Trans. JSASS Aerospace Tech. Japan Vol. 16, No. 1, pp. 35-39, 2018 DOI: 10.2322/tastj.16.35 Environmental-data Collection System for Satellite-to-Ground Optical Communications By Kenji SUZUKI, Dimitar KOLEV, Alberto CARRASCO-CASADO, and Morio TOYOSHIMA National Institute of Information and Communications Technology (NICT), Tokyo, Japan (Received June 7th, 2017) In satellite-to-ground optical communications, it can be assumed that the link will be established if site diversity is performed among two or more ground stations connected with the terrestrial network. But by storing the data based on weather survey data quantitatively and statistically, and carrying out analysis processing needs to show the validity. Therefore, the data from ten environmental-data collection stations throughout Japan has been collected and analyzed for a statistically-long time. This paper shows the overview of this system and the analyzed site diversity effect. Key Words: Satellite Communications, Optical Communications, Site Diversity Nomenclature Cr Cloudage Ct Cloud height H Height above sea level Ta : Ground surface temperature Tm : Ground surface average temperature (few-days average value) To : Sky temperature Ts Average sea level temperature verification of the effectiveness of the site diversity technique by the analysis of environmental data accumulated over a statistically-long period of time. For the collection of the environmental data, which is also necessary for practical free-space optical communication, we used the facilities of the National Institute of Information and Communications Technology (NICT) at ten geographically widespread locations at different longitudes and latitudes across the Japanese archipelago. 3)4) 1. Introduction As the resolution of Earth-observation satellite sensors increases, the quantity of data they produce also increases. In an optical-communication system of an Earth-observation satellite, even though communicating with a geostationary data relay satellite at 1.5-2.5 Gbps, the slower feeder downlink from the geostationary data relay satellite to the OGS (Optical Ground Station) implies that not all the acquired data can be downloaded in real time. Further study is needed for a broadband satellite communication system that uses light or millimeter waves for the feeder link from the geostationary data relay satellite to the ground station or for a direct downlink from the Earth-observation satellite to a ground station. However, light and millimeter waves are both susceptible to attenuation by rain or clouds, which can interfere with communications. Research on weather relevant to optical satellite communication, such as the extent of regions of clear skies, includes analysis of digital weather instrument data from the weather satellite Himawari, AMeDAS, and other sources of the Japan Meteorological Agency. 1) 2) However, there has been no verification of the site diversity effect taking actual satellite orbits into account by in-site long-term acquisition, storage, and analysis of data such as the statistical distribution of clear-sky regions, cloudage, and cloud height. We therefore took as our objective the Fig. 1. Configuration of an environmental-data collection system. Copyright 2018 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved. 35
2. Environmental Data Collection System 2.1. Overview Figure 1 shows the configuration diagram of the environmental-data collection system OBSOC (Observation system of the patch of Blue Sky for Optical Communication) installed at ten observation stations located across Japan and an environmental-data collection, analysis and display server installed at a center station. The collected data is transmitted to the center station via a terrestrial network at specified time intervals. The observation stations are controlled entirely from the center station, allowing a fully-automated operation. Figure 2 shows the ten locations at NICT facilities throughout Japan where the observation stations were installed. The locations have a line-of-sight elevation angle of 5 degrees or more to ensure a field of view as wide as possible for the whole-sky camera. When the station is installed on a rooftop, it is placed as far as possible from outdoor units of air conditioning systems to prevent effects on the cloudage and ceilometer sensors. Table 1. Specification of the measurement sensors. Sensor Specification 1) Whole-sky camera VGA image with the color CCD fisheye lens (170deg of viewing angle) 2) Cloudage/Ceilometer Angle of field 60 degrees infrared radiometer. 5 directions of the zenith and the north, south, east and west direction Amount of Cloud: 0~100%±6%, Ceilometer: 0m~8000m±200m 3) Temperature -60 ~60 (±0.2 @20 ) 4) Humidity 0%~100%RH,±1%RH(0~90%), ±1.7%RH(90~100%) 5) Pressure 500~1100hPa±0.15hPa (Resolution: 0.01hPa) 6) Illumination/insolation 0~2000W/m 2 7) Anemoscope/ 50m/sec Max (Survival wind speed Anemometer 100m/sec) 8) Rain gauge Tipping bucket type with heater: ±0.5mm (~20mm), ±3% (20mm~) Fig. 2. Location of the environmental-data collection stations. 