GCOM-C1 (Global Change Observation Mission- Climate 1) Mission/Shikisai
The GCOM-C1 program was approved by Japanese Space Activity Commission in December, 2009.
• The system design and EM design of GCOM-C1 including SGLI started in July 2009
• The SGLI PDR was over in March, 2010. The manufacturing of SGLI EM has been started.
• The CDR (Critical Design Review) of GCOM-C1 satellite system was held in Feb. 2013, and JAXA has started manufacturing the flight model components of GCOM-C1 satellite. 1)
In July 2017, the GCOM-C project received the nickname Shikisai (meaning colors in Japanese). The nickname was chosen by JAXA in a public contest. 2)
The purpose of the GCOM project is the global, long-term observation of the Earth's environment. GCOM is expected to play an important role in monitoring both global water circulation and climate change, and examining the health of Earth from space. 3)
GCOM consists of two satellite series, the GCOM-W and GCOM-C. The GCOM-C, carrying a SGLI (Second generation GLobal Imager), conducts surface and atmospheric measurements related to the carbon cycle and radiation budget, such as clouds, aerosols, ocean color, vegetation, and snow and ice. The GCOM-W, carrying an AMSR2 (Advanced Microwave Scanning Radiometer 2), observes water-related phenomena including precipitation, water vapor, sea surface wind speed, sea surface temperature, soil moisture, and snow depth. Global and long-term observations (10 -15 years) by GCOM will contribute to an understanding of water circulation mechanisms and climate change.
The GCOM-C1 spacecraft is 3-axis stabilized. Power of > 4.25 kW is provided at EOL (End of Life). The spacecraft on-orbit dimensions are (deployed configuration): 4.6 m (X) x 16.3 m (Y) x 2.8 m (Z).
The spacecraft has a mass of about 2093 kg at launch (dry bus mass of 1374 kg, propellant mass of 176 kg, SGLI mass of 400 kg). The satellite generates power of 4 kW. The design life is 5 years.
Figure 1: Photo of GCOM-C after completing environmental testing in May 2017 (image credit: JAXA)
Figure 2: Illustration of the deployed GCOM-C1 spacecraft (image credit: JAXA) 4)
Launch: The GCOM-C1 spacecraft was launched on 23 December 2017 (01:26:22 UTC) on an H-IIA vehicle from the Yoshinobu Launch Complex at TNSC (Tanegashima Space Center), Japan. The launch provider was MHI (Mitsubishi Heavy Industries, Ltd). 5) 6) 7)
H-IIA launch vehicle No. 37 incorporates JAXA's newly developed technology to insert GCOM-C1/Shikisai and SLATS/Tsubame into different orbital altitudes, respectively. It will expand opportunities of multiple satellite launch and take full advantage of the capability of H-IIA.
Orbit of GCOM-C1: Sun-synchronous orbit, altitude = 798 km, inclination of 98.6º, LTDN (Local Time on Descending Node) at 10:30 hours.
RF communications: The S-band is used for TT&C data transmission: TT&C data rates at 29.4 kbit/s (USB), 1 Mbit/s (QPSK,) and 1.6 kbit/s in SSA (S-band Single Access). Command data rates: 4 kbit/s (USB), 125 kbit/s (SSA). The payload data downlink in X-band (8105 MHz) with a data rate of 138.76 Mbit/s, modulation = OQPSK (Offset Quadrature Phase Shift Keying). Direct real-time downlink of payload data to receiving stations with agreement.
Real-time observation data over Japan are transmitted by X-band to JAXA's ground stations at Katsuura, or EOC (Earth Observation Center at Hatoyama, Saitama). The received data are distributed immediately after Level-1 data processing.
Global observation data observed by SGLI are transmitted in X-band to KSAT (Kongsberg Satellite Services) Station in Svalbard, Norway together with some HK data. KSAT is the commercial Norwegian company. GCOM-C1 transmits telemetry stored in the onboard recorder at relatively fast data rate of 1Mbit/s to KSAT/Svalbard by S-band/QPSK..
