| High resolution earth observation is one of the key technologies in the space optical remote sensing. In remote sensing, ground sample distance (GSD) is the spacing of areas represented by each pixel in a digital photo of the ground from air or space. The purpose of high resolution earth observation is to seek a digital photo with a less GSD. GSD is determined by several factors, such as the pixel dimensions of the camera and the resolution of the optical system. To improve the resolution, we must enlarge the aperture of the system. However, a larger aperture will result in difficulties in aberration correction, fabrication and adjustment. Meanwhile, a larger aperture means more mass, which will increase the cost of launch. Therefore, the developments of high resolution earth observation focus on two aspects:one is to apply image processing to the optical system with a small aperture; the other is to use a reflection optical system combined with modulation transfer function compensation (MTFC).How to estimate the MTF of the system accurately is the most important premise of MTFC. First, we introduce some methods that can be used to measure the MTF based on image analysis, such as pinhole method, slit method and edge method. Particularly, the slanted-edge method has been presented. Then, we analyze the factors, such as sample aperture of the sensor, finite differentiation of edge spread function (ESF), MTF compression in the frequency domain and noise in the image, which can affect the measurement. The corresponding correction methods have also been given. Finally, we propose a new analytical ESF fitting model to measure the MTF. By fitting the ESF data to the analytical expression, the LSF and the MTF can be easily calculated from the ESF fitting coefficients.Second, we consider an optical system as a linear shift-invariant system (LSI), and establish a model to character its degradation and restoration. Some typical restoration algorithms and objective image quality metrics have been presented. Particularly, we introduce a method based on MTF.Third, we analyze the degradation of an optical system with a circular aperture, and derive its optical transfer function (OTF) in both on-axis and off-axis fields. In simulation, four ESF fitting models and the ISO 12233 standard method have been used to estimate the MTF under different levels of noise (SNR=∞,30 and 20 dB). We compare them from both accuracy and complexity. Then we design and fabricate an imaging system, which is composed of a charge-coupled device (CCD) camera and an optical system with a small aperture. A resolution chart image and a remote sensing image have been used as objects, and we compute the MTF based on above five methods. Then, degraded images have been restored by Lucy-Richardson (RL), total variation (TV) and sparse deconvolution (SD), respectively. Finally, we assessed the restoration results by gray mean gradients (GMG), large entropy (LE), Laplacian summation (LS) and modulation transfer function area (MTFA), respectively.At last, we analyze the degradation of an optical system with an obscuration aperture, and derive OTF of the system in both on-axis and off-axis fields. Then we design and fabricate an imaging system, which is composed of a digital camera and an optical system with an obscuration aperture. MTF under different levels of obscuration (10.56%,20% and 30%) have been computed. Then, degraded images have been restored by four restoration algorithms and evaluated by three objective image quality metrics, respectively. Similar processes have been applied to MTF calculation under different levels of contrast (0.27,0.47 and 0.70).
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