GCOM (Global Change Observation Mission-Water)
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) 13)
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 16)
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) 17)
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. 18)
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. 19) 20)
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. 21) 22)
Figure 5: Schematic view of the payloads in the H-IIA launch vehicle (image credit: JAXA) 23)
Secondary payloads on this flight were: 24)
• 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. 25)
• 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). 26) 27)
Figure 6: The GCOM-W1 (Shizuku) satellite in the A-train constellation (image credit: NASA, JAXA) 28)
Mission status of GCOM-W1/Shizuku:
• October 18, 2017: Every year, the process is generally the same: the cap of sea ice on the Arctic Ocean melts and retreats through spring and summer to an annual minimum extent. Then, as the ocean and air cool with autumn, ice cover grows again and the cycle continues. But when we take a look at smaller regions within the Arctic, we get a more detailed picture of what's been going on. 30)
- The map of Figure 8 shows the extent of Arctic sea ice on September 13, 2017, when the ice reached its minimum extent for the year. Extent is defined as the total area in which the ice concentration is at least 15 percent. The map was compiled from observations by the AMSR-2 (Advanced Microwave Scanning Radiometer 2 instrument on the GCOM -W1 (Global Change Observation Mission 1st-Water)/Shizuku satellite mission operated by JAXA (Japan Aerospace Exploration Agency). The yellow outline in Figure 8 shows the median sea ice extent observed in September from 1981 through 2010.
- According to the NSIDC (National Snow and Ice Data Center) in Boulder, CO, the Arctic sea ice cover in 2017 shrank to 4.64 million km2, the eighth-lowest extent in the 39-year satellite record. Charting these annual minimums and maximums has revealed a steep decline in overall Arctic sea ice in the satellite era. But the decline is not the same everywhere across the Arctic Ocean. The Beaufort Sea north of Alaska, for example, is the region where sea ice has been retreating the fastest.
Figure 9: Regional ice extent in arctic seas, acquired in the period June 20-October 10, 2017 and analyzed by NSIDC (image credit: NASA Earth Observatory, images by Joshua Stevens, using data from the National Snow and Ice Data Center, story by Kathryn Hansen)
- This year, ice in the Chukchi and Beaufort and seas reached their minimum extents toward the end of September, later than the Arctic as a whole. The graph of Figure 9 shows the ice in these two seas was still declining while other regions had started freezing. The melting persisted the longest in the Beaufort Sea, which finally started to refreeze after reaching a minimum on September 27. Data for the graph come from the NSIDC MASIE (Multi-sensor Analyzed Sea Ice Extent) product, which is based on operational sea ice analyses produced by the U.S. National Ice Center. Note: the MASIE observations included also the SSM/I instrument on the DMSP series. In addition, in situ measurements were used.
- Ice loss is the Beaufort and Chukchi seas was not record-breaking this year, but the extents were much lower than usual. Notice in the map how the ice edge in these seas was farther north than average. According to Walt Meier, a scientist at the NSIDC (National Snow and Ice Data Center), the Chukchi and Beaufort seas entered the melt season with a lot of first-year ice. This ice type is generally thinner than multi-year or perennial ice (which survived the previous melt season); first-year ice tends to melt away more easily.
- Meier also notes that low-pressure weather systems persisted near the North Pole for much of the summer. "Low pressure will keep things cooler overall and generally will lead to a relatively higher ice extent overall," Meier said. "However, the position of the low this year led to winds blowing from the south and west that help move ice out of these regions. Also, the winds may have helped to bring in warmer ocean waters from the Bering Strait region as well."
