GCOM (Global Change Observation Mission)
GCOM is a JAXA (Japan Aerospace Exploration Agency) observation program consisting of a constellation of two medium-sized spacecraft with the provisional names of GCOM-W and GCOM-C. GCOM is seen as a follow-up program to ADEOS.-II (launch Dec. 14, 2002) with the overall objective to contribute to global change research through long-term (> 10 years) sustained observations with corresponding data sets. Three consecutive constellations of spacecraft are being planned, representing in particular the long-term Japanese contribution to the GEOSS (Global Earth Observation System of Systems) initiative. The prime goal of GEOSS is to achieve comprehensive, coordinated and sustained observations of the Earth environment. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)
The mission of GCOM is to achieve the following objectives:
• The primary goal of GCOM-W, a sea surface observation mission, is to contribute to observations related to global water and energy circulation. The payload consists of an AMSR2 (Advanced Microwave Scanning Radiometer-2) instrument.
• The primary goal of the GCOM-C mission is to contribute to surface and atmospheric measurements related to the climate change with emphasis on the carbon cycle and the radiation budget. The payload consists of a second-generation GLI (SGLI) instrument.
The GCOM series will be maintained toward the final mission goal which should be accomplished by the end of 13 year observations, processing and analysis and application research period. The GCOM program calls for the following goals:
• The establishment of long-term observation system for the global carbon cycle and radiation budget, integrated with other earth observation systems.
• Contribution to numerical climate models (driving force, outputs comparison, and parameter tuning).
• Contribution to operational use (weather forecast, monitoring of meteorological disaster, fishery..).
• Enhancement of new satellite data usability.
1) Global warming
• Understanding of the global warming by global and long-term measurement data on various parameters.
• Separation between natural variability and trends using the data set covering the 27 year period from the launch of the ADEOS or the 21 year period from the launch of the ADEOS-II.
2) Change of land environment
• Understanding of global forest dynamics
• Understanding of snow and ice changes
3) Clarification of sink and source of greenhouse gases.
Table 1: Overview of GCOM series investigation themes 15)
Table 2: Overview of some GCOM spacecraft parameters
Figure 1: Planned launch sequence of the two GCOM mission series of GCOM-W and GCOM-C (image credit: JAXA) 16)
Figure 2: Geophysical parameters in four categories (atmosphere, land, ocean and cryosphere) to be observed by GCOM (image credit: JAXA)
Legend to Figure 2: GCOM-W (blue border) and GCOM-C (green border). SST (Sea Surface Temperature) is observed by both GCOM-W and GCOM-C; hence, its border is a dotted line of blue and green.
GCOM-W1 (Global Change Observation Mission-Water 1) Mission / Shizuku :
The GCOM-W1 mission of JAXA is dedicated to sea surface monitoring and to contribute to observations related to global water and energy circulation. In September 2011, the GCOM-W1 mission received the nickname Shizuku - meaning a "drop" or a "dew" in Japanese.
Figure 3: Artist's view of the GCOM-W1 spacecraft (image credit: JAXA)
JAXA has decided to use medium-scale satellites for GCOM series observations. The GCOM-W1 spacecraft is 3-axis stabilized. The attitude of GCOM-W1 is controlled by 4 reaction wheels in response to the signal from IRU (Inertial Reference Unit) calibrated by Star Trackers and GPS receivers.
The EPS (Electrical Power Subsystem) has 2 redundant systems including batteries and solar paddles, and therefore the satellite can survive even if one solar paddle has a failure. Power of 4.05 kW is provided at EOL (End of Life).
The spacecraft on-orbit dimensions are (deployed configuration): 5.1 m (X) x 17.5 m (Y) x 3.4 m (Z). The spacecraft has a mass of about 1991 kg at launch (dry bus mass of 1324 kg, propellant mass of 151 kg, AMSR2 mass of 405 kg). The design life is 5 years. 17)
The PDR (Preliminary Design Review) of GCOM-W1 took place in March 2008. The spacecraft CDR (Critical Design Review) was completed in December 2009.
