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I.INTRODUCTIONMars is the most similar planet as the Earth in the solar system. So it is the most studied planets in the solar system. U.S.A., Russia and E.U. have launched more than 43 satellites or spacecraft. China has realized to surround and land on the Moon, but has never been to explore Mars. In the 2020, China will launch the first spacecraft to Mars independently. Though it is later, but China has started on the high level. In this exploration, it will realize three activities, which are to surround, land and rover on Mars. The scientific objectives and tasks of China Mars Exploration:
II.MISSION REQUIREMENTS OF HYPER RESOLUTION CAMERAThe shortest distance is 55,000,000 kilometers between Mars and the Earth. The longest distance is 400,000,000 kilometers. It will take 3 minutes for data to transfer from Mars to the Earth. Data rate is one of main engineer constraints because of the long distance. So it is important to get more information in the limited data rate to explore Mars. This mission will have three activities, which are to surround, land and rover on Mars. The spacecraft has surrounding segment, landing segment and rover. Mass is greatly limited to realize so many activities because of the limit of lunch rocket. The Hyper Resolution Camera (HRC) is mounted in the surrounding segment and to get the image of Mars. The tasks of HRC are:
A.The image characteristic of MarsMars and the Earth are planets of the solar system. They are quite different, as figure 1. The soil of Martian surface includes ferric oxide. The ferric oxide transfers into red and yellow oxide because of long term exposure to ultraviolet rays. So Mars is like a word full of rust. The reflectivity of Martian surface is 0.08-0.40 and the average reflectivity is about 0.25. On the surface there is a thin atmosphere, the pressure is no more than 0.01atm. The heat flux from sun to Mars is about 595 w/m2, which is about 1/2.3 of the heat flux from sun to earth, that is 1353 w/m2. B.The orbit anslysis of surrounding segmentThe surrounding orbit is elliptical orbit. The perigee of the orbit is about 265km and the apogee of the orbit about 15341.8km. The orbit inclination angle is 93.1 degree. The requirement of observation arc is from perigee to 1000km. Figure 2 gives a full orbit period analysis. From the figure, we can get the solar elevation angles of full orbit period and the distance variation between the surrounding segment and surface of Mars. C.Oribtal heat flux Analysis of surrounding segmentThe distance between the spacecraft and sun increases, the solar irradiance decreases. The average solar irradiance near the Earth is 1353 w/m2. The max is 1419 w/m2 and the min is 1317 w/m2. The average solar irradiance near Mars is 595w/m2. The max is 717 w/m2 and the min is 493 w/m2. Figure 3 gives the solar irradiance with distance variation. The orbit of surrounding segment is elliptic. The perigee is about 265km. The radiation from Mars varies with the height of orbit. Figure 4 gives Mars radiation varies with height of the orbit. III.DESIGN OF THE HYPER RESOLUTION CAMERAThe camera includes two equipment, one is Optical Main Body (OMB) and the other is Integrated Electronics. The OMB includes optical lens, focusing mechanism, and focal assembly and focal electronics. The Integrated Electronics includes image processing circuit, image compression circuit, management circuit, and second power circuit. Figure 5 gives the sketch of Hyper Resolution Camera. A.Compact design of cameraThe mass is one of the most important engineering constraints. The less mass is the better. Several ways are taken to get the lest mass.
B.Integrated CircuitsThe circuits include ADC unit, image processing unit, image compression unit, management unit, thermal control unit, power unit, and velocity measurement unit. All the circuits are all integrated into one equipment. Figure 9 gives the sketch of integrated circuits. Focal plane circuits include two same set of circuits, each of which include one multi-band TDI CCD, sequence circuit, drive signal circuit, analog filter circuit, and analog to digital circuit. All these functions are realized in one ASIC chip. ASIC chip reduces the dimension and power of circuits.
IV.CONCLUSIONSHyper Resolution Camera is the main payload of orbiter. The main objective of the Camera is to get hyper resolution image of key region of Martian surface. The image will provide the base data and scientific basis for the selection of land site and will give the formation and change process of Martian surface. Data rate and mass are the main engineer constraints to realize the scientific objective. Based on the two constraints and the characteristics of Martian surface, a candidate mission is provided, which has adopted high integrated design and intelligentialize. The Korch optical system, all composite structure and ASIC electronics are used to get the least mass. Image automatic identification based on feature extraction and image preview are used to take full advantage of data rate to get more valid data for scientific purpose. REFERENCEDavid Dorn, William Meiers,
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