• SLATS/Tsubame, a minisatellite of JAXA with a launch mass of 400 kg.
- The launch vehicle will insert the SLATS/Tsubame minisatellite into a lower orbit of ~ 500 km.
Figure 3: Launch photo of the GCOM-C1/Shikisai mission on an H-IIA vehicle from TNSC, Japan (image credit: MHI/JAXA)
• March 1, 2019: The onboard sensor SGLI (Second Generation Global Imager) can observe 19 bands of radiations from near-ultraviolet to thermal infrared region (380 nm-12 µm), which yield various physical properties related to cloud, water, snow, ice, aerosol, sea, land, vegetation, biomass, chlorophyll a, and photosynthesis. The spatial resolution and swath of SGLI are 250 m and greater than 1,000 km, respectively; the whole globe can be scanned approximately in every two days. 8)
- SGLI can observe 15 Essential Climate Variables (ECV) such as cloud, aerosols, vegetation, etc. and its data are expected to contribute to improve the projection accuracy of climate change and also to predict fishing grounds, yellow sands, red tides, etc.
- The released products can be downloaded via JAXA G-Portal (https://gportal.jaxa.jp/)
• December 25, 2018: JAXA released the GCOM-C/SGLI products on December 20th, 2018. The products cover the terrestrial, atmosphere, ocean and cryosphere and so on. 9)
- GCOM-C/Shikisai was successfully launched on December 23, 2017 from Tanegashima Space Center. Initial function verification and initial calibration and validation of the satellite and Second-generation Global Imager (SGLI) have been completed and GCOM-C/SGLI products were released.
- JAXA started the observation with SGLI in January 2018 and continues its nominal observation operation.
- JAXA plans to reprocess the past GCOM-C/SGLI products and provide products for the entire observation period by the end of June 2019. To check the reprocess plan, please refer to here.
• October 23, 2018: The GCOM-C mission entered and completed its in-orbit checkout phase, during which the science instruments and satellite systems are evaluated. The in-orbit checkout mission was through by March 2018, ensuring the product verification. The sample data of the GCOM-C standard product is now available on JAXA's G-Portal (Global Portal System). 10)
• August 3, 2018: JAXA has undertaken the initial checkout and calibration (i.e., verification with ground-based data) of GCOM-C1/Shikisai to start the satellite-derived data stream service in December 2018. The operation of the Shikisai satellite has been nominal since the launch on December 23, 2017. On July 25, JAXA started the test stream service measures and has provided Japan Fisheries Information Service Center (JAFIC) with three types of nearly real time data including sea surface temperature. 11)
- Observation images from Shikisai can yield higher resolution images compared with other Earth observation satellites which provide data used for fisheries. Additionally, the satellite's multiple remote sensing, capable of simultaneous observation of ocean color and water temperature, will make the data applicable both to fisheries and marine research. Sea surface temperature in the fishing grounds and other detailed information pertaining to marine environment are expected to advance searching for productive fishing grounds. Data from Shikisai is also expected to enhance the monitoring of coastal environment, making it possible to observe seaweed beds, tidal flats, and algal blooms. It will contribute to the management of coastal marine resources and studies.
- JAFIC will cooperate with JAXA in calibration and verification of the data by supplying on-site surface temperature and other data. The Center will also use the test data distribution to implement the service that provides information with stakeholders such as those engaging in commercial fishing and research institutions. JAXA will use on-site measurements for comparison provided by JAFIC and other sources and continuously ensure the ongoing checkout and calibration of the satellite.
