GISdevelopment.net ---> AARS ---> ACRS 1990 ---> Poster Session

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.

The leading-in of the velocity/altitude (w/h)
There are many ways to lead in the velocity/altitude ratio (W/H) many velocity/altitude meters are made according to various principles. Now we introduce a leading - in method, which is used in the remoting sensing satellite camera in our country.

In the satellite, there is always a set of satellite attitude control system of high precision. The system measures the important orbit parameters, which include the speed and height of the satellite. So long as the ratio of speed to height of the satellite is computed in real-time, the speed of the image-motion will be calculated by formula (1). This is done by the computer in the satellite.

A high accuracy of the speed of the satellite measured by the satellite attitude control system is required, however, under the influence of the earth topography fluctuation, the measuring accuracy of the flying height is relatively low. Thus it limits the given precision of the velocity/altitude (W/H.) Furthermore the component of the earth rotation speed in the direction of the satellite flying will also reduce the leading -in precision of the velocity/altitude (W/H)

Therefore the height and the speed need to be adjusted as follows:
  1. To adjust the satellite height by the elevation of the ground objects.

  2. To adjust the satellite speed by the component of the earth rotation speed in the direction of the satellite flying.
However, to achieve item (a), it is necessary to raise the precision of the given elevation precision of the ground objects, which is very difficult. Thus we introduce the concept of average elevation. We divided the photographic region into several smaller blocks and computed every average elevation, which is used to adjust the satellite flying height. Obviously, the smaller the block is, the higher the leading -in precision of the velocity/altitude (W/H).

The lading -in method of the velocity/altitude (W/H) was implemented by using the satellite orbit parameters (i.e.) the flying speed W and the flying height H) for real time calculation, and by using the average elevation of the photographic block to adjust the flying speed, and by using the component of the earth rotation in the direction of the flight to adjust the flying speed. Such a scheme was applied in the Chinese high-resolution space camera. Its precision is 1.5% which can match with the high quality photo-lens with static resolution 100 pl/mm. Unlike other leading -in and measured instruments such as the velocity/altitude meter, it uses the orbit parameters t calculate the velocity/altitude with high precision, high reliability, and low cost.

The image-motion compensation mechanism and control system
After the velocity/altitude (W/H) is introduced accurately a corresponding image-motion compensation mechanism and control system is needed. There are many ways. Normal methods are listed as follows:
  1. Optical Wedge Rotation Method: The drawback of the method is that the whole image plane compensation is not uniform, and its precision is very low.

  2. Photo-Lens Sway Method: The drawback of the method is that the speed of the image motion is related to the coordinates of the image points. Theoretically, only one image point in a picture (Usually, the main point of the picture) can be compensated completely, and its precision is very low.

  3. Optical Axis Rotation Method: The defects of this method is that image points of different view angle have complete different compensations, so its precision is low.

  4. The Moving Tilm Method: The film is moved at the speed of the image motion in the direction of image plane during exposure. There is no theoretical error, so its precision is high.

  5. The Photo-Lens Moving Method: The film is moved at the speed of the image motion in the opposite direction of the image plane during exposure. There is no theoretical error, It can obtain a satisfactory effect of image-motion compensation.
The first three methods have theoretical errors which affect their precision. The last two methods have no theoretical errors, their precision are high and could be achieved.

In the high-resolution space camera, the moving film method and the moving photo-lens method have been used widely for their high compensation precision. In this paper, we shall describe briefly the image-motion compensation mechanism and control system of the move film method.
  1. The Image-motion Compensation Mechanism of Moving Film Method

    As stated above, the basic starting point of the moving film method is to move the film at the speed of the image-motion in the same direction during expositing to compensate for the relative motion between the image points and the film. There is a film flattening plate at the focal plane of the camera. The flattening method is either negative pressure flattening or clamp plate flattening, and the negative pressure flattening is used more widely. The precision of the negative pressure flattening is higher and the flattening plate doesn't bear high pressure, At the moment of expositing, the film is flattened on the flattening plate, and makes a unified integrity with it. The flattening plate is moved by the electrical motor through transfer mechanism at the speed of the image-, motioning the same direction to compensate the image motion. A control system that controls the rotation speed of the electrical motor, which drive Flattening plate is needed to ensure the flattening plate moving accuracy in order to reach the speed.

  2. The Control System of the Image-motion Compensation

    The purpose of the control system of the image-motion compensation is to drive the motor of the flattening plate at the speed determined by the velocity/altitude (W/H). While the velocity/altitude changes, the speed of the motor also changes. Therefore, the control system of the image motion compensation should be of a high precision, steady speed and speed-control system. The functional diagram of the control system of the image-motion compensation of the camera is shown as follows:


    Fig. 1, the functional diagram of the control system of the image motion compensation

    Besides the leading-in precision of the velocity/altitude (W/H) and the method of the image motion compensation, the speed control precision of the motor of the image-motion compensation will also affect the precision of the image-motion compensation. In order to improve the speed-control precision is less than 0.01%. The satellite flying speed W and height H provided and adjusted by the satellite altitude control system is input to the computer on the satellite. The velocity/altitude (W/H) will be calculated in real time and transmitted to the code-frequency transformer in binary code. The output frequency signal is transmitted to the phase-frequency transformer in binary code. The output frequency signal is transmitted to the phase-locked loop as the rotation signal of the image-motion compensation, and compared with the feedback frequency signal of the tachometer in the phase-locked loop. The output signal of the phase-locked loop will drive the image-motion compensation motor by adjusting the loop and power amplification circuit.

    The transformer will change the binary code given by the computer into the given frequency signal needed by the phase-locked control system according to formula(3)

    F=pnf/D (W/H)---------------------------(3)

    Where
    F---the output frequency of the code-frequency transformer
    f---the focal length of the camera
    p----the number of the magnetic pole couple of the tachometer
    n ---the gar transmission ratio of the driving flattening plate mechanism
    D---the diameter ofthe flattening plate moving camera
Analysis of the precision of image-motion compensation
The precision of the moving film image-motion compensation is related to the leading-in precision of the velocity/altitude (W/H) and the speed-control precision of the flattening plate. The precision of the velocity/altitude may be up to 105%, the speed-control precision of the flattening plate is better than 0.1%, and the total image-motion compensation precisation Precision of the image-motion compensation system is better 2%. Let us ssee the example mentioned before, the flying height is 200 kilometers, the flying speed is 8 km/s, and the exposure time is 1/300 seconds. If there were no image-motion compensation system, the amount of the image motion would be 0.14 mm. After the compensation has been applied, the residual image motion is only 0.0028 mm

Conclusion
The image-motion compensation system described in this paper operated in a way by using the computer on the satellite to calculate in real time the leading-in velocity/altitude according to the satellite orbit parameters. It employs the digital phase-locked control system to drive the film flattening plate at the speed of the image-motion in the same direction to compensate for the image-motion during exposure. Tthe application of this system in remote sensing projects shows that is could completely satisy the needs of the image-motion compensation of the camera with the photographic resolution of 80-100 lp/mm and obtain satisfactory pictures of the ground objects. In a word, it is a high precision image-motion compensation system.

Reference
  • Milligan L. J. and Carnicelle, E, Phaseloockedloops privide accurate, efficient dc motor speed control. EDNN Vol 17, NO 15, pp 32-35 (1972)