Non-Rotationally Symmetric Field Mapping for Back-Scanned Step/Stare Imaging System

Abstract

Step/stare imaging with focal plane arrays (FPAs) has become the main approach to achieve wide area coverage and high resolution imaging for long range targets. A fast steering mirror (FSM) is usually utilized to provide back-scanned motion to compensate for the image motion. However, the traditional optical design can just hold one field point relatively stable, typically the central or on-axis field point, on the FPA during back-scanning; all other field points may wander during the exposure due to imaging distortion characteristics of the optical system, which reduces the signal to noise ratio (SNR) of the target. Aiming toward this problem, this paper proposes a non-rotationally symmetric field mapping method for the back-scanned step/stare imaging system, which can make all field points stable on the FPA during back-scanning. First of all, the mathematical model of non-rotationally symmetric field mapping between object space and image space is established. Then, a back-scanned step/stare imaging system based on the model is designed, in which this non-rotationally symmetric mapping can be implemented with an afocal telescope including freeform lenses. Freeform lenses can produce an anamorphic aberration to adjust distortion characteristics of the optical system to control image wander on an FPA. Furthermore, the simulations verify the effectiveness of the method.

Conceptual Design and Image Motion Compensation Rate Analysis of Two-Axis Fast Steering Mirror for Dynamic Scan and Stare Imaging System

“…Based on area array detectors, the imaging FOV can be expanded by scan motion of the gimbal. The scanning stops at the moment of exposure [2][3][4][5][6]. Reconnaissance and surveillance can be realized at the same time.…”

“…Large rotational inertia of the gimbal leads to a long settling time of start/stop motion, which results in low step frame frequency, low scanning efficiency and limited expansion of the imaging FOV [2][3][4][5][6].…”

“…Continuous scan motion of the gimbal enlarges the FOV. FSM is used to compensate the image motion caused by scan motion during the exposure [6][7][8][9]. Wide-area reconnaissance and target surveillance can be realized at the same time.…”

“…The structure of FSM, the compensation algorithm and the control method are complicated [6][7][8][9].…”

“…In the surveillance/tracking mode, the camera can obtain the target image and trajectory in real time, which requires the aerial reconnaissance camera to have the characteristics of a high frame frequency and positioning accuracy [ 1 , 2 , 3 , 4 , 5 ]. There are four common aerial camera imaging modes: staring imaging, linear array scanning imaging, step and stare imaging, and dynamic scan and stare imaging [ 6 , 7 , 8 , 9 , 10 , 11 ]. The advantages and disadvantages of these imaging methods are shown in Table 1 .…”