Figure 4: Boso Peninsula SST (Sea Surface Temperature) and the seine fishing ground as observed by Shikisai. In this image, the coolest waters appear in blue, and the warmest temperatures appear in red. Red circles are fishing spots for Japanese pilchard (Sardinops melanostictus). A band of waters at high temperature (in red) along the Japan current lies on the south of the fishing ground. Warm water (orange to green) veers north, countercurrent, off from the Japan current. Cold water (blue) is distributed along the Kashima coast. This suggests fishing grounds are formed where warm waters move north (image credit: JAXA)
Figure 5: Chlorophyll concentrations and algal blooms. The lower the chlorophyll concentration the cooler the color, the higher, the warmer. Algal blooms, commonly known as red tide occurred in the red circle, based on data from Kumamoto Prefecture Fisheries Research Center HP. High chlorophyll concentrations are visible on north and south along the inner Ariake Bay toward the offshore Kumamoto prefecture. In Isahara Bay, too, chlorophyll concentrations are high. The circle where algal blooms occurred, caused by diatoms and other organisms, is located where chlorophyll concentrations are also high (image credit: JAXA)
• GCOM-C1/Shikisai got into steady state operation from 28 March 2018. 12)
• March 23, 2018 (updated July 3 2018): The SGLI (Second Generation Global Imager) instrument aboard the JAXA satellite Global Change Observation Mission-Climate (GCOM-C) is an optical sensor capable of observations at wavelengths ranging from near ultraviolet to thermal infrared (380 nm to 12 µm). SGLI is optimized for polarimetric performance both front and back at red and near infrared wavelengths. Polarimetric measurement can provide data that helps researchers study the properties of light including the oscillation direction of electromagnetic waves, in addition to the magnitude of light. 13)
- These features are expected to characterize aerosols, particulate matter on Earth's surface more accurately. In the ultraviolet-visible-near infrared spectra, the surface reflectance is lower over land than over ocean. Vegetation and land cover affect the space based measurement, resulting in varied readings. Identifying types of aerosol only at the wavelengths is therefore hampered by difficulties.
- However, in the SGLI wavelength regions, the reflectance of the Earth's land is significantly lower. Compared with unpolarimetry, polarimetry is less susceptible to the glare from sunlight reflecting off Earth's surface in the ultraviolet-visible-near-infrared wavelength range. These factors are thought to improve the accuracy of the measurement, enabling to detect the properties of the tiny particles of the atmosphere and to measure the composition and other details of aerosols.
Figure 6: Colored image above China captured by GCOM-C on March 23, 2018 (image credit: JAXA)
• January 12, 2018: JAXA has released some sample observation first-light images of Earth acquired with the GCOM-C/Shikisai mission. Evergreen forests are seen in dark green in the true color image and cannot be discriminated, while in the false color image, evergreen forests are clearly visible in bright green colors (Figure 7). On the other hand, small yellow patches are seen in the enlarged false color image in the lower right of Figure 7. These are golf courses covered with faded grasses on winter. 14) 15)
Figure 7: Left: A true color composite image (reflectances of SGLI VN8, VN5, VN3 channels are assigned to red, green, and blue colors); Center: A false color composite image (reflectances of SGLI VN8, VN11, VN3 channels are assigned to red, green, and blue colors). The images have a resolution of 250 m and were captured over the Kanto area in Japan with SGLI around 10:30 JST on 6 January 2018. Lower Right: detail enlarged composite image (image credit: JAXA/EORC)
- Aerosol images over the Ganges river (Figure 8).
Figure 8: Left: The image is a true color composite (reflectances of SGLI VN8, VN5, VN3 channels are assigned to red, green, and blue colors);Middle: A near-ultraviolet (NUV) image; Right: Degree of polarization (POL) image. The images were captured over the Ganges river, India with SGLI onboard the SHIKISAI around 11:40 (JST) on 03 January 2018. Dense aerosols are seen around the mouth of Ganges river to coastal ocean in the NUV image. In the DPOL image, the solar light reflected at the ocean surface is seen to be highly polarized. SGLI can observe aerosols over land and ocean using the functions of NUV and polarization observations (image credit: JAXA/EORC)
- GCOM-C/Shikisai images of ocean color around Japan (Figure 9).