GCOM-W1 was integrated in 2010 and has already passed the most of the proto-flight test items in the spring of 2011. The AMSR2 proto-flight model provided desirable characteristics in the ground testing. 18)
Figure 4: Photo of the GCOM-W1 spacecraft (image credit: JAXA)
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 20 Mbit/s without convolution coding and with the QPSK scheme. 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.
Launch: The GCOM-W1 spacecraft (nickname: Shizuku - meaning a “drop” or a “dew”) was launched on May 17, 2012 [UTC, the local time in Japan was 1:39 hours on May 18, JST (Japan Standard Time)] on an H-IIA F21 vehicle from TNSC (Tanegashima Space Center), Japan. Launch provider: Mitsubishi Heavy Industries, Ltd. 19) 20)
Figure 5: Schematic view of the payloads in the H-IIA launch vehicle (image credit: JAXA) 21)
Secondary payloads on this flight were: 22)
• KOMPSAT-3 (Arirang-3) of KARI, Korea with a mass of ~1000 kg. Note: A contract between the launch service provider MHI (Mitsubishi Heavy Industries, Ltd.) and KARI was signed in January 2009. This represents the first satellite launch services order placed to MHI by an overseas customer. 23)
• SDS-4 (Small Demonstration Satellite-4) of JAXA with a mass of ~ 50 kg
• HORUYU-2 of KIT (Kyushu Institute of Technology) with a mass of 7.1 kg.
Orbit: Sun-synchronous orbit, altitude = 699.6 km, inclination = 98.2º, LTAN (Local Time on Ascending Node) at 13:30 hours (to continue the AMRS-E observations). Since the orbit is very similar to the one of the A-Train, it will join the A-Train constellation of NASA. The position of GCOM-W1 in the constellation is a few minutes prior to the Aqua spacecraft (Ref. 1). 24) 25)
Figure 6: The GCOM-W1 (Shizuku) satellite in the A-train constellation (image credit: NASA, JAXA) 26)
Mission status of GCOM-W1/Shizuku:
• The GCOM-W1 spacecraft and its payload are operating nominally in 2014. 28)
• On Dec. 18, 2013, the JAXA/EORC Tropical Cyclone Database of AMSR2 was released. 29)
• October 17, 2013: The GCOM Project Team of JAXA captured the 2013 Nikkei Global Environmental Technology Awards Prize for Excellence for its development of the Global Change Observation Mission 1st – Water “SHIZUKU” (GCOM-W1). 30)
Legend to Figure 8: The white line shows the 30-year (1981-2010) average minimum extent. The data were provided by JAXA from their Global Change Observation Mission-Water (GCOM-W1) satellite’s Advanced Microwave Scanning Radiometer-2 (AMSR2) instrument.
• In late September 2013, the ice surrounding Antarctica reached its annual winter maximum and set a new record. Sea ice extended over 19.47 million km2 of the Southern Ocean. The previous record of 19.44 million km2 was set in September 2012. 32) 33)
Figure 9: Antarctica’s sea ice on Sept. 22, 2013 observed by the AMSR2 instrument on JAXA's GCOM-W1 spacecraft (image credit: NASA, JAXA)
Legend to Figure 9: Antarctica’s sea ice is creeping further out in the ocean! New data from the GCOM-W1 satellite shows that sea ice surrounding the southern continent in late September reached out over 19.47 million km2. The extent — a slight increase over 2012's record of 19.44 million km2 — is the largest recorded instance of Antarctica sea ice since satellite records began, NASA said. While researchers continue to study the forces driving the growth in sea ice extent, it is well understood that multiple factors—including the geography of Antarctica, the region’s winds, as well as air and ocean temperatures—all affect the ice.