• July 26, 2017: An iceberg about the size of Mie Prefecture of Japan split off from Antarctica's Larsen C iceberg on July 12, 2017. The AMSR-2 (Advance Microwave Scanning Radiometer-2) on JAXA's Shizuku satellite captured the calving of the close to 5,800 km2 chunk of ice. The nascent iceberg created by the rift is estimated to weigh over a trillion tons. 31)
- AMSR-2 was instrumental in grasping this major calving event. It can turn to and observe the same area a few times a day regardless of time and weather. Antarctica is currently in its winter, the season of six-month-long darkness. AMSR-2's high temporal resolution could monitor the progression of the rift despite the absence of light, which the traditional optical sensors cannot.
Figure 10: AMSR-2 observation sequence of the Larsen-C iceberg in the period July 6 to 20, 2017 (image credit: JAXA)
• February 21, 2017: Global sea ice extent hit record low, according to observations from Shizuku on GCOM (Global Change Observation Mission) on January 14, 2017. It is an all time low in the history of satellite operation that started in 1978, JAXA continues operation of Shizuku and GCOM-C and monitoring arctic sea ice extent, off the coast of Greenland Sea and the rest of the arctic circle. 32)
Figure 11: Sea ice extent of Antarctica observed by Shizuku on January 14, 2017 (image credit: JAXA)
• The GCOM-W1/Shizuku mission is operational in 2016. 33)
Figure 12: Sample AMSR2 weekly image in the timeframe Feb. 5-11, 2016 (image credit: JAXA/EORC)
• November 2, 2015: JAXA/EORC (Earth Observation Research Center) has developed GSMaP (Global Satellite Mapping of Precipitation) realtime version (GSMaP_NOW) providing rainfall information of current hour, and released those information through a new webpage "JAXA Realtime Rainfall Watch". While GSMaP near-real-time version (GSMaP_NRT) is provided with a 4 hour data latency, which consists of 3 hour for data gathering and 1 hour for processing, GSMaP_NOW is provided in quasi-realtime and updated every half hour. For example, hourly GSMaP_NOW image and data during 09:30Z and 10:29Z is available at around 10:30Z through the web site. 34) 35)
- GSMaP_NOW produces rainfall map over the area of geostationary satellite "Himawari", using passive microwave observations that are available within half-hour after observation (GMI, AMSR2 near Japan, and AMSU direct receiving data). Furthermore, half-hour extrapolation of rainfall map toward future direction by using cloud moving vector from the geostationary satellite allows us to estimate "current" rainfall map at every half-hour.
Figure 13: Example image of the web site "JAXA Realtime Rainfall Watch" (image credit: JAXA/EORC)
• September 5, 2014: NOAA has begun to routinely utilize observational data acquired by the AMSR2 (Advanced Microwave Scanning Radiometer-2) aboard GCOM-W1/Shizuku to monitor the land, ocean and atmosphere globally and around the U.S. 36)
- NOAA is studying the possibility of utilizing data acquired by the AMSR2 instrument aboard the Shizuku as well as by the DPR (Dual-frequency Precipitation Radar) on the GPM (Global Precipitation Measurement) satellite. Both the AMSR2 and DPR instruments were developed by JAXA, and JAXA and NOAA have been discussing cooperation on the projects. As a first step, AMSR2 data are regularly provided to the U.S. for weather forecast applications. NOAA's recently signed MOU (Memorandum of Understanding) with NASA on the NASA-JAXA GPM mission will allow NOAA to receive the DPR data. As part of NOAA's participation with NASA on the GPM Science Team, a few NOAA investigators are testing the feasibility of utilizing the spaceborne radar information from DPR and its predecessor, the PR (Precipitation Radar) flown on the NASA-JAXA TRMM (Tropical Rainfall Measuring Mission) satellite, in regional forecast models to improve the forecasting of tropical systems.
- NOAA's National Hurricane Center is now routinely utilizing AMSR2 data, which have contributed to improvements in the center location and structure analysis for a number of hurricanes as shown in Figure 14. The cloud image captured through infrared observation (right) cannot clarify the internal structure of a typhoon or hurricane, but the microwave observation (left) clearly shows the dual structure of the eye and surrounding rain bands of Hurricane Arthur off the U.S. East coast through its observation of horizontally polarized brightness temperature (89 GHz-H) being naturally emitted from the Earth's surface and the atmosphere.
