Classification of space photography

There are many classification methods of space photography, which can be divided into panchromatic, color, multispectral, panchromatic infrared and color infrared photography according to the spectral effect of photosensitive materials. According to the direction of the main optical axis of the camera, it can be divided into vertical photography, oblique photography and cross photography. According to the imaging structure of the sensor, it can be divided into frame scanning, panoramic scanning, line scanning and array scanning photography. In addition, according to the recording form of the image, it can also be divided into photographic film, analog tape and high-density digital tape photography. Among them, the common method is to classify according to the imaging structure of the sensor.

Frame photography is to make the light in the object space pass through the whole field of view of the objective lens at the same time in flight, and instantly image on the whole focal plane of the camera according to the principle of central projection. Its characteristics are that the orientation elements in the camera are known, the image quality is good, the geometric relationship is simple, and it is easy to measure and process. Panoramic scanning photography belongs to dynamic scanning photography, which realizes panoramic scanning of ground scenery by rotating prism or objective lens or rotating optical rod. The aperture direction of panorama camera focal plane shutter is parallel to the flight direction, and the scanning direction is perpendicular to the flight direction. When the ground scene is scanned in flight, the object light passes through the objective lens and then passes through the focal plane gap, and is imaged on the continuously moving photosensitive film which is synchronous with the prism scanning but in the opposite direction. This panoramic photo is a central projection of the flight direction and a cylindrical projection of the scanning direction. The principle and imaging geometry of the panoramic scanning camera are shown in figs. 2a and 2b. S in fig. 2b is the projection center of the objective lens; F is the principal distance of the panoramic photographer; α is the half scanning angle; β is the deflection angle of the image point; S-O ξ η is the image coordinate system of panoramic photos; S-oxy is the image coordinate system of equivalent frame photos. Panoramic scanning has the advantages of large coverage, high resolution and rich ground information, and is suitable for reconnaissance and interpretation. Disadvantages are panoramic distortion and low measurement accuracy.

The information acquisition system of line scanning photography is shown in Figure 3. The characteristic of this instrument is that it can detect targets in multiple bands at the same time. The instrument is equipped with a scanning mirror, and the scanning direction is perpendicular to the flight direction. The incident light is sent to the detector through a mirror and an optical system and converted into an electrical signal. Its information is mapped after rough machining and color synthesis. Array scanning photography consists of charge-coupled devices arranged together at a very high density and imaged on the focal plane of a linear (or planar) optical system (Figure 4). This array scanner is also called solid-state scanner. Because the image transmission of CCD is very fast, the scanning time of each line is very short, and the actual image motion is very small, so there is no need for image motion compensation. The array scanner has no moving optical and mechanical parts and does not use electron beams, so its reliability and accuracy are better. The linear scanning sensor will be used for the recently launched French SPOT satellite and American MAPSAT satellite.