2.2. Environmental-data collection stations The environmental-data collection stations are equipped with the sensors listed in Table 1. The data from the whole-sky camera for identifying clear-sky regions (about 200 Kbytes per image), the cloudage and ceilometer instruments, and the various weather instruments is collected at a rate of 263 bytes per measurement. The weather instruments installed in the stations (3 to 5 in Table 1) were of the same grade as certified by the Meteorological Agency to ensure data reliability. A block diagram of the environmental data collection system is shown in Fig. 3. To protect the equipment, which is exposed to the weather outdoors, and to prevent effects from upstream equipment, a discharge-and-insulated-type SPD (Surge Protective Device) is used to protect the network system against lightning strikes, and a discharge-type SPD, breaker and UPS (Uninterruptible Power Supply) are used to protect the power supply. An example of a field installation of an environmental-data collection system is shown in Fig. 4. It is designed to withstand wind speeds of 50 m/s at the installation location with no part of the equipment blown away (maximum instantaneous wind speed; the design for the Okinawa installation is 90 m/s). Fig. 3. Fig. 4. Block diagram of the environmental-data collection system. Environmental-data collection station (Taiki-cho Hokkaido). 2.2.1. Whole-sky camera A highly-sensitive color CCD with a fish-eye lens captures VGA images of the entire sky. The images are output in JPEG compression format. The inside of the dome cannot be fogged by a heater and air-circulation fan that operate according to the outdoor air temperature and dome temperature. Other sensors include proprietary atmospheric 36
pressure sensors and an infra-red thermometer oriented toward the azimuth. The day and night whole-sky camera images acquired at NICT Koganei (Figures 5 and 6) show that regions of clear-sky and clouds are clearly distinguished. Here, a1, b1, a2 and b2 are coefficients estimated from the sea surface mean temperature and linear correlation (a1=0.0457, b1=1.0222, a2=-0.0253, b2=-27.507). Cloud height (Ct) is obtained from the ground-surface temperature and infra-red radiation temperature, with the reference value being -45 C at 8,000 m above sea level. 5) (5) Fig. 5. Whole-sky camera image (daytime). Fig. 7. Cloudage and ceilometer instruments. Fig. 6. Whole-sky camera image (night time). 2.2.2. Cloudage and ceilometer instruments A photograph of the cloudage and ceilometer instruments is shown in Fig. 7. Five infra-red temperature sensors that have a 60-degree field of view are arranged with one oriented to the zenith and the other four oriented to the north, south, east and west at elevation angles of 55 degrees. The sensors have the same specifications for power supply and control communication based on RS485. Figure 8 shows the cloudage (Cr) is obtained by calculating the relation of the ground surface temperature (Ta), sky temperature(to) and the whole-sky camera image data that has been accumulated on the basis of the coefficients of the statistically calculated correlation function. The coefficients a and b obtained by the first order correlation function obtained by statistically calculating the visual cloudage (1 to 10) from whole-sky camera past image data. The cloudage Cr estimation is calculated as follows: (1) a = a1 Ts+b1 (2) b = a2 Ts+b2 (3) Ts = Tm+0.0065 h (4) Fig. 8. Cloudage and cloud height estimation image. 2.2.3. Estimated cloudage and height The environmental trends indicated by the changes in data from the whole-sky camera, zenith-oriented infra-red thermometer, outdoor air temperature, and atmospheric pressure are presented in Fig. 9. The cloudage and cloud height can be estimated from changes in the ground temperature and upper air temperature, within the 60-degree field of view. When the temperature is low and clouds ap- 37
pear in a clear-sky region, the temperature rises and it is judged that clouds are present. Figure 10 shows an example of the whole-sky camera image, the estimated cloudage and the cloud height. The IEEE 1888 FETCH protocol is also supported. Furthermore, a past-data search and display function retrieves search results for the specified search conditions, which include the observation station, the time and day of the observation, and a range of instrument data such as cloudage and cloud height, and displays the results as a list or a graph, or the whole-sky camera images that correspond to the search results. Fig. 9. Environmental-data trends (Kashima). Fig. 11. Real-time environmental data display. (http://sstg.nict.go.jp/obsoc/?lang=e) Fig. 10. Whole-sky camera image, cloudage and cloud height (StarBED). 2.3. Environmental-data collection, analysis and display server The environmental data collected by the system at each observation station is stored on an environmental-data collection, analysis and display server of the center station located at NICT Headquarters, in Koganei (Tokyo). The data is analyzed to determine the degree to which windows for free-space optical communication between satellites and ground stations can be ensured. The environmental-data collection, analysis and display server has the functions described below. 