Intelligent camera for distributed reconfigurable remote sensing satellite system should be lighter to be installed on small or micro satellite. In this paper structure design of intelligent camera for distributed reconfigurable remote sensing satellite system was presented in detail. Three mirrors anastigmatic (TMA) optical system was used in the intelligent camera. Structure of the intelligent camera consists of the primary mirror module, the secondary mirror module, the tertiary mirror module, the focal plane adjusting module and the main framework module. Structure of the intelligent camera was designed so that mounting points of the camera were distributed more dispersedly and the camera's center of gravity was closer to the satellite platform. In this way better dynamitic characteristic was realized. Aluminum alloy framework was used to realize better thermal characteristic and lower weight. In order to compensate variation caused by vibration of spacecraft launch and change of temperatures on orbit, precision of focal plane adjusting module is no more than 10μm and the range of focal plane adjusting is ±2.5mm. Results of experiments indicate that total weight of the intelligent camera for distributed reconfigurable remote sensing satellite system is about 8kg and can satisfy the demand of the mission.
Distributed reconfigurable remote sensing satellite system can realize flexible and robust remote sensing network. Imaging system for distributed reconfigurable remote sensing satellite system should consume fewer power and be lighter to be installed on small or micro satellite. In this paper imaging system based on CMOS for distributed reconfigurable remote sensing satellite system was put forward. Circuits of CMOS sensor driving and CMOS data processing were detailed. In order to improve signal integrity circuits were divided into two sections which were connected with flexible printed circuit. CMOS sensor and its power circuits were placed in the focal plane printed circuit board. Imaging control box consisted of control and data processing circuits of CMOS sensor as well as communication and image data transferring circuits. Impedance matching was used to improve signal integrity of signals transmitted through flexible printed circuit between the focal plane printed circuit board and imaging control box. Results of experiments and analysis indicated that power dissipation of imaging system for distributed reconfigurable remote sensing satellite system was 4.05 W and the weight of imaging control box was 0.435 kg. SNR of each band of imaging system for distributed reconfigurable remote sensing satellite system was more than 41.28 dB as sun zenith angle is 20º and earth reflectance is 0.65.
KEYWORDS: Cameras, Sun, Signal to noise ratio, Reflectivity, Photography, Detection and tracking algorithms, Light sources and illumination, Space operations, CCD image sensors
Time delay and integration (TDI) CCD or CMOS sensors are widely used in space cameras to realize high resolution. Fixed TDI stages are often used when space cameras are working to avoid image saturation. As a result images obtained have problems such as short of digital number levels. In this paper an automatic on-orbit adjusting TDI stages method of space camera based on sun zenith angles was put forward. In this method radiance at entrance pupil of space camera was calculated in real time by nonlinear function fitting according to sun zenith angles of ground targets. Then optimal time of exposure could be obtained. Line transfer time for image compensation was calculated at the same time. Finally TDI stage was acquired by optimal time of exposure and line transfer time. In this way real-time calculation and adjustment of TDI stages were realized. Results of comparison of simulation by STK and computation by MODTRAN indicate that standard deviation of radiance fitting errors is about 0.175W/m2 ∙s and relative radiance fitting error is no more than 0.93% as sun zenith angle belongs to [5°, 65°]. Dynamic changes of SNR along with changes of time and sun zenith angle using adjusting TDI stages method and fixed TDI stages method were compared and analyzed. Results of analysis indicated that SNR would be improved from 23 dB to 39dB as sun zenith angle was 88.2° when automatic on-orbit adjusting TDI stages method was used in comparison with fixed TDI stages method.
