Airborne protected military satellite communications : analysis of open-loop pointing and closed-loop tracking with noisy platform attitude information

[摘要] U.S. military assets;; increasing need for secure global communications has led to the design and fabrication of airborne satellite communication terminals that operate under protected security protocol. Protected transmission limits the closed-loop tracking options to eliminate pointing error in the open-loop pointing solution. In an airborne environment, aircraft disturbances and noisy attitude information affect the open-loop pointing performance. This thesis analyzes the open-loop pointing and closed-loop tracking performance in the presence of open-loop pointing error and uncertainty in the received signal to assess hardware options relative to performance requirements. Results from the open-loop analysis demonstrate unexplained harmonics at integer frequencies while the aircraft is banked, azimuth and elevation errors independent of the inertial pointing vector and aircraft;;s yaw angle, and uncorrelated azimuth and elevation errors for aircraft pitch and roll angles of +/-10° and +/-30°, respectively. Several conclusions are drawn from the closed-loop tracking analysis. The distribution of the average noise power has a stronger influence than the distribution of the received isotropic power on the signal-to-noise ratio distribution. The defined step-tracking algorithm reduces pointing error in the open-loop pointing solution for a pedestal experiencing aircraft disturbances and random errors from the GPS/INS. The rate of performance improvement as a function of the number of hops is independent of the antenna aperture size and the GPS/INS unit. Pointing performance relative to the HPBW is independent of the antenna aperture size and GPS/INS unit for on-boresight, but not for off-boresight. With signal-to-noise ratios averaged over 100 hops and pointing biases less than or equal to 0.5 the half-power beamwidth, the step-tracking algorithm reduces the pointing error to within 0.1 the half-power beamwidth of the boresight, for all tested configurations. The overall system performance is bounded by the open-loop pointing solution, which is based on hardware selection. Closed-loop tracking performance is a function of the number of sampled hops and is for the most part independent of the hardware selection.

[发布机构] Massachusetts Institute of Technology