2.3.1. Monitoring and control The environmental-data collection, analysis and display server monitors the situation at the observation stations and sets the data acquisition intervals, etc. 2.3.2. Estimation of the proportion of time in which space-to-ground optical communication is possible The periodically-transmitted environmental data is used to estimate the total cloud cover and clear-sky region ratio (determination of clear-sky regions), visibility, etc. at each observation station, and statistical data processing to determine daily averages, etc. is performed. The resulting data is registered in a database. 2.3.3. Data display function Figure 11 shows the real-time environmental-data display. Trend graphs that include the most recent environmental data for all the observation stations are displayed by an Internet browser (Fig. 12). Fig. 12. Trend graphs of environmental data. 2.3.4. Estimation of laser communication windows for optical communication considering the satellite orbit This function takes the TLE (Two-Line Element) orbit elements of the assumed Earth-observation satellite as the input and estimates the visibility at each observation station (clear-sky regions). In addition to searching and displaying environmental data from the observation stations that are in the range of visibility of the satellite and the observation time, statistical processing of the proportion of times at which optical satellite communication is possible is performed (Fig. 13). 3. Site-diversity Technique Figure 14 shows annual clear-sky rate (1 st June 2014-31 st May 2015) of ten observation stations. In 2016, the rainy season in Kanto region happened from 5 th to 18 th of June. 38
Figure 15 shows the strong impact this season has on light propagation, e.g. the annual clear-sky rate in the Koganei OGS is usually 56.4%, but during the 2016 rainy season was only 11%. Using OBSOC environmental-data to measure the atmospheric transmittance and the cloud coverage applied to the NICT SOTA mission, it is possible to estimate the link probability in each pass over the ground stations (Figure 16). When using 2 OGSs, the probability increases from around 50% up to 77%. If three OGSs can be selected in every pass, the probability increases up to 100%. This means that three OGSs could be enough to mitigate the effects of the clouds completely, even in the rainy season. 6) Fig. 16. Link probability per SOTA link during 2016 rainy season. 4. Conclusion Fig. 13. Estimation of laser communication windows. Satellite optical communications will likely change the way we transmit information from space. However, the atmospheric-related issues are not yet fully solved. Site diversity is the most important technique to guarantee continuous operation. For site-diversity techniques to work properly, a thorough characterization of the local weather of each site is necessary to monitor the conditions in real time and predict the handovers from one ground station to another. In this paper, environmental-data collection system developed in NICT was described. We plan to verify the correlations between the laser wavelengths used in free-space optical communication and environmental data, collect, store and analyze environmental data that cover a period of at least three years, predict site-diversity line of sight at the time the satellite is passing for establishing free-space optical communication between satellites and ground stations, and investigate algorithms for optimum ground station selection. References Fig. 14. Annual clear-sky rate (1st June 2014-31st May 2015). Fig. 15. Clear-sky rate average and rainy season. 1) Takayama, Y., and Toyoshima, M.: Estimation of Frequency of Laser Communications between a Low Earth Orbit Satellite and Ground Stations, IEICE Transactions B, 94 (2011), pp. 402-408 (in Japanese). 2) Ninomiya, H., Takayama, Y., and Fukuchi, H.: Diversity Effects in Satellite-Ground Laser Communications using Satellite Images, AIAA ICSSC-2011, 2011-8033, pp. 1-5. 3) Suzuki, K., Kubooka, T., Fuse, T., Yamamoto, S., Tsuji, H., Morikawa, E., Takayama, Y., Kunimori, H., and Toyoshima, M.: Experimental System for Site Diversity through Statistical Processing of Environmental Data for Optical Communication between Satellites and Ground Stations, IEICE 2013 Society Conference, B-3-21, p229, Sep. 2013 (in Japanese). 4) Suzuki, K., Kubooka, T., Fuse, T., Yamamoto, S., Kunimori, H., and Toyoshima, M.: Environmental-data Gathering System for Satellite-to-Ground Stations Optical Communications, ICSOS2014, Poster Session, May 2014. 5) Suzuki, K., Kunimori, H., and Toyoshima, M.: Report of the Environmental-data Statistical Processing for Satellite-to-Ground Stations Optical Communications, IEICE 2015 Society Conference, B-3-9, p194, Sep. 2015 (in Japanese). 6) Carrasco-Casado, A., Kolev, D., Suzuki, K., and Toyoshima, M.: Improvement of Link Availability with Site-Diversity Techniques and efforts on Standardization of Weather Characterization, IEICE 2017 General Conference, B-3-32, p263, Mar 2017. 39