| Aerial remote sensing technology is a new subject developed in the 20th century.It has been widely used in many fields such as resource exploration,topographical surveys and military reconnaissance.As a representative aerial photoelectric instrument,the aerial panoramic camera usually uses Time Delayed Integration Charge-Coupled Devices(TDI CCD)as the imaging detector,and it sweeps perpendicularly to flight track to acquire ground scene information.In order to obtain the high quality aerial remote sensing images,it is required that TDI CCD charge shifting row by row in the same direction and the same velocity as the sweep image motion.Therefore,the mismatch error between the directions of TDI charge shifting and the sweep image motion should be minimized by the calibration of the TDI direction for focal plane assembly during the assembly procedure of an aerial panoramic camera.And it is of great significance to the improvement of imaging quality of the aerial panoramic camera.In this paper,the calibration method of TDI direction for focal plane assembly in aerial panoramic camera is studied.It mainly includes the following aspects:According to the working principle of TDI CCD,a mathematical model of velocity mismatch between the TDI charge shifting and the sweep image motion is established.Then the expression of mismatch smear modulation transfer function(MTF)is derived.And the parameters(velocity ratio or direction mismatch angle,phase and stage)that influence the MTF is analyzed in the case of only velocity mismatch and only direction mismatch respectively.It is pointed out that velocity mismatch affects the imaging quality of TDI CCD in TDI direction,and direction mismatch affects the imaging quality of TDI CCD along the row direction.Then the direction mismatch smear imaging simulation is carried out.It is pointed out that the the TDI direction calibration for focal plane assembly is quite necessary.The mathematical model of aerial panoramic camera imaging is established by coordinate transformation method,and the formula for calculating the included angle between the direction of TDI and sweep image motion for different pixel positions on TDI CCD is derived.The variation of the included angle at different pixel position is analyzed by given examples.It is shown that mechanical interleaving butting focal plane assembly is not recommended for short focal length aerial panoramic camera,and it is considered that the TDI charge shifting and the sweep image motion are in the same direction in the ideal assembling status.The included angle between the direction of TDI and sweep image motion,named direction mismatch angle,is calculated in the actual assembling status,and the direction mismatch angle is exactly the sum of angular deviations of the scan head and the focal plane assembly relative to the ideal assembling status.The direction mismatch angle is defined as the calibration factor of TDI direction for focal plane assembly.By measuring and eliminating the direction mismatch angle,the calibration of the TDI direction for focal plane assembly is realized.According to the measuring principle of theodolite,the principle of calibrating the TDI direction for focal plane assembly by theodolite is analyzed,and a calibration method of TDI direction for focal plane assembly based on the theodolite is proposed.The concrete operation steps of this method are given,but the method has the disadvantages of uneven illumination,difficult fine-tuning of scan head and low efficiency of alignment.Then the factors that affect the calibration accuracy of the method are analyzed.The error expressions of TDI direction calibration for the optical butting focal plane assembly and the mechanical interleaving butting focal plane assembly are derived respectively.The calculation results show that the method can be used for short focal length aerial panoramic camera,and the calibration of TDI direction for the focal plane assembly meets the operating requirements.However,for the long focal length aerial panoramic camera,the calibration accuracy is low due to the difficulty of fine adjustment of the scan head(large volume and weight),and it is unable to meet the requirements of long-focus aerial panoramic camera.Combined with the imaging mathematical model of aerial panoramic camera,the trajectory of the image point varying with the scan mirror on the photosurface of TDI CCD is deduced as a hyperbolic curve,when the TDI CCD operates in the area mode.By analyzing the hyperbolic equation,a method for measuring the direction mismatch angle of optical butting focal plane assembly and another method for calculating the direction mismatch angle of mechanical interleaving butting focal plane assembly are presented.And then a calibration method of TDI direction for focal plane assembly based on TDI CCD area mode is proposed.After measuring the direction mismatch angle,a precise adjusting mechanism is used to control the rotation angle of the focal plane assembly to complete the calibration of TDI direction.The operation steps of the method are given,and the calibration method based on TDI CCD area mode has the advantages of convenient operation and adjustment,no repetition,high alignment efficiency and high calibration precision,but there is a high requirement for the calibration environment for this method.Then the factors that affect the measure accuracy of the direction mismatch angle are analyzed.and the measurement accuracy of the direction mismatch angle measurement methods for the two types focal plane assembly is simulated by Matlab software.The calculation results show that this method has high calibration accuracy,and it can be applied to the calibration of TDI direction for long focal length aerial remote sensor,which can meet the application requirements.Finally,the experimental verification is carried out.Firstly,the validity of the TDI direction calibration method of short focal length aerial panoramic camera based on theodolite is verified.And the TDI direction for an optical butting focal plane assembly in a long focal length aerial panoramic camera is calibrated by theodolite.Then the calibration accuracy of the calibration method based on theodolite is verified by using the calibration method based on TDI CCD area mode,and the calibration of TDI direction for the optical butting focal plane assembly in the aerial panoramic camera is completed by using the method based on TDI CCD area mode.By comparing the calibration results and the calibration time of the two calibration methods,it can be concluded that the calibration error is reduced at least from 9’59.5"to 19.5" by using the calibration method based on TDI CCD area mode,and the efficiency of alignment is improved 60 percent.A mechanical interleaving butting focal plane test piece is designed.The long focal length aerial panoramic camera is used to verify the calibration method of TDI direction for mechanical interleaving butting focal plane assembly.After the calibration is completed,the calibration error is 11.8"± 24.6"(3σ),which is significantly higher than the computed result of the calibration error using the theodolite method.The long focal length aerial panoramic camera has been carried out the laboratory dynamic imaging test and flight test,which meets the requirements of the designed performance parameters,It confirms that the proposed method in this paper is feasible and effective. |
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