The image data formats in lunar explorations

The image data formats in lunar explorations Ruiling Li1, Jianzhong Liu1, Chunlai Li1

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The National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012 E-mail: [email protected]

Abstract.

We can determine such key lunar characteristics as surface mineral composition, surface

chemistry, and topography and produce moon global mosaic map with lunar image data. That is, lunar image data are the significant resource in lunar scientific research. We review the image data formats in a series of lunar explorations and propose for Chang’E project image format in the future. In earlier lunar explorations such as Lunar Orbiter, Apollo and Surveyor missions, the image data were transmitted to Earth as analog data and were scanned by video camera to create digital images after images were received from spacecraft. These images were usually saved as photos to distribute to users in hard copy forms. In Clementine and Lunar Prospector programs, the data are transmitted to the PDS nodes with downlink and PDS centers are responsible for organizing and distributing of the data. Several principles should be applied in storage and distribution image data. We suggest the image data in Chang’E project be PDS format.

INTRODUCTION Many lunar exploration programs were developed by The United States and the Former Soviet government within 1958-1976 years. The Clementine spacecraft and the Lunar Prospector spacecraft were launched in 1994 and 1998 respectively. Numerous lunar data and images are achieved in these projects. Lunar image data are the significant resource in lunar scientific researches, and we can determine such key lunar characteristics as surface mineral composition, surface chemistry, and topography and produce moon global mosaic map with lunar image data. The archive and distribution format of image data is the precondition of data fully being used. At the same time, the image database can be updated and added frequently only saved in given format. In Chang’E project, well-documented and permanent data archives are required, and the images should be consistent in content and organization with other planetary image collections and archives in lunar explorations.

THE IMAGE DATA IN FORMER LUNAR EXPLORATIONS

The format of Lunar Orbiter’s image data The Lunar Orbiter program consisted of the investigation of the Moon by five identical unmanned spacecraft. The five Lunar Orbiter spacecraft returned over 1654 high-quality photographs, of which, 840 were taken from low flight altitudes and provided detailed coverage of 22 areas along the equatorial region of the nearside of the Moon. The remaining photographs were taken from 1

approximately 44 kilometers over the nearside to approximately 6000 kilometers over the farside（LPI ，2005）. They provide broad coverage of essentially the entire Moon and detailed coverage of 88 areas on the nearside. Lunar Orbiter images were photographic products acquired on the spacecraft during five missions. LO data were transmitted to Earth as analog data after onboard scanning of the original film into a series of strips (Thomas et al., 1970). These data were exposure again by camera and formed new pictures after the visual signal was returned to the earth.

The format of Surveyor image data Surveyor consisted of seven unmanned lunar missions that the important goal was to obtain lunar and cislunar photographs and both scientific and technological information needed for the Apollo landing program. The data transmissions were converted to a standard television signal for closed-circuit and public broadcast television. They were processed by analog-to-digital conversion of data (NASA, 2005).

The format of Apollo image data Each Apollo spacecraft was equipped with different lens camera. The Apollo film was scanned using a video camera to create a digital file (Grant Heiken et al., 1991). Each frame was digitized in Targa format. The Targa images were then processed to produce the image in JPEG format. These pictures were resided in cold storage in NASA data repositories. Lunar and Planetary Institute digitally scanned these data and are now available in an easy to use online digital resource.

The format of image data in recent lunar explorations The objective of the Clementine mission was to make scientific observations of the Moon and the near-Earth asteroid image. The observations included imaging various wavelengths including ultraviolet and infrared, laser ranging altimetry (K.E. Edwards et al., 1996). The Clementine mission mapped most of the lunar surface at a number of resolutions and wavelengths and the images covered over 99% of the entire lunar surface. The Clementine images were compressed onboard the spacecraft using a space-hardened Matra chip. The compression technique performed "lossy" compression using a Discrete Cosine Transform (DCT) algorithm. The images were kept in the compressed format when archived to reduce the total volume of the archive collection (Eric, 1995). The Clementine images were produced by NRL and PDS, and stored in EDR archives (USGS, 2005). Each Clementine EDR data product consists of a single camera observation. The image data have been reformatted with standard PDS labels. The image data was distributed as PDS and TIFF format. In addition to these explorations, Lunar Prospector that equipped with instruments rather than cameras obtained numerous data. The data in Lunar Prospector were saved in PDS format. Smart-1 is the first space probe ESA has ever sent to the Moon and it also is the first of ESA's missions to test advanced technology needed for future scientific planetary missions. In order to integrate with former planetary database, Smart-1 is ready to organize and distribute data in PDS format (B.H.Foing et al., 2005).

