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Dynamic environment testing system for space remote sensor

Liao Zhichun, Yang Binxing
Beijing Institute of Space Machine and Electricity
P.O. Box 9201,Beijing China
Tel:(86)(010) 68756432;Fax: (86)(010)68382807

1 Preface
For developing space remote sensor with high quality, apart from solving various theoretical questions of remote sensor itself, it is necessary to solve the problems of ground test and environmental experiment for remote sensor at the same time to ensure high quality and high reliability. No one of the two points can be neglected. This paper describes some aspects about Dynamic Environment Testing System for space remote sensor developed by Beijing Institute of Space Machine and Electricity under Chinese Academy of Space Technology.

2 Necessity of developing Dynamic Environment Testing System (DETS)
For space remote sensor manufacturing, fitting, adjusting, and testing, all the activities are being done in the ground surface under the environmental condition of standard atmosphere pressure. All factors such as continuous convention and fluttering of the air around the working site upon the effect of heat (temperature field), and various kinds of disturbances imposed by people, ground vibration, gravity field etc., will deface the performance of sensor to a considerable extent. However, when remote sensor traveling in space, the around environment is at high vacuum (or low air pressure of the payload bay of satellite), weightlessness, no artificial disturbances and no ground vibration etc., but the factors of solar radiation and cosmic rays etc. are existed. Because of the difference between the two environmental conditions, the functional performances of remote sensors are very different in space and in ground surface. Obviously, the most concern of us is the functional performance of the remote sensor in space orbit. Generally speaking, under the environmental condition of satellite orbit, the performance of remote sensor will decline by 30%or more, comparing to that of in ground test and measurement. Along with enlarging of focus length and optical aperture, and increasing of resolution, these affections will show up more obviously. Hence, through a certain ground experiments, to find out factors influencing the performances of the sensor and the degree of effectiveness, becomes special concern of designer. Building Dynamic Environment Testing System is important means to solve this topic.

The DTES is a large integral ground testing equipment. It is used to simulate satellite orbital environment, and to provide real-time testing conditions for remote sensor working under such conditions. In the period of engineering model development, remote sensor systems will be taken in an integrated environmental experiment to comprehensively check the functional performances of optics, mechanics, electronics, and heat subsystems, so that the real performances of the remote sensor in orbit can be estimated in advance. The significance of these experiments, first of all, in the period of engineering model manufacturing, is to discover existent faults of the sensor itself early, concluding the problems in designing, manufacturing, fitting and adjusting etc., so as to correct them in time, and secondly, to return experimental result back to the designers, so as to redesign the remote sensor system and to adjust relevant parameters to have a good match to improve the performance of redesigned system. As saying above, the performances of the remote sensor in orbit will fall by 30% or more, comparing to that of in ground testing. However, after such experiment and adjustment, the functional performance will recover by 15% or more.

The DETS developed by our country for space optical remote sensor environment experiment can fulfill experimental needs of optical remote sensor of all kinds. During the time of engineering development, by absorbing successful experiences of similar systems in domestic, and adopting the merits of similar systems abroad, combining the two factors with the practice of China, we decided the overall schemes of the DTES. The emphasis is to fulfill the needs of optical remote sensor testing, and the needs of other experiments which need vacuum cold and dark environment are also considered to a certain extent.

3 Composition of Dynamic Environment Testing System
The DTES is composed of three subsystems, Thermal Vacuum Simulator (TVS), Optical Testing System (OTS) and Micro-Vibration Isolation System (MVIS). The DTES is about 23m in full length, 13m in height (7m above ground, and 6m underground). The principal part of the system is mounted in a big hall with area about 700m2. The environmental conditions in the hall about temperature (22±1) °C, humidity (50±10)%, air purity 100 thousand degree. Clearance height of the hall is 12m. There are optical and electronic testing instrument room, mechanical fitting work place, 5T crane, and a variety of laboratory equipment, to fulfill the needs of experiments. There exists an auxiliary room near the hall, with the auxiliary facilities in it. In a word, the distinguishing feature of the DETS is that it is an integration of optical, mechanical, electrical, and thermal subsystems, vibration-isolation, clean, pollution-free, and large scale, and it is only one in domestic, special for space remote sensor environmental experiment.