Figure 9: These images are color composite (reflectances of SGLI VN7, VN6, VN4 channels are assigned to red, green, and blue colors) images around the Island of Tsushima (middle) and around the Kanto area (right) observed with SGLI onboard the SHIKISAI around 11:10 (JST) on 01 January 2018. The locations of the images are shown in the left image. SGLI can observe the spatial distribution of ocean colors with the spectral channels of high sensitivity designed for ocean color observation in order to retrieve the concentrations of suspended matter and phytoplankton in water. These observations are useful for fishery prediction and the monitoring of red tide occurrence (image credit: JAXA/EORC)
Figure 10: This image is a true color composite (reflectances of SGLI VN8, VN5, VN3 channels are assigned to red, green, and blue colors) image of 250 m spatial resolution captured over the Okhotsuk Sea and Japan Islands with SGLI onboard the SHIKISAI around 10:20 (JST) on 6 January 2018. Snow, sea ice, and clouds are shown in white. Land and ocean areas are seen in dark brown and blue colors (image credit: JAXA/EORC)
Figure 11: This image is a false color composite (reflectances of SGLI SW3, VN11, VN8 channels are assigned to red, green, and blue colors) image of 250 m spatial resolution captured over the Okhotsk Sea and Japan islands with SGLI onboard the SHIKISAI around 10:20 (JST) on 6 January 2018. Snow and sea ice are shown in deep blue while water and ice clouds are seen in white and light blue, respectively. Sea ice are formed along the eastern coast of the Eurasia Continents and spreads along the east side of Sakhalin flowing down to the south (image credit: JAXA/EORC)
- The project will continue the initial functional verification (for about three months after launch,) then confirm data accuracy by comparing it with observation data acquired on land, and perform initial calibration and inspection operations including data correction.
• December 24, 2017: JAXA received telemetry data from GCOM-C1 /SHIKISAI and SLATS/TSUBAME, confirming that their satellite attitude control system had transitioned to the steady state. The current status of both satellites is stable. 16)
- Subsequently, the following procedure occurred – power generation that supports the satellites' operation by the deployed solar array wings, ground communications and sound attitude control that maintains those operations. Combined by the completion of the series of other operations, such as powering up of the bus and mission equipment, the satellites have entered the state where they can be sustained in orbit. This concludes their critical operations phase.
- SHIKISAI and TSUBAME will move on to the next operations phase, where the functions of the satellites' onboard apparatus will be examined approximately in the next three-month period.
- JAXA conveys deep appreciation for the support by all for the satellites' launch and tracking.
Figure 12: Eventual operational orbital altitudes of GCOM-C1 and SLATS (image credit: JAXA)
• The reception of telemetry data from JAXA's SHIKISAI satellite was made at 10:44 a.m. (JST, or 19:44 UTC) at the JAXA Mingenew Station, Australia, confirming SHIKISAI's solar array deployment above Australia. 17)
Figure 13: Images captured by the Shikisai onboard cameras following solar array deployment. Left: Solar array paddle 1 (+Y side); Right: Solar array paddle 2 (-Y side), image credit: JAXA
Sensor complement: (SGLI, a single instrument is flown)
SGLI (Second-generation Global Imager):
SGLI is an advanced multi-purpose visible/infrared (VNIR, SWIR, TIR) imager of GLI heritage, flown on ADEOS-II. The objective is to measure ocean color, SST (Sea Surface Temperature), land use and vegetation, snow and ice, clouds, aerosols and water vapor, etc. 18) 19)
• The prime goal of SGLI is to retrieve global aerosol distributions. To achieve this target, SGLI will have 2 polarization channels with 3 directions
• SGLI is mainly focused to land and coastal areas. There are 11 channels with an IFOV of 250 m. GLI on ADEOS-II had only 6 channels of 250 m resolution.
The SGLI assembly features two separate sensors (radiometers) labeled VNIR (Visible Near Infrared) and IRS (Infrared Scanner). Note, the VNIR device is also referred to as VNR in the text.