• July 2014: GCOM-W1 data is being captured at the KSAT Svalbard ground station and assembled into APID packets. Using the JPSS (NPP) infrastructure, the GCOM raw data APID packets are routed to the NOAA IDPS (Interface Data Processing System) in near-realtime. Once received at the IDPS, the APID packets are reformatted into RDRs (Raw Data Records) and sent to the NDE (NPP Data Exploitation) system for distribution to the Environmental Satellite Data Processing System where further processing to brightness temperatures (Level 1)/SDRs (Sensor Data Records) and geophysical products (Level 2)/EDRs (Environmental Data Records) are performed. The RDRs are processed to SDRs utilizing software provided by JAXA. The EDRs are generated utilizing NOAA's AMSR2 product processing system. - The goal of the product processing system is to provide validated operational L2 products from the AMSR2 instrument that address the GCOM-W1 requirements in the JPSS L1RD supplemental for distribution to operational users. 37)
• July 2014: AMSR2 standard products are distributed through the GCOM-W1 Data Providing Service . Level 1 data has been distributed to public since January 25, 2013, and Level 2 and 3 products since May 18, 2013. The GCOM-W1 Data Providing Service System also distributes AMSR and AMSR-E standard products. Also, registered users can obtain near-realtime data by applying special user form. 38)
• In Feb. 2014, the U.S. NOAA (National Oceanic and Atmospheric Administration) announced that it will utilize observation data acquired by the AMSR2 instrument aboard GCOM-W1, starting in June 2014, to monitor the birth and development of a tropical low pressure system. JAXA and NOAA signed the MOU (Memorandum of Understanding) on GCOM data application in 2011. 39)
• The GCOM-W1 spacecraft and its payload are operating nominally in 2014. 40)
• On Dec. 18, 2013, the JAXA/EORC Tropical Cyclone Database of AMSR2 was released. 41)
• 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). 42)
Figure 15: This image shows the extent of Arctic sea ice on September 12, 2013, the day before NSIDC estimated sea ice extent hit its annual minimum (image credit: NASA) 43)
Legend to Figure 15: 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. 44) 45)
Legend to Figure 16: 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. 46)
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. 47)
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).
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. 48)
• 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 17). 49)
• 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. 50)
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. 50).
• 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. 51)
- 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. 52) 53)
Figure 18: Rainfall image of the Typhoon No.11 (Haikui) approaching the east coast of China, observed by Shizuku on August 7, 2012 (image credit: JAXA) 54)
Figure 19: 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. 53)
• July 4, 2012: JAXA released some observation images of Earth acquired by GCOM-W1 (Shizuku). 55)
Legend to Figure 20: 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. 56)
Figure 21: 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. 57)
• 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. 23).
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. 58) 59) 60) 61) 62) 63)
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 22: Channel Specifications of AMSR2 64)
Table 4: Overview of AMSR2 parameters
Figure 23: Three views of the AMSR2 instrument (image credit: JAXA)
Figure 24: Overview and components of the AMSR2 instrument (image credit: JAXA)
Figure 25: Conical scanning configuration of AMSR2 on GCOM-W1 with a swath width of 1450 km (image credit: JAXA)
Figure 26: 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 27.
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. 19).
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. 59)
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 28: 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 29):
- 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.
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. 66).
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: 65)
• 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. 17)
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. 66)
Table 6: Definition of AMSR2 processing levels
Assessment of the C-band RFI (Radio Frequency Interference):
Figure 30 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 30, 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. 66).
Figure 31 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 30. 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 31: 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. 66).
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 32: Overview of the GCOM ground segment elements (image credit: JAXA)
Figure 33: 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 34: Cooperation between JAXA and NOAA regarding AMSR2 data of GCOM-W1 (image credit: JAXA, Ref. 29)
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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).