The massive size of the satellite system and its component parts greatly reduces the number of vulnerable critical components. Three areas of concern are the sensors, the primary power antenna, and the solar cell arrays. The power satellites are relatively invulnerable to a ground-based laser threat but could be vulnerable to a space-based system because of decreased laser-to-target range and the absence of atmospheric absorption. Sensors. Sensors (in particular, the star and Sun sensors of the attitude control system) are vulnerable to laser attack. However, as these sensors constitute a negligible fraction of the system weight, volume and cost, they can be hardened by prudent design using a variety of passive and active countermeasure schemes to withstand the anticipated threats. Sensor vulnerability is therefore considered an important item which must be evaluated in detail with specific design specifications, but also an item of small system impact. Primary Power Antenna. The primary power transmitting antenna is a critical component which is easily identifiable as an aimpoint for laser attack. The large size of the antenna, the distributed nature of its power module/klystron transmitters, and the shielding effect of the radiating face, all combine to increase the antenna's survivability. Assuming some redundancy in the power distribution network across the antenna, the large size and distributed nature of the antenna indicate that damage would be localized to the laser-irradiated region. Therefore, significant system degradation would require a large irradiated area. This increases the energy requirements of the incident laser beam. The power module design is such that damage would probably require melting of the aluminum face. The conditions under which melting could occur can be approximated as follows. Neglecting the internal heat source of the module as small compared with the minimum laser irradiance required for melting (it can be shown to be less than 10%) , a simple equilibrium energy balance can be written for an antenna section as where A is the irradiated area, E is the absorbed laser irradiance, e is the a emissivity for thermal radiation, a is the Stefan-Boltzmann constant, and T is the surface temperature in degrees Kelvin. The above assumes a radiation loss surface
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