The effective number of dipoles per unit volume decreases as the chaff cloud expands with time. Thus the attenuation per unit path length will decrease. (See Equation 1 above.) However, since the cloud size increases with time, the signal path length through the cloud increases and the total signal attenuation will not decrease as rapidly with time as does attenuation per unit path length. A variety of factors such as variations in chaff dipole dimensions and coating, atmospheric conditions, wind, altitude, etc., can all affect chaff cloud growth and transport. These factors, along with the placement of the chaff cloud relative to the pilot beam and the pilot beam elevation angle, will affect the variation of signal attenuation with time and the length of time the pilot beam will intercept the chaff cloud. The effects of these factors are indicated in Table C.4-1, which considers one pod of chaff deployed at 10,000 ft. altitude by a single small airplane flying from south to north through the pilot beam under very light southerly wind conditions. (This is the worst case from the viewpoint of SPS vulnerability, short of chaff deployment at saturation levels by large squadrons of aircraft arriving in relays a few minutes apart. For a rectenna sited within the territorial limits of the United States or offshore within its Air Defense Identification Zones (ADIZ), military response to penetration of the ADIZ would be a massive scramble of fighter interceptors long before the chaff could be dispensed.) The fall rate at sea level for one-mil aluminized-glass chaff will vary from approximately 0.6 to 1.0 fps due to variations in chaff mechanical tolerances. The fall rate increases exponentially with altitude and is approximately twice the fall rate at sea level at about 40,000 ft. Wind speeds also tend to increase exponentially with altitude. The wind model used in Table C.4-1 doubles the wind velocity for each 5000-ft. increase in altitude. The elevation angle for the pilot beam signal path to the SPS in synchronous orbit decreases for transmitter locations at higher latitudes, increasing the path length through the chaff cloud. For the elevation angles of interest, the SPS pilot beam would be most vulnerable under no wind or very light southerly wind conditions. For higher wind velocities, even if from the south, wind transport of the chaff cloud will carry the cloud beyond the signal path before the chaff settles to the ground, as indicated in Part B of Table C.4-1. The chaff threat to the pilot beam is thus very limited in time and would be almost exclusively from a terrorist group. Any aircraft dispensing chaff in the
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