Operation of HDTV onboard KAGUYA(SELENE) in the Extended Mission and its Observational Results

Transcription

1 Operation of HDTV onboard KAGUYA(SELENE) in the Extended Mission and its Observational Results Rie Honda 1, Junichi Yamazaki 2, Seiji Mitsuhashi 2, Junichi Tachino 3, Masahito Yamauchi 4, Motomaro Shirao 5 and Koichi Miyadera 6 1 Kochi University, 2 NHK (Japan Broadcasting Corporation), 3 NHK Engineering Service, 4 NHK Mito Broadcasting Station, 5 Planetary Geological Society of Japan, 6 Chiyoda Video Abstract: HDTV onboard KAGUYA (SELENE) succeeded in acquiring 595 movies and 309 still images during the 19- month long mission that completed on June 10th, 2009(UT). More than a half of the data was obtained during the extended mission period that started in November The summary of the HDTV operation, the coverage, the status of the instrument, and the result of observations during the extended mission period together with the other periods are described in this paper. Introduction JAXA s lunar explorer KAGUYA (SELENE) was launched from Tanegashima in Japan on September 14th, KAGUYA completed its 19-month long mission by the controlled impact on the Moon on June 10th, Along with fourteen scientific instruments exploring the Moon and its surrounding environment, KAGUYA had the High Definition Television System (HDTV) developed by NHK (Japan Broadcasting Corporation) on board for public outreach and educational purposes. The HDTV system was designed to acquire the videos of the Earth and the Moon on the lunar revolving orbit, in particular, the Earthrise and the Earth-set above the lunar horizon. The mission period of KAGUYA was divided into the three phases: the initial checkout phase between October 20th, 2007 and December 20th, 2007, the nominal mission period between December 21st, 2007 and October 31st, 2008, and the extended mission period between November 1st, 2008 and June 11th, HDTV succeeded in acquiring highquality movies and still images throughout the mission period. In particular, most of the movies and images are obtained in the initial checkout period and the extended mission period. This paper reports the operation of the HDTV in the extended mission period and summarizes the observational result throughout the whole mission period. Overview of HDTV Specification Figure 1 represents the overview of the HDTV. HDTV was composed of a telephoto camera, a wideangle camera (hereafter abbreviated by TELE and WIDE, respectively), and the common data processing unit. HDTV obtains high definition videos of in pixel format with the depth of 8 bits for each of RGB color at the frame rate of 30 fps. The system was attached on the spacecraft s XY plane with the +Z axis directed toward the center of the mass nominally, so that each of TELE and WIDE observed the lunar surface in the foresight and aftsight direction. Both cameras were designed to capture the lunar horizon in the center of FOV at the spacecraft altitude of 100 km. The spacecraft orbited around the moon in the circular polar orbit at the altitude of km until the beginning of the extended mission, thus HDTV was constantly able to capture the lunar horizon during these periods. For more detailed description of HDTV, see Honda et al. (2009) and Mitsuhashi et al. (2008). Figure 1. The overview of the HDTV. The picture in the left is the STM (structured thermal model) and the figure in the left shows the configuration of TELE and WIDE of HDTV with respect to the spacecraft (S/C). HDTV Operation Although the chances of the observation of the Earth by HDTV came every half-month, the illumination condition changed gradually throughout a year. Figure 2 represents the temporal variation of solar angle, which is the angle between the spacecraft orbital plane and the Sun s direction. The full-earth was able to be observed when the Earth, the Moon and the spacecraft were aligned in the same viewing direction of the camera, in which condition is around 0 degree. This situation occurs just twice a year, in April and in September 2008 in the nominal mission period. Thus HDTV observation is determined referring the visibility of the Earth the illumination condition, i.e., the solar angle. The full Earth observations in April 2008 and September 2008 were prioritized among all the observations. Figure 3 represents the image of the full Earthrise thus obtained on September 30th, 2008.