• VNIR is a pushbroom instrument providing 14 channels in the VNIR spectral region (actually also in the UV), 11 channels are termed VNIR-NP (VNIR Non-Polarized), and 2 channels are called VNIR-P (VNIR-Polarized). The VNIR-P channels of the polarimeter provide 3 polarization angles at: 0º, 60º, and 120º.
The VNIR-NP channels are divided into three 24º pushbroom type telescopes configured in the cross-track direction to realize the wide FOV (70º) requirement with wide spectral range (380 nm to 865 nm). Each telescope has refractive telecentric optics and 11 channels CCD on which the '11 channel bandpass filter assembly' is mounted. 20)
To realize the VNIR-P polarization observation, three linear polarization channels (0º, 60º and 120º) are set for two pushbroom telescopes which are dedicated for 670 nm and 865 nm observation. A tilting operation around the Y-axis of ±45º is required for VNIR-P to observe aerosols (scattering angle requirement). The scattering angle observation is calculated using the satellite orbital position, sun and observation target direction. A scattering angle direction between 60º and 120º is required for the aerosol retrieval over the land surface.
• IRS is a whiskbroom type scanning radiometer (mechanical method) covering the SWIR (Shortwave Infrared) and TIR (Thermal Infrared) spectral regions.
SGLI has a capability of simultaneous nadir and slant observations. In addition, the sensor has a capability of along-track multiangle observation. A chance of multi-angle observations on forest areas with less cloud influence will increase comparisons with cross- track observations. In the GCOM –C1 project, global AGB (Above Ground Biomass) data will be provided as a standard product that is estimated by taking advantage of the multiangle observation capability.
Figure 14: Schematic view of the SGLI instruments (image credit: JAXA)
The key VNIR observation channels such as 670 nm and 865 nm are being observed with both low and high dynamic range independently according to the requirements (Table 2). The total spectral channels for SGLI are optimized to 19 channels including tilting polarization observation (there were 36 channels for GLI instrument). On the other hand, the SGLI standard products are increased from 22 products of GLI to 29 products.
The basic IFOV (Instantaneous Field of View) is set to 250 m - compared to GLI's 1 km requirement. Using this higher resolution with a wide FOV (1150 km for VNR and 1400 km for IRS), it is expected that the human activity influence on Earth's environment can be studied.
Table 1: Key parameters of the SGLI instrument
Table 2: Radiometric specification of the VNIR channels of SGLI
Table 3: Specification of the IRS (SWIR and TIR) channels of SGLI
The optical SGLI instrument is being designed and developed at NEC Toshiba Space, Tokyo, Japan. In turn, NEC Toshiba Space selected Sofradir of France to provide the infrared detectors for SGLI. As of 2008, Sofradir is providing concept studies for the cooled infrared MCT (HgCdTe)focal plane array detectors of the SGLI instrument. The two TIR arrays are centered on 10.8 and 12 µm wavelengths respectively, which are hybridized on a single readout circuit for accurate registration. 21) 22) 23)
Figure 15: Illustration of the SGLI VNIR instrument (image credit: NEC Toshiba, JAXA)
The IRS whiskbroom scanner features six channels in the region of 1.05 µm to 12 µm (Table 3). The 45º tilted scan mirror is rotated around the X-axis continuously to realize a scan of 80º for Earth observation; in addition, the onboard calibrator (blackbody, solar diffuser, and inner light source) and deep space are being scanned on each scanner revolution. Compared with the double-sided mirror employed on GLI and MODIS, the constant incident angle to the IRS scan mirror represents an advantage for the calibration function.
Figure 16: Illustration of the SGLI IRS instrument (image credit: NEC Toshiba, JAXA)
The observation light is directly focused onto the focal plane using a Ritchey-Chretien type telescope without any relay optics. The infrared spectral range is divided by the dichroic filter for the SWIR and TIR regions in order to optimize the detection process.
