Optical Correlator for Image Motion Compensation in the Focal Plane of a Satellite Camera

Transcription

1 15 th IFAC Symposium on Automatic Control in Aerospace Bologna, September 6, 2001 Optical Correlator for Image Motion Compensation in the Focal Plane of a Satellite Camera K. Janschek, V. Tchernykh, - TU Dresden, Germany 1. Problem Description 2. State of the Art 3. Proposed Solution 4. Image Motion Detection 5. Image Motion Compensation 6. Expected Performances 7. Conclusions IFAC Symposium

2 Problem Description Image Motion in the Focal Plane of Satellite Camera Short exposure Image motion due to: orbital motion of satellite attitude control errors Long exposure Low SNR Motion blur IFAC Symposium

3 State of the Art Time Delayed Integration (TDI) Principle: The charges, accumulated by photosensitive cells of imaging device, are shifted electronically according to the motion of projected image Advantage Longer exposure possible - SNR improvement pixels Image shift Charges shift Limitations One dimensional discrete image motion compensation - residual image blurring Limited number of TDI steps - SNR improvement limited Larger pixels of image sensor - larger optical system IFAC Symposium

4 Proposed Solution Mechanical Image Motion Compensation with Visual Feedback Movable Focal Plane Platform Image motion due to attitude instability and Earth rotation Image Sensor Image Data Image motion due to orbital motion Image Motion Detector Controller Piezoelectric Actuators Advantages 2D stepless motion compensation - no residual image blurring Longer exposures possible - better signal to noise ratio Smaller pixels of image sensor - compact optical system IFAC Symposium

5 Image Motion Detection Principle of the Image Motion Detection Requirements Solutions Low integration time Subpixel accuracy Low SNR 2D correlation of sequential images from matrix sensor Optical Correlator Low processing time IFAC Symposium

6 Image Motion Detection Image Motion Detection by 2D Correlation X(t 1 ) - X(t 0 ) Image sensor Sampling interval - t RAM module Image (t 1 ) t 1 - t 0 = t Image (t 0 ) Optical Correlator Y(t 1 ) - Y(t 0 ) time delay - t IFAC Symposium

7 Image Motion Detection Joint Transform Optical Correlator Current Image Fourier Lens OFP1 Image Sensor Image shift x Reference Image Laser Diodes Collimators Spatial Light Modulators Spectrum Image Spectrum Image Correlation Image OFP2 Image Sensor 2 x Optical correlator performance compared to the Digital Signal Processor Correlations / sec Standard Components 200 Special Components 500 Digital Processing Unit x 0 DSP Optical Correlator IFAC Symposium

8 Image Motion Detection Motion Detection Performances with Optical Correlator Image shift determination error - σ < 0.2 pixel Errors of shift determination, pixels Acceptable SNR of the input images - 0 db Sampling frequency Hz Response time - within 10 ms SNR, db IFAC Symposium

9 Image Motion Compensation Image Motion Compensation with Optical Correlator Movable Focal Plane Platform Image motion due to orbital motion Image motion due to attitude instability and Earth rotation Image Sensor Image Data Image Motion Sensor Opt. Correlator Image shift X, Y Controller Piezoelectric Actuator Position Sensor Platform position X, Y IFAC Symposium

10 Image Motion Compensation Laboratory Models of System Elements Laboratory Model of Movable Focal Plane Platform Image Sensor Linear Stage with Piezo Drives Laboratory Model of Optical Correlator Funded by European Space Agency IFAC Symposium

11 Control Loop Structure Image Motion Compensation Imaging αv sat δ V 0 Image Motion Controller Platform controller Piezo platform V pl V image Reference Velocity Generator V image V pl Platform sensor Delay T S Optical Correlator 10 ms 5 ms V sat - ground track velocity of the satellite α - scaling factor of the imaging system V pl - velocity of the platform with respect to satellite frame V image - velocity of the image with respect to the platform δ V - variations of the image velocity due to attitude instability, planet rotation, etc. IFAC Symposium

12 Image Motion Compensation Velocity Profile Velocity, mm/s 17 Image disturbance velocity Pre-set platform velocity Time, ms Actual platform velocity Velocity Acquisition Pre-setting Imaging Re-position Pre-setting Continuous Imaging Disturbance velocity Actual platform velocity Pre-set platform velocity IFAC Symposium

13 Expected Performances Expected Performances (Image Motion Compensation) Reference mission: orbit altitude 600 km; ground resolution 2 m per pixel Residual error of image motion compensation: within 0.5 pixels Acceptable angular velocity of the satellite: 0.3 degrees/s Acceptable angular acceleration of the satellite: 0.5 degrees/s 2 Initial synchronisation interval: 30 ms Platform travel distance: ± 2 mm Platform velocity: up to ± 25 mm/s Imaging interval: 50 ms IFAC Symposium

14 End-to-End Imaging Performance Simulated Satellite Images Expected Performances No image motion compensation Exposure time ms SNR - 6 db Electronic TDI (64 steps) Exposure time - 19 ms Residual shift - 2 pixel Mechanical motion compensation Exposure time - 50 ms Residual shift pixels Orbit altitude km Ground resolution - 2 m / pixel Residual angular velocity (w.r.t. nadir) º/s IFAC Symposium

15 Conclusions A new concept of a system for compensation of the image motion in the focal plane of a satellite camera has been proposed. The system includes an image motion sensor and optical correlator for precision measurement of the motion of dark and fast moving image. Implementation of proposed system allows to: increase the quality of the obtained images reduce the requirements to the attitude stability of the satellite reduce the optical system dimensions IFAC Symposium