2 From January 2009 until the end of May 2009, the spacecraft altitude was decreased to 50 20km. With this low altitude, TELE camera was no longer able to capture the lunar horizon in the FOV and most of the FOV of WIDE camera was also covered by the lunar surface. Thus we focused on the observation of the lunar surface in this period (e.g., Figure 4). Figure 2. Temporal change of theβangle of the spacecraft throughout the mission together with the description of mission phases and HDTV operational events. cause the observations by the other scientific instruments are prioritized during this period. More data transmission time was allocated to HDTV during the initial checkout period and the extended mission period. In particular, HDTV was permitted to obtain 2 movies per day during February 2009, and 4-6 movies per day after March However, the β angle between February and April in 2009 were too high to observe the lunar features in the low and middle latitude area with the moderate contrast and shadows (see Figure 2). Thus we conducted a global mapping of the high-latitude area in the latitude higher than 55 degree during this period. Figure 6 represents the coverage of the footprint of HDTV observations. The Mare, the area around the South Pole Aitken Basin and the high latitude area in the northern hemisphere were well observed by WIDE camera. Figure 5. The nubmber of obtained movies per month. Figure 3. The full Earthrise acquired using TELE on September 30th, Figure 4. A snapshot of the area around Clabius crater (345 E, 75 S) extracted from the movie obtained by using WIDE on January 19th, 2009 at the spacecraft altitude of 50km. Figure 5 shows the number of the movies obtained throughout the mission. 595 movies and 309 still images are obtained in total. The total number of frames reached 106 million and the amount of the images for each frame reached 6.3 TB in TIFF format. During the nominal mission period, HDTV observation was limited to the acquisition of several movies per month except for the full-earth imaging, be- Figure 6. Coverage of footage obtained by HDTV. The footprint of WIDE movies, TELE movies and sequential still images (WIDE) are shown by the rectangles in yellow, magenta, and cyan colors, respectively. Clementine s LDIM composite map is used as the basemap. Specialized Observations After July 2009, we conducted four types of specialized operation The first was the observation of the lunar surface while the spacecraft attitude was fixed in the inertial coordinate system. This type of attitude control was conducted on July 4th and December 11th in 2008 for the calibration of the other scientific sensors. HDTV conducted observation in both opportunities and acquired dynamic footages in which the lunar

3 horizon traverses in the FOV of HDTV and the images in which the lunar surface was captured in the nadir direction with high spatial resolution. Figure 7 represents an example image of the area around the central peak of Pythagoras acquired during inertial navigation. The spatial resolution of this image was as good as 14m/pixel that is comparable to the resolution of Terrain Camera and Multiband Imager of KAGUYA. horizon were observed. The obtained footage appears similar to the Earth s diamond ring observed during the lunar eclipse on the Earth. Figure 8. The Earth and the Sun rising from the lunar horizon acquired using TELE with gain 0dB at 15:14:25 (UT) on February 9, The third specialized operation was the imaging of the lunar surface at the extremely low altitude. The spacecraft experienced its lowest altitude of 11 km in the middle of April except for the S/C controlled impact operation. We also conducted the HDTV observation at this opportunity. Figure 9 shows a snapshot from the footage of the area around (262 E, 50 S) acquired using WIDE on April 16th, 2009, at the spacecraft altitude of 11km. The best spatial resolution of this image is about 9m/pixel. Small new craters and the structure of the crater wall are well recognized in this image with extremely oblique view. Figure 7. The upper image is a snapshot of the area around the central peak of Pythagoras crater (central position, (63.5 N, -63 E), D=142km) obtained using TELE at 8:36:00 on December 12th, 2009 (UT). The spatial resolution is 14m/pixel. The image in the lower left is the mosaic of lunar orbiter image (photo number IV-176-H1 and IV176-H2) for reference together with the QL images of HDTV in the right. The second specialized operation was the observation of the Earth and the Sun during a penumbral lunar eclipse on February 9, For this observation, the condition in which the Sun was temporarily hidden behind the Moon and only the Earth was observed by HDTV was explored, and found the opportunity at 15:14:25 on February 9, 2009 (UT). The spacecraft attitude was changed so that the Earth was located in the center of the FOV of TELE, and fixed in the inertial coordinate system. The imaging of the Earth and the Sun was then started just before the Earthrise. Figure 8 shows a close-up view of the snapshot extracted from the movie obtained with TELE. The sun-lit Earth s atmosphere and the Sun partially covered by the Earth and the Moon rising from the lunar Figure 9. Image of the area around (262 E, 50 S) acquired at the S/C altitude of 11 km by WIDE camera on April 16th, The last specialized operation was the imaging just prior to the controlled impact of the spacecraft on the lunar surface at 18:25 on June 10th, 2009 (UT). The impact point was calculated to be (65.5 S, 80.4 S) near Gill crater, which was on the night side at impact time unfortunately. Thus the last imaging was conducted 14 minuets prior to the impact while the spacecraft was on the dayside. Since there was no time to transmit the video (it required 20 minutes to transmit 1 minute-long video) to the ground station, a series of operation of acquisi-

