September 5, 2025 (JST)

Japan Aerospace Exploration Agency

 The Global Observing SATellite for Greenhouse gases and Water cycle “IBUKI GW” (GOSAT-GW)^*1 was successfully launched on June 29, 2025 (Japan Standard Time) and entered its nominal operational orbit on July 20. Antenna of the Advanced Microwave Scanning Radiometer 3 (AMSR3)^*2, which is one of mission payloads, was set to rotate at a speed of 40 revolutions per minute to ensure stable observations, and AMSR3 started collecting observation data^*3 as part of the satellite’s initial functional verification operations activity^*4 since August 11.

 The Advanced Microwave Scanning Radiometer (AMSR) series is a sensor to observe weak microwaves naturally emitted and/or scattered from the land and ocean surfaces and in the atmosphere. Since it can estimate various water-related geophysical parameters, including sea surface temperature and precipitation, its observation data contributes to wide areas, which are operational utilization for weather forecasting, fisheries, safe ship navigation, etc., as well as monitoring of global water cycle variation and climate change. AMSR3 succeeds continuous observations by AMSR-E on board the US Aqua satellite, launched in 2002, and AMSR2 on board the GCOM-W satellite, launched in 2012, and has additional five new channels (see Figure 1, to be more precise, 10.3 GHz Vertical Polarization (V-Pol.) and Horizontal Polarization (H-Pol.)^*5, and V-Pol. of 165.5, 183.3±7 and 183.3±3 GHz).

 Previous microwave radiometers including AMSR2 have difficulty in observing snowfall in high-latitudes, however, observation channels of 165.5 to 183.3 GHz newly added to AMSR3 enables to estimate complete view of global precipitation (both rain and snow) including high-latitudes in detail. Furthermore, those channels can also provide water vapor profile information, which is important for weather forecasting. This is expected to contribute to improving the forecast accuracy of heavy rainfall and extent, track and intensity changes of tropical cyclones. Also, additional observation channels in 10 GHz with improved temperature resolution and reduced noises help to produce robust sea surface temperature, which is utilized in fisheries, in higher spatial resolution.

Figure 2 presents a global observation image captured by AMSR3 onboard “IBUKI GW” (GOSAT-GW,) emphasizing cloud and precipitation regions across the planet. Figure 3 shows detailed observation of the Arctic region, also obtained by AMSR3, highlighting environmental features specific to high-latitude areas.
In addition, Appendix includes the following observation images for reference:
• Figure 4: Data from AMSR3’s 21 observation channels
• Figure 5: AMSR3 sea surface temperature observations
• Figure 6: AMSR3 sea ice observations in both the Arctic and Antarctic regions

Figure 2 is RGB composite image of AMSR3 observation data from August 15 to 17, 2025 by assigning data of 18.7 GHz V-Pol., 89.0 GHz V-Pol., and 165.5 GHz V-Pol. to three primary colors (red (R), blue (B) and green (G), respectively), in visually understandable way. Clouds and precipitation areas are especially highlighted with light green in this composite image. Since the 165.5 GHz channel, newly added to AMSR3 and used in Figure 2, can capture signals from ice particles in the upper layer of clouds, AMSR3 can observe clouds and precipitation areas in more detail. AMSR3 has about 1500 km swath width and covers 99.9 % of the whole Earth’s surface within 3 days as shown in Figure 2. Thus, AMSR3 can be considered the world’s best advanced microwave radiometer thanks to its ability of large area coverage in short-term, high-spatial resolution resulted by large antenna, and multiple observation channels covering low-to-high frequency bands.

