THE EFFECTS OF A SATELLITE POWER SYSTEM ON GROUND-BASED ASTRONOMY A. R. Thompson National Radio Astronomy Observatory*, P. 0. Box 0, Socorro, NM 87801 Bands allocated to radio astronomy occur at roughly octave intervals across the radio spectrum. Threshold levels for harmful interference in these bands are specified in CCIR Report Number 224-4, and are based upon observations with a single antenna which are the most sensitive observations to radio interference. They also assume reception of an unwanted signal in sidelobes of 0 dBi gain. These levels for the principal radio astronomy bands are shown in Figure 1. In considering the effects of the SPS on radio astrononjy the following components of the SPS spectrum must be included. The Power Signal The typical level at^an observatory not nearer than 100 km to a rectenna is estimated to be 0.01 Wm-2, increasing by 5 dB if phase lock is lost at a satellite. This level, received in 0 dBi sidelobes of radio telescope, produces a signal of 10-5W which is 35 dB above the overload threshold for a typical parametric amplifier, or 10 dB above the overload threshold for an FET amplifier. At present most radio astrononjy receivers contain little or no filtering between the antenna and the first amplifier stage, to avoid the noise resulting from loss in ambient temperature filters. Cryogenically cooled filters, when developed, will prevent overload from the SPS power signal with little loss in sensitivity for most bands other than 2.69-2.70 GHz and 4.99-5.0 GHz. In these last two bands, which are close to the power signal and its second harmonic, impaired performance is likely to result. Harmonics of the Power Signal Of several harmonics that fall close to radio astronomy bands the second presents a serious problem since it is likely to cause overloading when antennas are pointed close to the satellites. Transmitter-Generated Noise Bands of noise generated by the power transmitting system will be centered on 2.45 GHz and low-order harmonics. For klystrons, Arndt and Leopold (1978) estimate that the noise should be less than the CCIR- 224-4 level in the 2.69-2.70 GHz band. Crossed-field tubes and transistors are also being considered as power generating devices and may have significantly different noise properties. Thermal Noise From The Collector Arrays The collector array on each satellite subtends a maximum solid angle of 0.5 sq arcmin at the earth and operates at a temperature of approximately 360 K. The thermal emission from 60 satellites, assuming unit emissivity is shown in Figure 1 for an observer's local midnight when the 60 satellites appear most nearly broadside-on. The actual emissivity of the cell arrays is not known, but probably results in a flux density level 3 to 10 dB lower than shown in Figure 1, i.e., very close to the CCIR 224-4 levels. Thus, for pointing angles closer than the separation of the 0 dBi sidelobe level from the main beam of the radio astronomy antenna, interference can occur. Intermodulation Products Intermodulation products are to be expected from interaction of the power signal in nonlinear elements such as corroded joints in towers and fences, receiving systems and possibly the ionosphere. Widely distributed signals such as television broadcast signals are most likely to be *The National Radio Astronomy Observatory is operated by Associated Universities, Incorporated, under contract with the National Science Foundation.
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