Objective  Space target detection is the basis of space situation awareness, and space-based space target observation can be independent of the geographical location of ground-based observation stations, with unique advantages. For geosynchronous orbit (GEO) targets, the current space-based system mainly uses optical observation means. In order to improve the detection sensitivity of distant and dim targets, it is usually necessary to increase the aperture of the visible light camera to increase the light collecting capacity. As a result, the weight and volume of the camera will be significantly increased, and the resource occupation and development cost of the satellite platform will be greatly increased. Therefore, it is necessary to study a new camera observation method, which can not only achieve higher sensitivity detection, but also meet the requirements of small and light payload for satellites.

  Methods  In order to meet the requirements of observation GEO target from LEO, the time delay integration (TDI) push-scanning image technology of linear array CCD usually used in the earth observation camera is applied to the area array CCD, and the time delay integration (TDI) imaging mode of the area array camera is designed (Fig.2). According to the orbit characteristics of low-orbit zero inclination satellite, the GEO target speed with different inclination angles is analyzed (Tab.1), and the CCD charge transfer speed is designed to match the target speed. The point target signal is enhanced through charge accumulation, so as to improve the detection sensitivity of the camera. The principle of TDI imaging with area array is described, the calculation model of camera signal to noise ratio is derived, the main imaging parameters such as target motion, integration time, detection sensitivity and signal to noise ratio are calculated, and the influence on point target detection imaging is analyzed. In TDI mode, the east-west relative velocity of the target is compensated to zero (Fig.3), the north-south vertical image movement and the maximum integration time of the typical inclination target are calculated (Tab.2). It can be concluded that the longer the integration time is, the greater the north-south vertical image movement is. The larger the target dip angle is, the greater the north-south velocity component is, and the greater the image movement is. The maximum integration time allowed by the camera is 3.2 s when observing a target at an inclination of 15° under extreme conditions (Tab.2). Finally, the design parameters of the camera are determined. The angular resolution of the camera is 15", the aperture is 160 mm, the integration time is 3 s, and the detection sensitivity is 15 Mv (Tab.3).

  Results and Discussions   A principle prototype is developed. The star simulator is used as the dim point target in the darkroom. The principle prototype is installed on a high-precision turntable to image the point target. The turntable is used to simulate the relative speed of the camera and the target. The turntable speed is adjusted to match the charge transfer speed of the camera. The camera works in the area array TDI mode to image the target and collect images. The point target image signal-to-noise ratio formula is used to calculate the camera SNR, and then complete camera sensitivity and signal-to-noise ratio index tests. Different TDI stages are adjusted to complete the camera response linearity test under different integration times. The test results show that the camera has 96 TDI stages, the sensitivity is better than 15 magnitude, the signal-to-noise ratio is greater than 5 (Fig.4), and the linearity response is good (Fig.5), which verifies the correctness of the camera design indicators.

  Conclusions  A visible light camera based on the area array TDI imaging mode is studied for observing GEO targets from low-orbit zero inclination satellites. Compared with the natural rendezvous and array staring imaging modes, area array TDI mode can increase exposure time through time delay integration without increasing the camera optical aperture or adding gimbal, so as to improve the camera detection sensitivity. On the one hand, it improves the camera detection sensitivity when the camera aperture is unchanged; On the other hand, the camera adopts the area array output mode, which can ensure the observation arc length requirements and facilitate the ground system to determine the target orbit. It provides a new technical approach for realizing higher sensitivity detection of GEO target from LEO.