Strapdown Inertial Measurement Units For Motion Compensation For Synthetic Aperture Radars

Strapdown Inertial Measurement Units For Motion Compensation For Synthetic Aperture Radars

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STRAPDOWN INERTIAL MEASUREMENT UNITS FOR MOTION COMPENSATION FOR SYNTHETIC APERTURE RADARS Dr. Thomas A. Kennedy Senior Staff Engineer Hughes Aircraft Company, Radar Systems Group P.O. Box 92426 Los Angeles, CA 90009 ABSTRACT Synthetic Aperture Radar (SAR) motion can be compen- sated by using an antenna mounted strapdown inertial measurement unit (IMU) as the motion sensing system, but sensor and system errors affect SAR image quality. Strap- down IMU errors include gyro and accelerometer scale fac- tor and bias errors, velocity error, platform tilt and errors induced by limited inertial sensor bandwidth. The effects of these-errors on SAR image quality are presented in terms of the SAR impulse response and a motion compensation system conceptualization is described where an antenna- mounted strapdown IMU transfer-aligns to the aircraft’s master navigator. INTRODUCTION High-resolution SAR require antenna motion compensation to maintain the required coherent phase history of the radar return signal [1,2,3]. This motion compensation data must provide knowledge of the precise position of the antenna phase center during the SAR integration interval. If the nonlinear components of antenna phase center position error exceed a small fraction of the carrier wavelength (approx- imately 1.2 inches at X-Band) across the SAR window, the SAR image can be severely degraded. A SAR motion com- pensation system corrects for the antenna motion induced phase distortion by applying inertial sensor derived motion compensation phase correction terms to the radar return signal. An integral part of a SAR motion compensation system is the motion sensing system. Although the need to define SAR imposed requirements for the motion sensing system has been previously addressed in the open literature [4,5], this paper provides a systems approach to specifying the SAR related motion sensing system requirements. This paper reviews motion compensation for SAR and presents the effects of strapdown IMU sensor and system er- rors on SAR image quality. These errors include: * gyro and accelerometer scale factor and bias errors velocity error platform tilt, and errors induced by limited inertial sensor Presented at the IEEE 1988 National Radar Conference 0885-8985/88/1000-0032 $1.00 @ 1988 IEEE bandwidth. The effects of these errors on the SAR image quality are presented in terms of the SAR impulse response. IMU errors that cause low frequency phase errors (less than one cycle per array time) are categorized in terms of quadratic and cubic phase errors. IMU errors that cause high frequency phase errors (greater than one cycle per array time) are categorized in terms of the integrated sidelobe ratio (ISLR) and peak sidelobe ratio (PSLR). Finally, a motion compensation system conceptualization is presented where a strapdown IMU is attached to an antenna and transfer-aligns to the aircraft’s master navigator.
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Strapdown Inertial Measurement Units For Motion Compensation For Synthetic Aperture Radars

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