4 tion of a still image and its transmission was conducted at the time interval of 1 minute. Figure 10 shows a part of consecutive images acquired by TELE between 18:10:59 and 18:13:59 of June 10th, 2009 while the spacecraft altitude was decreased from 28km to 21 km. Figure 11. Temporal change of the population of white blemish together with the temperature. The data was obtained by dark images acquired at the gain of +12db. Figure 10. Consecutive still images obtained just before controlled impact on the Moon at 18:25 on June 10th, 2009 (UT). Images are taken at one-minute time interval between 18:10:59 and 18:13:59 of June 10th, 2009, by TELE. Status of the Instrument Since HDTV adopted commercial products for CCDs (Panasonic IT CCD), the high production rate of white blemish and the deterioration of images with time were firstly expected (e.g., Sasada et al., 1997, Yamauchi et al. 2001). Thus, a white blemish correction device was developed by NHK in preparation of the data processing on the ground. We also monitored the status of the CCD including the changes of the population of white blemish throughout the mission period, by acquiring the dark images at the gain mode of +12dB with the longest integration time (1/63.3sec). The population of the white blemish was obtained by counting the number of pixels with the intensity larger than 43% of the saturation level, which threshold was adopted to become higher than the nominal dark noise level ranging from 0 to 20%. Note that no correction was conducted for each image so far. Figure 11 shows the temporal change of the population of white blemish of all bands of TELE together with the temperature. The result of the observation suggests that the number of the white blemish had strong temperature dependence, however, if we compare the observations at the similar temperature, it was kept constant despite of the effect of radiation. However, it should be noted that the solar activity was kept at the lowest level during the mission period. Thus the defect of the CCD might increase when the solar activities are at the higher level and with the vigorous radiation effects. In addition, we also observed the increase of dark noise level for CCDs of WIDE, in particular, for blue CCD. We conducted the offset adjustment (referred as ABB; auto black balance) to improve the image quality when the dark noise level exceeded 20% of the saturation level. Conclusion HDTV onboard KAGUYA succeeded in acquiring 595 movies and 309 still images during the 19-month long mission. Throughout the whole mission period, HDTV acquired many valuable images such as full Earth rise/set, the Earth s Diamond ring-like phenomenon, imaging at the altitude as low as 11 km. The quality of the images was kept good during the mission period in spited of the radiation effect in the space. Acknowledgement The authors are grateful to Dr. Y. Iijima, Dr. H. Otake, H. Maejima, Dr. S. Nakazawa and H. Konishi of JAXA for their support on the operation, Mr. H. Terada and Mr. T. Kato of NEC for Earth s Diamond ring observation, Dr. S. Sobue (JAXA) for public outreach, and Prof. A. Nakamura (Kobe Univ.), and Prof. T. Takata (Miyagi Educational Univ.) for their advises on planning. We are also grateful to the engineers of Meisei electronics, Fujinon, Ikegami Tsushinki, SONY, SUGAWA VIDEO ENGINEER-ING, NTSpace, and Fujitsu for their cooperation in the development of HDTV. This research was funded by a grant given by the Houso Bunka Foundation. References R. Honda, J. Yamazaki, S. Mitsuhashi, J. Tachino, and M. Shirao, Proceedings of the 41st ISAS Lunar and Planetary Symposium, 4pp. (2008) S. Mitsuhashi, J. Yamazaki, J. Tachino, M. Yamauchi, K. Tanimoto, R. Honda, M. Shirao, Y. Iijima,.H. Maejima, H. Ohtake, S. Sobue, SMPTE (2008) Sasada, T. Saito, M. Fujii, and N. I. Zelentchicov, Radiation Measurements, 28 (1-6), (1997) G. R. Hopkinson, C. J. Dale, P. W. Marshall, IEEE Transaction on Nuclear Science 43 (2), (1996) M. Yamauchi, J. Yamazaki, T. Watanabe, S. Mitsuhashi, T. Ando, A. Yokota, S. Kuboyama, T. Aburaya, T. Suzuki, Y. Iwata, T. Murakami, ITE Technical Report 25 (75), (2001) (in Japanese)

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