the SPS traveling in the same direction of the SPS will have a velocity relative to the beam at the point of crossing of 1.16 km/sec (and 5.85 km/sec for a retrograde orbit). From the position of the SPS, these relative velocities represent angular rates of 0.1 16 and 0.585 mrad/sec, respectively. To follow a target of the lower angular rate a beam would have to be rotated about its origin at a rate of 360° in 895 minutes (0.4 degrees per minute), and the higher rate 2.0 degrees per minute. In orbits 1000 km below the SPS, these rates would be 0.36 to 20.7 degrees per minute. The angular rates of maneuvering targets or satellites in retrograde orbits at close range generally would be expected to exceed the track capabilities that could be designed into the MPTS. Targets traveling toward the SPS could be followed at much slower slew rates except at close range when a miss or ”fly-by” is involved (see figure 3.1). The effectiveness of an energybeaming weapon depends on the power in the beam, the beam intensity versus time profile, the angular cross-section of the beam, required beam acceleration and slew rates, and the accuracy to which a target can be tracked and the beam pointed. The tracking/pointing error of a system against a moving target (high information rate required) will usually be significantly greater than the error of that same system against a stationary or nearly stationary target (under mechanically damped low information rate conditions). 3.1.1.2.1 Microwave System The accuracy to which the microwave beam of the reference design can be pointed toward the rectenna site (table 2.1) is considered adequate for a ’’weapon mode" if this accuracy could be maintained during target track. The antenna, a low-density structure, is large and massive but probably could be used to track non maneuvering orbital targets at rates of one or so mrad/sec, provided target detection and/or designation is completed early enough to allow the antenna to be brought on target moving at the appropriate rate without exceeding acceleration limits as established by antenna structural characteristics and figure requirements. Figure tolerances may be as stringent as one tenth the wavelength (+0.1 A = + 1.22 cm) for a frequency of 2.45 GHz. From figure 2.1, it is only after a target has closed to within 12,000 km that irradiance on target is greater than one solar constant. Therefore, use of the microwave system in geostationary orbit (GEO) against earth and near-earth targets does not seem practical. However, at ranges of 4,000 km or less, the rate of temperature rise for highly absorptive targets being irradiated could be significant, causing a damaging heat buildup.
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