Its technical features are as follows:
Sub-meter large format imaging technology
The two cameras of Gaofen-2 satellite designed an optical system with long focal length, large F number and light miniaturization. The focal length is 7.8m, and the f number 15 is the largest among the remote sensing satellites in orbit in China. The full-color and multi-spectral and five-spectral integrated TDI CCD devices are used to push-scan the ground scene, and the field of view of a single camera is 2. 1 through device splicing. The field of view is further expanded by two cameras, so that the resolution of ground pixels under the satellite can reach panchromatic 0. 8 1m, multispectral 3. 24m, and the observation width can reach 45.3km It is a sub-meter high-resolution satellite.
Optimal design of wide coverage and high revisit rate orbit
Through the optimization design of orbit and yaw maneuver parameters, Gaofen-2 satellite has an orbital inclination of 97.9 and a regression period of 69 days, which can ensure the observation coverage of a vast area between 80 degrees of the north and south latitude of the earth, and can run 102 1 circle in a regression period, thus achieving seamless global coverage. When the satellite tilts 23, the revisiting period anywhere in the world can be less than 5 days, and when the satellite tilts 35, the revisiting period will be further shortened. The wide coverage and revisiting rate of the satellite have reached the leading level of China's remote sensing satellites. ?
High Stability and Fast Attitude Yaw Maneuver Control Technology
Gaofen-2 satellite is the first satellite in China equipped with high-precision APS star sensor made in China. The actuator adopts a hybrid structure of control moment gyro and momentum wheel to realize the rapid attitude deflection maneuver and stable control of the satellite. Through the improvement of single-stage algorithm of star sensor, the optimization design of system-level attitude determination algorithm and control algorithm, the high-precision and high-stability attitude control of all domestic components is realized for the first time. The measured data in orbit show that the satellite attitude stability can reach 1.5e-4/s, and the attitude determination accuracy can reach 0.0025. Gaofen-2 satellite has achieved 35-degree maneuver and stability in orbit 150s under the constraint of "avoiding flexible vibration as much as possible", which is the highest level of domestic remote sensing satellites at present.
Design of high positioning accuracy image
In order to meet the quantitative application requirements of users, Gaofen-2 satellite has taken many measures to improve the positioning accuracy of images. Among them, the whole satellite high-precision time synchronization scheme ensures that the time synchronization accuracy error is less than 50μs, so that the camera, control and measurement GPS work under a unified time reference during imaging. Various high-precision attitude measurement schemes, such as direct attitude determination by domestic high-precision star sensor, joint attitude determination by star sensor and gyro, integrated installation of star sensor and camera and high-precision temperature control, make it possible to accurately obtain satellite imaging direction; The special vibration reduction and isolation device suppresses the influence of micro-vibration and ensures the stability of imaging orientation. Various measures have made many indexes related to image positioning accuracy, such as orbit measurement and attitude measurement, reach the international advanced level, among which the orbit measurement accuracy reaches 10m in real time, 0.5m afterwards and 0.003 in attitude measurement. The preliminary evaluation of on-orbit image test shows that the positioning accuracy of non-control points reaches 20-35m, which is better than the design index of 50m and reaches the international advanced level.
Image high radiation quality design
Gaofen-2 satellite has taken many measures to improve the image radiation quality. The optical system and imaging circuit of the camera are optimized, and the modulation transfer function MTF is designed, which achieves a high MTF number under the condition of relatively small aperture. At Nyquist frequency, the static MTF full spectrum is not less than 0. 12, the multi-spectrum is not less than 0.2, and the on-orbit dynamic MTF is not less than 0. 1. On the basis of image sampling 10bit quantization, the internal hood is used for the first time to further improve the stray light suppression, the high-precision low-temperature focal plane circuit and CCD control are used to reduce the electronic noise, and the gray code coding is used to reduce the system noise to ensure that the image signal noise is as small as possible. Under various conditions of high solar angle and ground reflection, the panchromatic signal-to-noise ratio reaches 23-43dB, and the multispectral signal-to-noise ratio reaches 25-43dB, all of which are leading satellites in China.
In order to adapt to the complex imaging illumination conditions in different regions of the earth's surface, a scheme with wide dynamic range, good linearity and multi-level adjustable imaging series and gain is designed, which has five-level adjustment function, the gain of each spectral segment can be adjusted independently, and the integration series can be adjusted by 1-96, so that the dynamic range of the system can reach 20-800DN and the linearity is better than 3%. According to the on-orbit test histogram statistics, the gradation values are widely distributed, the image information is rich, and the visual effect is excellent, reaching the international advanced level of similar satellites.