• The GCOM-W1/Shizuku spacecraft and its payload are operational in mid-2013. Full operational service (level 2 and level 3 geophysical data products) are available since May 17, 2013. 34)
JAXA has started offering eight kinds of products whose physical quantity concerning water on the Earth, including precipitable water and sea surface temperature, is calculated based on the observation data acquired by the AMSR2 (Advanced Microwave Scanning Radiometer 2) aboard the Shizuku (GCOM-W1) after its initial calibration operation was completed. 35)
These products will contribute to capture environmental changes on a global scale such as worrisome decreasing ocean ice areas in the North Pole as well as the El Nino and La Nina Phenomena. The products can also be utilized for various fields including weather and precipitation forecasts for storms and downpours by global meteorological agencies such as the JMA ( Japan Meteorological Agency) and the U.S. NOAA (National Oceanic and Atmospheric Administration).
• In January 2013, JAXA has started offering brightness temperature products from AMSR-2 after its initial calibration operation was completed and the brightness temperature products (level 1B and level 3) with good quality and stability have been available to users at the GCOM-W1 Data Providing Service since late January, 2013. 36)
• Nov. 2012: The AMSR2 SST (Sea Surface Temperature) product is validated by comparing with various buoy SST observations reported through the GTS (Global Telecommunication System) operated by WMO (World Meteorological Organization). Each match-up data will include AMSR2 footprints around buoy stations within radius of 30 km and 2 hours. Root mean square error (RMSE) between AMSR2 and Buoy SSTs from August to December 2012 is currently 0.56 °C and correlation coefficient (R) is 0.998 (Figure 10). 37)
Figure 10: Early validation results of AMSR2 SST. Upper: AMSR2 SST in descending orbit on Nov. 10, 2012. Lower: Comparison of AMSR2 and buoy observations (image credit: JAXA)
• October 2012: JAXA and JAMSTEC (Japan Agency for Maritime-Earth Science and Technology ) are collaborating in the Earth environment field by combining space and oceanic technologies such as merging data acquired by an Earth observation satellite and on-the-spot data obtained through an observation system deployed in the ocean. 38)
The project started to provide data on the Arctic Ocean area acquired by the Shizuku spacecraft to the NiPR (National Institute of Polar Research) for its voyage to observe and investigate the area using JAMSTEC’s Oceanographic Research Vessel “MIRAI”. The provided data is expected to contribute to NiPR’s research and safe navigation. - The project is sending MIRAI data, including sea ice distribution, ocean temperature distribution and sea ice concentrations observed by Shizuku, which can conduct observations regardless of weather conditions, three times a day. Shizuku’s data is being used to find the best sailing routes and for selecting observation areas.
The data on the Arctic Ocean area acquired this time by MIRAI, including the ocean surface temperature, will also be provided to JAXA in order to verify the accuracy of Shizuku’s data as well as to study environment changes such as analyzing recently decreasing Arctic sea ice (Ref. 38).
• On August 10, 2012, JAXA completed the initial functional verification of GCOM-W1 and has moved to the regular observation operation as scheduled. The project will first perform the initial calibration and checkout during which the acquired data will be compared with observation data on the ground to confirming the data accuracy. 39)
- AMSR-2 standard products will be distributed to public for research and educational purposes though web site (https://gcom-w1.jaxa.jp/) after calibration/validation phase. The current data distribution schedule (Sept. 2012) is to distribute Level 1 products (brightness temperature) to the public in January 2013, and Level 2 (geophysical parameters) in May 2013. 40) 41)
Figure 11: Rainfall image of the Typhoon No.11 (Haikui) approaching the east coast of China, observed by Shizuku on August 7, 2012 (image credit: JAXA) 42)
Figure 12: Image of AMSR2 Level-1B products showing brightness temperature at vertical and horizontal polarization of each frequency bands of one scene on July 22, 2012 (image credit: JAXA, Ref. 41)
• July 4, 2012: JAXA released some observation images of Earth acquired by GCOM-W1 (Shizuku). 43)
Figure 13: Global color composite image observed by AMSR2 (image credit: JAXA)
Legend to Figure 13: The one-day image was observed on July 3, 2012 using the brightness temperature (in vertical and horizontal polarization) of the 89.0 GHz channel and the vertical polarization of the 23.8 GHz channel. In this image, whitish-yellow color parts indicate areas with heavy rain or sea ice, light blue color areas are with little water vapor in the atmosphere or thin clouds, the dark blue color sections are areas with more water vapor in the atmosphere or thicker clouds, and the black color parts are areas that were not observed.
