Data Processing Plan of Imaging Interferometer of the Chang’E Project Liyan Zhang1, Chunlai Li1, Jianzhong Liu1, Jianjun Liu1 1
National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China, 100012 Phone: +86-10-64888774Email:
[email protected]
Abstract: In the first step of the Chinese lunar exploration project, the main scientific objective of the Imaging Interferometer instrument is to acquire the global distribution and abundance of lunar rocks and minerals. The Imaging Interferometer complement the X-ray and Gamma-ray Spectrometers to explore the global distribution and abundance of the utilizable elements on the moon, which is the second scientific objective of the first step of the Chinese lunar exploration project. We designed the ground data processing system for the Imaging Interferometer data in order to make efficient use of the Imaging Interferometer data. It includes the Preprocessing Subsystem and the Science Application and Research Subsystem. Keyword: Chinese lunar exploration project, Imaging interferometer, Data processing, Global distribution and abundance, Lunar rocks and minerals
INTRODUCTION The first step of the Chinese lunar exploration project, namely circumlunar exploration project, will launch Chang’E-1 satellite in 2007. Imaging InterferoMeter(IIM) is one of the eight payloads embarked on Chang’E-1 satellite. IIM is an optical instrument for imaging, multi-band spectral mapping in the circumlunar exploration project of China. Its main scientific objective is to acquire the global distribution and abundance of lunar rocks and minerals. Besides, IIM assists X-ray/ Gamma-ray spectrometer to explore the global distribution and abundance of the usable elements on the moon. IIM together with X-ray/ Gamma-ray spectrometer will achieve the second scientific objective of the circumlunar exploration project of China, namely acquirement of the global distribution and abundance of lunar rocks, minerals and utilizable elements (Ziyuan Ouyang, 2003). All the science telemetry data from Chang’E-1 satellite will be processed, archived and distributed at the Science and Application Center of the Moon and Deep Space Exploration (SACMDSE), National Astronomical Observatories, Chinese Academy of Sciences. So science exploration data of IIM will be also processed, archived and distributed at SACMDSE. An estimated amount of standard data products of IIM for one-year nominal operation is about 5-10 TB. Taking into account of the large amount of data, efficiency of data processing and product generation are important. In order to make efficient use of IIM data and achieve the second scientific objective of circumlunar exploration project, we designed the ground data processing system for the IIM data which includes preprocessing subsystem and science application subsystem.
PREPROCESSING The preprocessing subsystem of IIM raw telemetry data consists of level 0A, level 0B, level 1, level 2A, level 2B and level 2C processing and produces six levels data products which include level 0A, level 0B, level 1, level 2A, level 2B, level 2C data product.
The level 0A data processing The IIM raw telemetries in the Consultative Committee for Space Data Systems (CCSDS) transmission data frame format will be applied channel processing. Then the compressed/uncompressed IIM data frames will be extracted. In the course of the level data processing, Channel processing consists of frame synchronization, descrambling, ReedInternational Lunar Conference 2005
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Solomon decoding and extraction of the transmission data frame including the IIM compressed/uncompressed data blocks according to virtual channel information.
The level 0B data processing On the Chang’E project there are two ground acquisition stations. One is built in Beijing, the other is built in Kunming, Yunnan Province. So each same transmission data frame is acquired by the two separate stations. At this step the optimal data frame are selected from the level 0A data of two stations. Then the compressed/uncompressed IIM data blocks are extracted from the transmission data frame including them. At last, IIM compressed data frames will be decompressed. After the above processing is carried out, IIM level 0B data product is produced.
The level 1 data processing The level 1 data processing includes dark current subtraction, a relative radiometric calibration and spectrum reconstruction. Then all the subsidiary data are also merged into level 1 data at this step. Firstly the data have to be applied a relative radiometric calibration to remove sensor-dependent systemic errors caused by the course of IIM design, manufacture and assembly. The systemic errors are classified into the errors of the optical system, the mechanical system, the electronic circuit and CCD chip. Then the data have to be applied spectrum reconstruction to reconstruct from interference image to spectral image by the Fourier Transform. At last, subsidiary data including spacecraft ancillary and instrument supplement data are also merged into level 1 data at this step. After the above processing is carried out, IIM level 1 data product is produced.
The level 2A data processing Spectral radiometric correction will be carried out on the basis of the absolute radiometric calibration matrixes (Ying Dong, 2001).
The pixel value of level 1 data presents the relative spectral intensity. Then in order to get target absolute radiance, the level 1 data will be applied an absolute radiometric calibration. The coefficients for the absolute radiometric calibration are generated by the radiometric calibration file. The radiometric calibration file is prepared by pre-flight performance tests. After the data are applied the above absolute radiometric calibration processing, we obtain spectral image data which are provided with target absolute radiance.
The level 2B data processing In order to correct geometric distortion from satellite orbit, attitude and instrument itself, geometric coarse correction will be carried out according to the correction model and geometric parameters. How to construct geometric coarse correction model and obtain the geometric parameters are very important at this step. The geometric parameters include the information of satellite orbit, attitude and instrument parameters, etc. They can be obtained from the ephemeris file and the calibration file. Concerning the geometric coarse correction model, we plan to construct it on the basis of the geometric coarse correction model of earth remote sensing and relationship between image point coordination and lunar point coordination. International Lunar Conference 2005
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After the geometric coarse correction, satellite bottom point coordination can be obtained.
The level 2C data processing At this level, we will use photometric function and suitable parameters to carry out photometric correction in order to remove illumination difference among images. Photometric correction is necessary to compare and mosaic images at different illumination geometries (Hirata et al., 2002). With regard to parameters of photometric correction, we can obtain the previous step. Concerning photometric function, we plan to construct it on the basis of large numbers of experiments and analysis. After photometric correction processing, we will obtain all IIM preprocessing data products which include level 0A, level 0B, level 1, level 2A, level 2B, level 2C data product.
SCIENCE APPLICATION AND RESEARCH SUBSYSTEM On the basis of preprocessing data product, we can carry out geometric precision correction using lunar control points, image mosaic and spectral analysis successively. At last, we will carry out classification of minerals and rocks, and develop an algorithm to estimate mineral abundance and chemical composition using correlation between the abundance of certain mineral(s) and the ratio of absorption band depth.
CONCLUSIONS This paper represents IIM data preprocessing subsystem and science application subsystem. And the course of the preprocessing is represented in detail, which includes production of level 0A, level 0B, level 1, level 2A, level 2B and level 2C data product. At last the course of the science application processing is introduced in brief.
REFERENCES Hirata, J. Haruyama, H. Demura, et al., Data Processing Plan of Lunar Imager/Spectrometer on the SELENE Project, Lunar and Planetary Science XXXII (2002), 1493.pdf. Ying Dong, Study on the Imaging Interferometer Calibration Scheme, Master’s Degree Dissertation of Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, 2001, pp.37-49. Ziyuan OUYANG, International Lunar Exploration Progress and Scientific Objectives of Chinese Lunar Exploration Program, Journal of Guizhou University of Technology (Natural Science Edition), 1-7(2003)
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