Design technology of light and small camera
The optical system design of Gaofen-2 satellite camera adopts a three-mirror coaxial scheme with a relatively small aperture (f number 15) for the first time, and the camera aperture is greatly reduced. By optimizing the distance between the primary mirror and the secondary mirror and the folding angle, a larger focal length ratio (the ratio of focal length to optical system length) can be obtained. For the first time, the main support structure design of framing support and damping vibration isolation is adopted to ensure the higher stability of the camera and the mechanical environment resistance of the rocket active section, making the camera more compact, smaller and lighter. The main mirror and lens barrel of the camera are made of new ceramic-based carbon fiber composite materials, which can not only ensure the stability of the camera mechanism, but also further reduce the weight of the camera. Compared with the traditional design, the volume and weight of the camera are reduced by about 1/20. The camera adopts three-mirror focusing mode and five-spectrum integrated TDI CCD device scheme. Compared with similar cameras abroad, the size and weight of the focal plane assembly, three-mirror assembly and focusing mechanism are greatly reduced, and the light and small state has reached the international advanced level.
Design of high integration, high dynamic and low noise imaging circuit
In order to adapt to the light and small design of the camera, the camera imaging circuit adopts advanced high dynamic, low noise and high integration design technology. Aiming at the characteristics of camera electronics, such as the splicing of focal plane multi-color CCD devices, many pixels, high line speed, CCD working frequency reaching 10MHz, image original data rate reaching 2Gbps, high signal-to-noise ratio and timing accuracy, high device power consumption, and the need to adapt to multi-mode conditions, the camera imaging circuit adopts a customized highly integrated five-spectrum integrated TDI CCD device, which improves the photoelectric conversion efficiency and MTF index of the device. TLK27 1 1 high-speed serial data interface technology is adopted for the first time on the satellite to realize high-speed transmission of image data, and an efficient image compression algorithm is adopted. Noise reduction design mainly adopts low noise focal plane CCD and readout circuit design technology, focal plane CCD timing control and driving design technology, variable rate timing signal generation and control technology, high dynamic signal processing technology and adaptive image processing and transmission design technology; In order to solve the influence of the dark current variation of CCD devices on the image quality, increase the dynamic range of the system and improve the image quality, the working temperature of CCD is set between -5~ 10 degrees, and the actual on-orbit temperature is between -2~5 degrees, so that the dark signal part of CCD signal is effectively suppressed. The above measures have realized the high integration, high dynamic and low noise design of the imaging circuit, greatly reduced the signal noise level and effectively ensured the camera imaging signal quality.
Design of various flexible working modes of load
In order to meet the requirements of users, Gaofen-2 satellite has designed various flexible load working modes such as imaging transmission, imaging recording and data playback. When the camera images and the satellite can transmit data with the ground data receiving station, the satellite performs imaging transmission. Imaging transmission modes can be divided into three modes: quasi-real transmission, full-color image real transmission and multi-spectral image real transmission. Among them, the quasi-real transmission mode is the conventional imaging mode of Gaofen-2 satellite in orbit, and the real transmission mode of panchromatic image and multispectral image is the special imaging mode, which is mainly used for testing or storing the emergency of system failure.
When data transmission with the ground station is impossible, the satellite can work in the imaging recording mode, and record the image data, auxiliary data and service system data imaged by the camera in solid-state memory in real time. When the satellite can transmit data with the ground data receiving station, the satellite can play back the data stored in the solid-state memory to the ground station. At this time, the satellite works in data playback mode, which can be divided into image data playback mode and service system data playback mode according to different types of playback data.
For imaging transmission and data playback mode, the data transmission operation between Gaofen-2 satellite and ground data receiving station can be carried out by single-station relay transmission, double-station relay transmission and mobile station reception according to different situations of ground stations. A variety of flexible load working modes are convenient for users to use satellites and receive data, which further improves the efficiency of satellite use.
Intelligent on-orbit health detection and fault handling capabilities
Gaofen-2 satellite has intelligent on-board autonomous management capability, including various on-orbit safety control strategies such as battery charging autonomous control, battery discharge depth safety control, forced load shutdown, and emergency control of satellite attitude abnormality. At the same time, an autonomous fault handling mechanism is established for three different levels of faults: general, emergency and fatal, which improves the survivability and reliability of satellites in orbit. Gaofen-2 satellite also has the ability of relay satellite measurement and control, and the coverage of coordinated measurement and control between heaven and earth has nearly doubled, which has improved the monitoring ability of the satellite and effectively guaranteed its operation. In addition, the optimized design of on-orbit working strategy also makes the working intensity and stress environment of battery, momentum wheel of control system, CMG, solar wing driving mechanism and data transmission antenna reasonably balanced, avoids the occurrence of early failure problems, ensures the service life to meet the design index requirements, and makes the long-life design technology of low-orbit remote sensing satellites in China reach the international advanced level for the first time. (Excerpted from Baidu Encyclopedia)