The four SWIR channels employ an InGaAs photodiode detector array cooled to -30ºC using a Peltier thermo electronic cooler. The two TIR channels use a photovoltaic type HgCdTe (PV-MCT) detector array cooled to 55 K by a Stirling-cycle cooler. The bandpass filters corresponding to the spectral channels are mounted on the focal plane in the detector packages.
The solar diffuser (made of Spectralon), the inner light source using LEDs (Light Emitting Diodes) for the SWIR channels and a high-emissivity blackbody for the TIR channels, are used as the onboard calibrator. These calibration sources and a deep space window, arranged around the scan mirror, make it possible to obtain calibration data on every scan.
Table 4: The SGLI level 2 products 24)
Figure 17: Photo of the SGLI instrument (image credit: JAXA)
Figure 18: Spectral reflectances of several observation targets and the atmospheric transmittance. Locations and widths of the SGLI channels are shown in blue bars. Black dots indicate the channel to be directly used for the retrieval of each SGLI product (image credit: JAXA/EORC)
Ground segment of GCOM-C
The GCOM-C ground system has many subsystems for planning the SGLI observation and satellite operation, tracking control, receiving the SGLI observation data, deriving physical quantities for a product, and distributing our products. The system overview is shown in Figure 19. 25)
Figure 19: GCOM-C ground system overview (image credit: JAXA)
Table 5: The outlines of main component systems
The satellite control system makes the SGLI observation plane. It has a function to adjust observation requests, various constraints such as downlink plan, latency requirements and data storage. The observation plan is sent to mission operation system and used to check the completeness of the planned observations.
GCOM-C observes the Earth based on observation plan uplinked with S-band. The SGLI observation data is downlinked via the Svalbard ground station or the X-band domestic station and transmitted to the GCOM-C mission operation system in Tsukuba Space Center of JAXA. They are processed to be a product and delivered to cooperative agencies and JAXA data distribution system, G-Portal.
Observation data processing: The mission operation system is a key system for observation data processing and products transmission (Figure 20). The system control function in the mission operation system makes the products shown in Table 6 from downlinked data. At this time, the system control function can process the data in consideration of the priority. GCOM-C can use more than 3000 CPU cores and executes parallel processing. This system makes it possible to process and transmit products that satisfy the latency requirements for near-real-time products despite the complex processing flow. The products are kept in large storage more than 2.5-PB and transmitted to users.
The downlinked data are about 100 GB/day and approximately 1 TB products are handled per day. In nominal phase of operations, about 2 PB products will be made.
Figure 20: Process flow in mission operation system (image credit: JAXA)
Table 6: Product levels
GCOM-C/SGLI products: In the level-1 processing, radiometric correction and geometric correction processing are carried out to calculate satellite observation radiance data that are input to higher-level processing. The accuracy of radiance data is maintained by reflecting the SGLI in-orbit evaluation results in the correction parameters. 26) The 28 products of physical quantities (level-2) and statistics (level-3) are made across the land, atmosphere, ocean, and cryosphere using the level-1 and the level-2 product as input. The list of the SGLI level-2 products is shown in Table 7. The processing flow of the level-2 products is shown in Figure 21. The mission operation system controls this complex flow to wait for input products, which is effective for maintaining the accuracy of higher level products.
Table 7: List of SGLI level-2 products
All products are transmitted to the G-Portal and stored in folders for each physical quantity and statistical period. There are many folders compared to other satellite products. There are 156 folders including near-real-time products and standard products. Figure 22 shows samples of the GCOM-C/SGLI principal products. These products are expected to be used in academic research as well as practical use related to fishery and sea passage information, and meteorological prediction.
Future updates are planned two times to improve product accuracy during the five-year mission period. The first update is scheduled in 2020.
Figure 21: The processing flow of level-2 products (image credit: JAXA)
Figure 22: August, 2019, Top: Chlorophyll-a concentration and normalized difference vegetation index. Bottom: Sea and land surface temperature (image credit: JAXA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).