KEYWORDS: Cameras, Electron multiplying charge coupled devices, CMOS sensors, Signal to noise ratio, Imaging systems, Quantum efficiency, Cooling systems, Back illuminated sensors, Mechanics, Systems modeling
Electron Multiplying Charge Coupled Device(EMCCD) can realize read out noise of less than 1e- by promoting gain of charges with the charge multiplication principle and is suitable for low light imaging. With the development of back Illuminated CMOS technology CMOS with high quantum efficiency and less than 1.5e- read noise has been developed by Changchun Institute of Optics, Fine Mechanics and Physics(CIOMP). Spaceborne low light detection cameras based on EMCCD CCD201 and based on CMOS were respectively established and system noise models were founded. Low light detection performance as well as principle of spaceborne camera based on EMCCD and spaceborne camera based on CMOS were compared and analyzed. Results of analysis indicated that signal to noise(SNR) of spaceborne low light detection camera based on EMCCD would be 23.78 as radiance at entrance pupil of the camera was as low as 10-9 W/cm2/sr/μm at the focal plane temperature of 20°C. Spaceborne low light detection camera worked in starring mode and the integration time was 2 second. SNR of low light detection camera based on CMOS would be 27.42 under the same conditions. If cooling systems were used and the temperature was lowered from 20°C to -20°C, SNR of low light detection camera based on EMCCD would be improved to 27.533 while SNR of low light detection camera based on CMOS would be improved to 27.79.
Mapping precision of space-borne stereo mapping camera is primarily determined by attitude angle errors of the satellite. Time synchronization errors of space-borne stereo mapping camera will bring on extra attitude angle errors. In this paper model of space-borne stereo mapping camera was established in satellite tool kit (STK) to obtain the regularity of attitude angles changing with time. Influence of space-borne stereo mapping camera’s time synchronization precision on attitude angle errors was analyzed by combing the regularity of attitude angles changing with time and the sampling theory. As a result digitalized model of extra attitude angle errors and time synchronization errors of space-borne stereo mapping camera was put forward. In validation experiments real attitude angle data of a stereo mapping satellite were collected and extra attitude angle errors caused by specific time synchronization errors of space-borne stereo mapping camera were obtained. Results of the experiments and analysis indicated that extra attitude angle errors caused by specific time synchronization error could be reduced from 0.01939 arc second to 0.00003879 arc second as time synchronization precision was optimized from 1ms to 20μs.
Multilinear CCD Sensor was often used on space cameras to obtain multispectral images with each line representing
different band channels. However images of different band channels obtained at the same time didn't coincide as there
were spaces between lines. Pixel numbers to be adjusted between images of different channels varied when the space
camera worked by swaying forward and backward or adjusted row transfer period to compensate image movement. An
automatic multispectral images synthesis algorithm of space camera was put forward on the basis of analysis of such
phenomenon. In this algorithm a new evaluation function was used to determine pixel numbers to be adjusted and the
image regions of each band channel to be clipped. In this way images of different band channels could be synthesized
automatically to obtain an accurate colorful image. This algorithm can be used to dispose a large mount of images from
space camera directly without any manual disposal so that efficiency could be improved remarkably. In validation
experiments the automatic multispectral images synthesis algorithm was applied in synthesis of images obtained from
outside scene experiment of a multispectral space camera. Result of validation experiments proved that the automatic
multispectral images synthesis algorithm can realize accurate multispectral images synthesis of space camera and the
efficiency can be improved markedly.
Interline transfer CCD camera can be designed to work in time delay and integration mode similar to TDI CCD to obtain
higher responsivity and spatial resolution under poor illumination condition. However it was found that outputs of some
pixels were much lower than others' as interline transfer CCD camera work in TDI mode in laboratory radiometric
calibration experiments. As a result photo response non-uniformity(PRNU) and signal noise ratio(SNR) of the system
turned for the worse. This phenomenon's mechanism was analyzed and improved PRNU and SNR algorithms of
interline transfer CCD camera were advanced to solve this problem. In this way TDI stage was used as a variant in
PRNU and SNR algorithms and system performance was improved observably with few influences on use. In validation
experiments the improved algorithms was applied in radiometric calibration of a camera with KAI-0340s as detector.
Results of validation experiments proved that the improved algorithms could effectively improve SNR and lower PRNU
of the system. At the same time characteristic of the system could be reflected better. As working in 16 TDI stages,
PRUN was reduced from 2.25% to 0.82% and SNR was improved about 2%.
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