• On June 28, 2012 (UTC), the GCOM-W1 spacecraft was inserted into a planned position on the international A-Train orbit. GCOM-W1 (Shizuku) is flying in front of the Aqua satellite. The satellite will remain in this position until the OCO-2 spacecraft of NASA/JPL joins the constellation sometime in 2014. - JAXA will increase the rotation speed of the AMSR2 aboard the Shizuku from the lower rotation mode (11 rpm) to the regular observation mode of 40 rpm to verify its observation performance. 44)
Figure 14: Artist's view of the A-Train spacecraft (image credit: NASA)
• JAXA confirmed that the solar array paddle deployment was successfully performed for the Global Change Observation Mission 1st - Water "SHIZUKU" (GCOM-W1) somewhere over Australia via image data. 45)
• After the separation of KOMPSAT-3 spacecraft at 16 minutes into the flight, the GCOM-W1 spacecraft separated from the launch vehicle 23 minutes after launch (Ref. 21).
Sensor complement: (AMSR2, a single instrument is flown)
AMSR2 (Advanced Microwave Scanning Radiometer-2):
AMSR2 is a follow-on JAXA radiometer of AMSR and AMSR-E heritage (passive instruments) installed on the ADEOS-II (JAXA) and the Aqua (NASA) missions, respectively. The objective is to achieve measurement of: sea surface temperature (SST), soil water content (moisture), sea wind speed, water equivalent of snow cover, precipitation intensity, sea ice distribution, precipitable water, etc. The observables are the microwave emissions from the atmosphere, ocean, sea ice, and land which are being measured at multiple frequencies. 46) 47) 48) 49) 50) 51)
The following improvements of the AMSR2 instrument were implemented based on experience gained in the AMSR-E mission:
1) Deployable main reflector system with 2.0 m diameter
2) Frequency channel set is identical to that of AMSR-E, except for the additional 7.3 GHz channel for radio frequency interference mitigation
3) Two-point external calibration with the improved HTS. In addition, deep-space maneuver will be considered to check the consistency between the main reflector and CSM (Cold Sky Mirror).
The instrument employs a parabolic offset antenna (antenna aperture of 2 m diameter) providing a conical scan with a swath width of ~ 1450 km (from a 700 km orbit). The incidence angle is 55º nominally. AMSR2 is a total power microwave radiometer with a two point external calibration method:
1) Deep space using a cold sky mirror
2) An on-board hot load.
For the absolute calibration, deep space observations will be done using the main mirror. AMSR2 is able to provide global observations in just 2 days.
The AMRS2 frequency channels are identical to those of AMRS-E except the 7.3 GHz channel which is being used for RFI (Radio Frequency Interference) mitigation in the 6.925 GHz channel. Intensive efforts were made to improve the performance of the HTS (High-Temperature noise Source, a hot load), which has been the greatest challenge in the AMSR-E calibration. A redundant momentum wheel was added to increase the reliability of the instrument.
Table 3: Frequency channels and resolution of the AMSR2 instrument
Figure 15: Channel Specifications of AMSR2 52)
Table 4: Overview of AMSR2 parameters
Figure 16: Three views of the AMSR2 instrument (image credit: JAXA)
Figure 17: Overview and components of the AMSR2 instrument (image credit: JAXA)
Figure 18: Conical scanning configuration of AMSR2 on GCOM-W1 with a swath width of 1450 km (image credit: JAXA)
Figure 19: Artist's view of GCOM-W1with the AMRS2 instrument (image credit: JAXA)
AMSR2 has two calibration targets, named HTS (High Temperature noise Source) and CSM (Cold Sky Mirror). The configuration of these targets are shown in Figure 20.
CSM is a 0.6 m diameter offset parabolic reflector, whose manufacturing process is the same as that of the main reflector to ensure consistency between the two reflectors. The antenna patterns of CSM were measured as is the case with main reflector to be verified the performance requirement. In addition to the antenna pattern, VSWR (Voltage Standing Wave Ratio) of the combination of CSM and each feed-horn was measured (Ref. 18).