 Observation channels of 165.5, 183.3±7 and 183.3±3 GHz (here after called as “high-frequency channels”) are newly added to AMSR3, and they have more sensitivity to ice particles floating inside clouds, surface snowfall and water vapor in mid-to-high layers in the atmosphere compared to observation channels available in the previous sensor, AMSR2. Figure 3 shows RGB composite image focusing on the Arctic on August 16, 2025 (ascending path) using same composite method as Figure 2. To retrieve precipitation, atmospheric analysis data, such as air temperature and water vapor, are used as reference, and precipitation inside yellow dot-line is determined as snowfall and that on outside is determined as rain in this case. In fact, clouds and precipitation have appeared differently in the RGB composite image of Figure 3. Light purple areas inside the yellow dot-line seem to be ice clouds consisting of ice particles (ice crystal and snow) mainly, and light green areas outside are supposed to be rain clouds with rich water particles (cloud droplet, rain). Please note that for Greenland, environmental conditions such as elevation and surface temperature differ from those over sea ice. Consequently, in this RGB composite image, the land surface appears bright blue, making cloud identification difficult. By utilizing high-frequency channel observations in AMSR3, it becomes possible to obtain information on cloud properties (water or ice) and snowfall beneath clouds, which cannot be captured by AMSR2.

 JAXA, in collaboration with the prime contractor Mitsubishi Electric Corporation and various partner companies involved in satellite operations, is currently conducting a three-month initial functional verification phase.
 During this period, it is being confirmed that the satellite “IBUKI GW” (GOSAT-GW) and its onboard sensors are operating nominally as expected. Upon successful verification, JAXA will conduct the initial calibration and validation of the observational data and make transition to the nominal observation operation.
 For the calibration and validation of AMSR3, JAXA will verify the product accuracy in cooperation with AMSR2 and the JAXA’s Global Satellite Mapping of Precipitation (GSMaP)^*7, which will use AMSR3 data as input.

[Appendix]

 As shown in Figure 4, AMSR3 has 21 observation channels within frequency bands from 6.9 to 183.3 GHz including V-Pol. and H-Pol. to observe weak microwaves naturally emitted and/or scattered from the land and ocean surfaces and in the atmosphere. Since each frequency has different sensitivity to different geophysical parameters, it is expected to be utilized in various research and operational application areas related to the atmosphere, ocean, land and cryosphere.

 Since the AMSR series can obtain information under clouds both day- and night-time, it is widely used in oceanic applications. Figure 5 shows sea surface temperature distribution observed by AMSR3 descending paths from August 15 to 17, 2025 and indicates that AMSR3 successfully observes sea surface temperature over the tropical Pacific Ocean where frequently covered by clouds. Please note that light grey areas over the ocean are missing due to heavy rainfall and/or strong wind.

 The region off the coast of Peru in South America, shown in black rectangle in Figure 5, is an important region where El Niño and La Niña phenomena^*8, which affect weather of Japan and/or the world, are generated. In Figure 5, zonally expanded lower temperature signals (yellow-to-orange,) which are typical conditions in non-El Niño period, are well captured in the figure. Channels that have sensitivity to sea surface temperature in higher spatial resolution are enhanced in AMSR3, and further expansion of utilization in abnormal weather prediction and fisheries are expected.

 AMSR3 can observe the Arctic and Antarctic Oceans, which are far from Japan and have difficulty in developing and maintaining in-situ observations. Sea ice covering these polar oceans is a critical and essential indicator for monitoring climate change. Figure 6 shows the sea ice distribution over the Arctic and Antarctic for ascending paths on August 15, 2025, and white-colored areas indicate sea ice extent. The Arctic region, currently in summer like Japan, is approaching the time of year when sea ice extent reaches its minimum. In contrast, the Antarctic region is midwinter and sea ice areas are spreading widely.

 Space-based observations of sea ice have continued since 1978, and the AMSR series has made significant contributions to long-term and high-spatial resolution monitoring of sea ice areas since 2002. Previous observation results have revealed that the Arctic sea ice extent in summer season has decreased by almost 40 % over the past 46 years*9. Now, AMSR3, the latest microwave radiometer, will succeed to the role of sea ice monitoring and provide more detailed information on sea ice conditions by leveraging its full sensor capability. These advanced observations will contribute to continuous monitoring of climate change.