The outlines of the three subsystems are in below.

3.1 Thermal Vacuum Simulator (TVS)
TVS is used to imitate vacuum or lower atmosphere pressure, cold and heat environment in satellite orbit. It consists of a q4m*8.2m vacuum tank plus various auxiliary devices and instruments. The efficient working space of the tank is q3.4m*6m. It can form a 6*10-6 Pa extreme vacuum (non-load) and £100k cold and dark environment. Vacuum degree and temperature field are adjustable according to needs. Specially in the time of optical remote sensor experiment, the temperature around the sensor can be continuously adjusted at the range of -20°C ~ +50°C, with the non-uniform of the temperature field £±0.3°C, the stability of the temperature field £0.5°C/24h. The vacuum volatilization organic pollution £1*10-7g/cm2 strictly ensures the optical system out of pollution. Hence, the performance of TVS is on the leading position in similar domestic apparatus. Three-dimensional adjustable platform in vacuum tank is used for adjusting the attitude of the experiment object mounted on it to match the optical system being checked. All in all, the TVS has enough experimental space, good vacuum condition, clean and neat environment with high or low temperature, to fulfill the needs of every kind of remote sensor experiment.

3.2 Optical Testing System (OTS)
OTS, it has collimators with various focus lengths and auxiliaries, standard plane mirror, and moving target simulator. The integrated wave-front aberration of collimate (p-v value) £ l/8. The internal thermal control system of collimate guarantees the temperature differential of the points £0.5°C. Combining with TVS, it is used to test the functional performance of optical remote sensor under normal temperature, normal air pressure, and the environmental condition similar to satellite orbit. There for, the DETS can be used to do heat vacuum, heat balance, heat circulation tests as well as vacuum calibration experiment, and to measure the static and dynamic photographic resolution of remote sensor, and MTF of optical system. Through analysis of these experiments, the actual functional performance of the remote sensor in orbit flying, ground resolution for example, will be predetermined rather precisely.

3.3 Vibration Isolation System (VIS)
VIS is used to isolate ground vibration caused by people activities (such as moving of vehicle, operation of construction site, running of machine etc.), ensuring normal optical testing. The VIS consists of vibration-isolation base, and air spring vibration- isolation platform. The platform is 23m long and 4.5m wide, adopting 28 large air springs to suspending it. All the experimental apparatus and experimental object are placed on this platform. Under control of operation desk, the platform suspends total mass of hundreds tons of tested objects and experimental equipment, isolating them from ground. The platform is the domestically developed one with the largest dimensions and comparatively good performances.

4 Distinguishing features of DETS
Combining optical testing apparatus (long focus length collimator etc.) with space environment simulator, to fulfill the task of performance test of remote sensor under orbital environment condition.

Out fitting a complete high precision optical testing equipment, getting rid of air fluttering and convection in the testing optical path.

Uniformity of temperature field, small temperature gradient, and very low organic volatilization pollution.

A perfect micro-vibration isolation system.

The whole system fits up all kinds of necessary equipment, having good potential application. Not only carry out remote sensor experiment but also realize space environment experiment for middle or small earth observation satellite.

5 Function of DETS
Simulation experiment (heat balance, heat vacuum, heat circulation etc.) of space emote sensor under simulated orbital working condition and integrated performance test, static and dynamic resolution measurement and MTF measurement.

Adjusting parameters of remote sensor
according to orbital working condition of satellite (especially doing vacuum focus calibration)to reach the good match.

Fulfill environmental simulation for middle or small satellite.

The successfully development and operation of the Dynamic Environment Testing System will further the development of Chinese space remote sensing technology.