Figure 20: Configuration of calibration assembly; TCP and Sun Shade are installed on HTS/AMSR2 only (image credit: JAXA)
HTS is a microwave absorber shrouded in a thermally-controlled box and panel. HTS/AMSR2 thermal design: The accuracy and the reliability of AMSR2 has been improved compared with the designs of AMSR and AMSR-E. There were major problems in HTS thermal design of AMSR and AMSR-E,and they caused large temperature gradient on the absorber surface of HTS. For the thermal design of HTS/AMSR2, the following two major items were requested to improve: (Ref. 47)
1) Uniformity of the absorber surface (2.5ºCp-p)
2) Thermal measurement accuracy (0.4ºC (3σ)).
Uniformity of the absorber surface of HTS/AMSR2.
To improve the temperature uniformity, thermal design of HTS/AMSR2 has been improved. The thermal design of HTS/AMSR, AMSR-E, shown in Figure , has the following features:
- Controlling the absorber temperature using heater rods inside the absorber corns
- The surface at the facing side of the absorber covered with MLI whose temperature was not controlled.
- The thermal connection between the absorber surface and outside(space, sun etc.) was large because there were big chinks between HTS and the MLI of the facing side. This design caused the large temperature gradient on the absorber surface.
The improved thermal designed was analyzed using on-orbit thermal analysis and two thermal vacuum tests. As a result of the on-orbit thermal analysis with the AMSR2 model, which adopted these improvements, the temperature uniformity of the absorber is satisfying the request.
Figure 21: Heater control of HTS/AMSR-E (image credit: JAXA)
To improve the temperature uniformity of its surface, The thermal design around HTS/AMSR2 had been improved in the following items (shown in Figure 22):
- Changing thermal control methodology; covering all aspects of the absorbers with panels whose temperatures are uniformly controlled. (instead of the Heater rod in the AMSR-E/HTS)
- Installing the TCP (Thermal Control Panel) to control the temperature of the facing side of absorber surface
- Installing “Sun Shade” to minimize the sunlight heat input which comes into HTS through the chinks between HTS and TCP.
Figure 22: Heater control of HTS/AMSR2 (image credit: JAXA)
Characterization and Calibration of Early Orbit Data:
By the extensive effort in improving HTS in terms of thermal control mechanism, in-orbit performance of HTS was significantly improved. Therefore, the project applies the simple two-point calibration method for deriving AMSR2 Tbs in the current processing system, with corrections such as for detector non-linearity and antenna beam characteristics (e.g., spillover factor and cross-polarization ratio), based on the pre-launch laboratory measurements and analyses. Also, the measured antenna patterns were used in deriving coefficients to produce Level-1R product. Although the satellite system was designed to enable deep space calibration maneuver during the initial checkout phase, it was cancelled because of the potential risk of strong RFI which could damage the AMSR2 receivers (Ref. 54).
Intercalibration with other microwave radiometers:
Currently (2013), the project is testing several intercalibration methods for version 1.1 AMSR2 Tbs. The first one is to intercalibrate multiple polar orbiting microwave radiometers by utilizing the TRMM (Tropical Rainfall Measuring Mission) TMI (Microwave Imager) as a transfer radiometer. Since TMI can cover various observing local time by TRMM’s precession orbit, one obtains simultaneous observations by TMI and each polar orbiting radiometer and then indirectly compare among polar orbiting radiometers via TMI.
The same concept has been studied and tested by GPM X-CAL team. Because of different sensor characteristics such as in the observing center frequency and Earth incidence angle, these differences must be compensated before comparison. To do this, RTMs (Radiative Transfer Models) are being used and global analysis data produced by meteorological agencies. The project is currently using RTTOV 10.2 distributed by the Satellite Application Facility for Numerical Weather Prediction as RTM 6). In simulating Tbs, we are usingsurface emissivity model/atlas built-in RTTOV10.2: FASTEM 5 for ocean and TELSEM for land surface emissivity. We are using ERA-Interim analysis produced by the European Centre for Medium-Range Weather Forecasts and the Global Daily Sea Surface Temperatures produced by the Japan Meteorological Agency. The analysis procedure is as follows for the case of AMSR2 and TMI intercalibration.
