A image-motion compensation system of high precision space camera
Yang Pei Jun
Beijing Institute of Space Machine and Electricity,
Beijing, China
Abstract
This paper describes an image-motion compensation system of high precision space camera. It discusses the automatic lead-in method of the velocity/altitude ratio of the system, the mechanism of the image-motion compensation, as well as the design though of the control of the image-motion compensation. Finally, it briefly discusses the compensation accuracy of the image-motion compensation system. We applied the high precision image-motion compensation system to the visible bands of remote sensing satellite of china, and got satisfactory results.
Introduction
In the designing process of an aerial or space camera, one of the important problem worth of consideration is the displacement of the image points on the film which is a result of the relative motion between the earth and the camera's photographic platform such as airplane or satellite, The displacement makes the film unclear and defeats the image quality of cameras. As we know, a high quality space camera must be equipped with a high resolution photo-lens However merely depending on it, we couldn't get satisfactory imaging quality without an image-motion compensation system to match the lens.
In the early years, optical image-motion compensation technique was used in space camera of Chin. A pair of revolving optical wedges were applied t deflect the imaging beam and to reduce the motion of the image points on the film due to the relative motion. But this technique suits only to space cameral with lower resolution. There exist inherent defects for the optical image motion compensation system, such as the unevenly distributed compensation on the whole image plane etc, which constrains its application in high-resolution space camera. Today, we used a low cost and high precision automatic image-motion compensation system introduced in this paper. It coordinated with a high-resolution photo-lens and a control technique of a perfect optimum exposure, and obtained a great deal of high quality pictures of the ground objects.
Image-Motion and its speed
Here are given the speed (v) and the amount (
d) of the image-motion for the vertical frame space camera at the exposure:
V = fW / H-------------(1)
d = fWt / H-----------(2)
where,
f----the focal length of thecamera;
W ----satellite velocity;
H ---the height of satellite;
t----the exposure time of the camera.
If the focal length f of a camera is 1 m, the exposure time t is 1/300 sec., the height of the satellite H is 200 km, the speed of the satellite is 8 km per second, then the amount of the image-motions 0.14 mm image - motion would make the pictures unclear and would cause a very bad photographic resolution in spite of the high resolution 0of the camera photo-lens. Therefore, it is very important to use the image-motion compensation technique in space photography, the higher the photographic resolution of a camera, the less the amount of the image-motion is permitted during exposure, and the higher the image-motion compensation precision of the camera is required. It is evident from formula (1) that he focal length of the camera is ka constant. Thus, from the ratio of the speed to the height of the satellite, usually called velocity/altitude ratio (W/H), we can calculate the speeds of the image -motion, and in turn we use it to compensate for the image-motion. The space camera uses the satellite as a photographic platform, so the real-time velocity/altitude (W/H) of the satellite is the prerequisite for a starting point of designing automatic image-motion compensation system of the camera.
