Platform motion disturbance filtering for strapped-down electronically scanned array seekers with disturbance observer

Strapped-down electronically scanned radio frequency seekers are currently being introduced as alternatives to mechanically driven gimballed seekers. As the sensing frame of strapped-down seekers does not have any mechanical isolation from the missile body, the guidance signal extracted from such devices tends to get corrupted by a non-negligible fraction of the missile platform motion despite subtraction of the body motion using rate gyros and decoupling loops. This conventional signal extraction scheme is described, and the motivation for the decoupling loop is established. It is shown that for practical system with finite gains, some residual body rate component remains in the guidance signal. In this contribution, a disturbance observer (DOB)-based scheme has been described which can reduce such body motion corruption to a negligible amount. This scheme is capable of completely eliminating the corrupting body motion component in the nominal condition but does not require additional hardware and uses only a reasonable signal processing load. The DOB is used here in an unconventional way, where the guidance signal is reconstructed as if it is a 'disturbance.' Quantitative aspects of the proposed signal processing scheme are analyzed by deriving sensitivity expressions for the proposed scheme with respect to parameter perturbations. Validity of the theoretical results of the sensitivity studies is demonstrated and compared with a case study using simulation. The efficacy of the DOB-based filter is tabulated for different frequencies of platform motion. For a few typical cases, the advantage of the proposed scheme over the conventional is also tabulated. Effective beam shifter digitization noise mitigation is assumed for all the cases studied.