- Create spatio-temporal match-up Tb dataset between AMSR2 and TMI observations.
- Compute differences between observed and calculated Tbs (O-C) for both AMSR2 and TMI, over rainforest and cloud-free/calm ocean areas. Global analysis data and RTM are used to derive calculated Tbs.
- Further create “double difference” to cancel out the differences in frequency and incidence angle: AMSR2 (O-C) – TMI (O-C).
The following parameters are part of the GCOM-W1 standard products: 53)
• Brightness temperature
• Total vapor power
• Total cloud liquid water
• SST (Sea Surface Temperature)
• Sea surface wind speed
• Sea ice concentration
• Snow amount
• Soil moisture
Table 5: Overview of AMSR2 standard products and their target accuracies (Ref. 16)
Product level definition: Table 2 shows a definition of AMSR2 processing levels. Tb products are available in three processing levels: Level-1B, -1R, and -3. The Level-1B product contains Tb values in swath format with native spatial resolution of each frequency channel. On the other hand, the Level-1R product provides resolution-matched Tbs by some sort of beam pattern matching procedure with the Backus-Gilbert method . Because of the significant differences in the spatial resolution, it is not straightforward to combine Tbs at different frequency channels. This Level-1R product aims to ease this difficulty for data users. Four resolution sets (6, 10, 23, 36 GHz) and raw swaths of 89 GHz A/B scan are included in a Level-1R granule. For example, in the 10.65 GHz resolution set, Tbs at 10.65, 18.7, 23.8, 36.5, and 89.0 GHz channels are stored, by lowering the spatial resolutions of the channels at 18.7 GHz and higher. Spatio-temporally averaged Tbs are available in the Level-3 product at 0.1/0.25 degrees and 10/25 km resolutions in the equidistant cylindrical and polar stereo projection methods, respectively. 54)
Table 6: Definition of AMSR2 processing levels
Assessment of the C-band RFI (Radio Frequency Interference):
Figure 23 shows the spatial distribution of the Tb difference between 6.925 and 7.3 GHz vertical polarization channels. Since the frequency difference of these two bands are small, the Tbs emitted from natural targets should be similar. Therefore, large differences may indicate potential RFI signals, although these differences in some areas such as over ice sheet and intense precipitation areas may be the real natural signal. In Figure 23, the red color (blue color) indicates the area where Tbs at 6.925 GHz (7.3 GHz) are potentially contaminated by RFI. A significant amount of RFI signatures over the U.S., Japan, and some parts of Europe to India in the 6.925 GHz vertical polarization channel is similar to those of AMSR-E. The RFI signatures at the 7.3 GHz channels seem to be more widespread. Over land, they are evident over Southeast Asia, Eastern Europe, Russia, and so forth. Also, the frequency of occurrences of the 7.3 GHz RFI is higher over the ocean. However, the most important fact is that the spatial distributions of the RFI signatures at these two bands are quite different. Also, there are still many areas with small Tb difference between the two bands, indicating the areas free from RFI (Ref. 54).
Figure 23: Spatial distribution of AMSR2 Tb difference between 6.925 and 7.3 GHz channels of descending passes on July 25, 2012 (image credit: JAXA)
Figure 24 shows statistics of Tb difference between 6.925 and 7.3 GHz vertical polarization channels from July 2012 to January 2013. From the average panel (upper left), the same characteristics can be confirmed as in Figure 23. Typical RFI signatures indicate periodical fluctuations, some from actual time variations and some from observing the azimuth angle dependence. Therefore, the standard deviation panel (lower felt) also indicates potential RFI signatures.