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THE IMAGE DATA FORMAT COMMONLY USED HDF (Hierarchical data format) format HDF is a library and multi-object file format for the transfer of graphical and numerical data between machines. HDF supports data types such as raster image, multi-dimensional arrays and text files. Features of HDF format include (Li Jing, 2005): It is self-describing, allowing an application to interpret the structure and contents of a file without any outside information. Scientists can know file structures well about data, as well as manage and analyze the data. It is extensible. It can easily accommodate new data models, regardless of whether they are added by the HDF development team or by HDF users. HDF files can be shared across most common platforms, including many workstations and high performance computers. An HDF file created on one computer can be read on a different system without modification.

FITS (Flexible Image Transport System) format FITS is the standard format used in astronomy, and it is used for the transport, analysis, and archival storage of scientific data sets. It can distribute on-line. It supports multi-dimensional arrays, spectrum data and text data. The data is portable which can be used in different platform and is greatly suited to use for storage of astronomic instrument data (R. J. Hanisch et al., 2001). However, this data cannot be compressed and the data storage required tremendous capacity. The data in this format must be processed with astronomic procedure.

PDS (Planetary Data System) format The PDS is sponsored by NASA's Office of Space Science. The PDS acquired and distributes scientific data from NASA planetary missions, astronomical observations, and laboratory measurements. Its purpose is to ensure the long-term usability of NASA data and to stimulate advanced research (NASA, 2005). The PDS described and stored data using standards that are designed to enable future scientists who are unfamiliar with the original experiments to analyze the data, using a variety of computer platforms, with no additional support. To reduce the data capacity, the data are stored in compressed format. The spacecraft attitude, ephemeris and decompression software are available in data volume.

CDF (Common Data Format) format The Common Data Format (CDF) is a self-describing data format for the storage and manipulation. CDF is a scientific data management package that allows programmers and application developers to

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manage and manipulate scalar, vector, and multi-dimensional data arrays. It usually used to organize (NASA, 2005) .

CONCLUSIONS Many significant achievements have been acquired in the lunar explorations. The image data format are varied with the technology and theories of space exploration development. In former lunar explorations such as Lunar Orbiter, Apollo and Surveyor missions, the image data were transmitted to the earth as analog data and were scanned by video camera to create digital images. These images were usually preserved as pictures and distributed to users with hard copy forms. There are disadvantages in the pictures format. Firstly, because of the diversities of missions and cameras in the spacecraft, it is difficult that the images in this format acquired in different explorations can be matched in coordinate. Secondly, it is different to digitize the picture and the images will be faded and distorted with the time going. Therefore the image data can’t be preserved perfectly for long time.

In Clementine and smart

-1 program, the image data are transmitted to the PDS node with downlink and PDS center are responsible for organization and distribution of the data. In Chinese Chang’E project, it is very important that image data format should be chosen. We think several principles should be applied in storage and distribution of image data. One is the image data can be updated and extended continuously to ensure the long-term usability of data and stimulate advanced research. The other is the data can be expediently used in different platform and are available for not only scientific organization but also general interested user. Apart from two points, the data format should be consistent with the international data format organization standard to conveniently manipulate and manage. In Chang’E project, well-documented and permanent data archives are required, and the images should be consistent in content and organization with other planetary image collections and archives in other lunar explorations. So we suggest that PDS format should be choose for the image data in Chang’E project.

Acknowledgements Xin Ren and Liyang Zhang are acknowledged for their insightful discussion.

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