around boresight. This 10 meter circle is the geometrical area subtended in the Ionosphere by the 1 Km transmit array at the geosynchronous satellite as shown below. The corresponding angle (0) subtended by the 1 Km array is (0) = sin’1 i Km/36,000 Km = .0016° which, for an ionospheric mean height of 360 Km, subtends a 10 meter diameter area. The ionosphere can induce several types f degradations to the uplink pilot beam including differential phase delays (which results in boresight misalinement), random phase jitter (which must be included in ne RMS phase error budget of 10° for the phase control system), and amplitude variations (resultii < in AM-to-PM conversion at the receivers in the tr :.-mit array). In order to determine the maxim differential phase delays permissible by the i mosphere, let us first look at the downlink power bijin. All the beams from the 7220 subarrays will also go through a 10 meter spot in the ionosphere tc reach a point source (one dipole) at the recte ia. For a beam displacement at the rectenna of 5v meters (which would be in addition to the 50 meter displacement produced by the previously discussed 2° mean phase buildup specification on the transmit antenna), the corresponding differential phase delay, 8, in electrical degrees is 8 = 360° (X/.1225 m) where X/10 meters = 50 meters/360 Km or 8 = 4.08° across the 10 meter area. On the uplink beam, if there is a corresponding 4° differential delay through the 10 meter area, then a distance, D, where D - (36,000 Km) (50/360 Km) = 5 Km. Thus the uplink boresight beam will be shifted 5 Km from the transmit array as shown in the diagram. The 10 meter spot in the ionosphere that is actually incident upon the satellite's transmit array is shifted from boresight by (360 Km ”| AD = [36,000 KnJ 5 Km = 50 meters in the ionosphere. The allowable differential phase delay (on both uplink and downlink signals) through a 10 meter diameter area in the heated ionosphere is 4°. Preliminary studies indicate the actual delay may be smaller by several orders of magnitude (Ref 12). The pilot beam/ionosphere differential delay relationships may be summarized as follows: The uplink pilot signal and the downlink power beams from each of the 7220 subarrays to a single dipole element at the rectenna will intersect a 10 meter diameter area in the ionosphere. For a differential phase delay of 4° across this 10 meter area, the downlink beam will be shifted by 50 meters (which has been taken as the alinement requirement for each source of error). The spatial portion of uplink pilot signal that intersects the transmit antenna will be shifted 50 meters from true boresight. This shift will introduce an additional 50 meter alinement error in the retrodirective phase control system. (The fact that the uplink pilot and downlink power beam do not transverse the same paths through the ionosphere does not present a problem. After the retrodirective beam leaves the satellite antenna, the required areas of coherence are limited to 10 meter diameter sections in the ionosphere.) The ionosphere puts an additional constraint on the uplink pilot beam signal. The present pilot beam system has double-sideband, suppressed carrier modulation which is symmetrical about the downlink power beam frequency, fDL, as shown below:
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