Figure 24: Spatial distribution of statistics of AMSR2 Tb difference between 6.925 and 7.3 GHz channels of descending passes during from July 2012 to January 2013. Average (upper-left), standard deviation (lower-left), maximum (upper-right), and minimum (lower-right), image credit: JAXA
The maximum (upper-right) and minimum (lower-right) panels show potential RFIs in 6.925 and 7.3 GHz channels, respectively. In addition to RFI over land, clear signatures can be found over ocean, particularly around Japan and Hawaii in the 7.3 GHz channel, and around Ascension island in the 6.925 GHz channel. As shown in the minimum panel over the tropical to mid-latitude areas, Tbs at 7.3 GHz channel are much more sensitive to precipitation signals than at 6.925 GHz. On the other hand, the Tbs at the 6.925 GHz channel are significantly higher than those at the 7.3 GHz channel over the Antarctic and Greenland ice sheets, indicating volume scattering. Probably for similar reasons, the Tbs at 6.925 GHz are slightly higher over the desert, Tibetan Plateau, and high-latitude land regions. Those natural signals should be distinguished from the RFI signals. Currently, the project is testing simple RFI identification methods by using the Tb difference between the 6.925 and the 7.3 GHz channels. The method seems to work well over the United States, where the RFI at the 6.925 GHz channel is dominant. However, it may not work over the areas with RFIs at both frequency channels, such as Eastern Europe and India. To construct a robust method of RFI identification, it would be necessary to use other channels in combination with the Tb difference between the 6.925 and 7.3 GHz channels. The project is working on the revised method to flag the RFI contaminated footprints (Ref. 54).
GCOM-C1 (Global Change Observation Mission- Cimate 1) Mission
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. 55)
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 design life is 5 years.
Figure 25: Illustration of the GCOM-C spacecraft (image credit: JAXA) 56)
Launch: A launch of GCOM-C1 is scheduled for 2016 on an H-IIA vehicle from TNSC (Tanegashima Space Center), Japan.
Orbit: 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..
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. 57)
• 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. 58)
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 26: 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 8). 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 7: Key parameters of the SGLI instrument
Table 8: Radiometric specification of the VNIR channels of SGLI
Table 9: 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. 59) 60) 61)
Figure 27: 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 9). 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 28: 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 10: The SGLI level 2 products (Ref. 1)
GCOM ground segment and data distribution:
There will be two categories of observation data from the GCOM payload instruments: a) the global observation data set, which will be downlinked to the Svalbard station (Spitzbergen, Norway) on every orbit; b) the regional observation data around Japan (a subset), which will be downlinked to the JAXA domestic station on every pass of station visibility.
• The global observation data will be sent from Svalbard to TKSC (Tsukuba Space Center). They will be archived and processed at TKSC and be delivered to researchers and practical fields users.
• The regional observation data around Japan will be sent from the JAXA domestic station to TKSC. They also will be archived and processed at TKSC and be delivered to researchers and practical fields users.
JAXA will provide JMA (Japan Meteorological Agency) and JAFIC (Japan Fisheries Information Service Center) with the observation data of AMSR2 and SGLI, respectively. JMA and JAFIC will use them for weather forecast and sea condition information, respectively.
The TT&C (Telemetry Tracking & Command) data will be downlinked to Svalbard via X-band, and to the JAXA ground network via S-band, and be sent to TKSC. TKSC is in charge of spacecraft monitoring and control including operations planning.
Figure 29: Overview of the GCOM ground segment elements (image credit: JAXA)
Figure 30: Overview of the GCOM-W1 ground segment (image credit: JAXA/EORC)
• The AMSR2 may serve as a potential substitute for JPSS (Joint Polar Satellite System), the former NPOESS MIS (Microwave Imager Sounder).
• SGLI (Second Generation Global Imager) to provide ocean color capability not accomplished by NPP, plus augment other VIIRS capabilities.
GCOM-W and -C cooperation directly contributes to the Disaster, Water, Weather and Climate SBA (Societal Benefit Areas) by providing critical meteorological, climate and environmental observation data. The cooperation also will contribute indirectly to the other SBAs of Health, Energy, Ecosystem, Agriculture and Biodiversity.
Figure 31: Cooperation between JAXA and NOAA regarding AMSR2 data of GCOM-W1 (image credit: JAXA, Ref. 27)
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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.