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The advanced microwave sounding unit (AMSU) is a multi-channel microwave radiometer installed on meteorological satellites. The instrument examines several bands of microwave radiation from the atmosphere to perform atmospheric sounding of temperature and moisture levels.
Level-1 radiance data are calibrated brightness temperatures.
Level-2 geophysical data from AMSU include:
AMSU data is also used together with infrared radiances from HIRS, AIRS, or IASI to produce blended MW/IR level-2 geophysical products such as:
AMSU data is used extensively in weather prediction. Brightness temperatures are processed as quickly as possible and sent to numerical weather prediction (NWP) centers around the world. This data helps keep the assessment of the current state of the atmosphere correct, which in turn helps make predictions more accurate.
Long-term AMSU records are also used in studies of climate.
AMSU-A + AMSU-B have flown together on the 3 NOAA KLM satellites: NOAA-15 (NOAA K), launched May 13, 1998; NOAA-16 (NOAA L), launched September 21, 2000; and NOAA-17 (NOAA M), launched June 24, 2002.
For NOAA-18 (NOAA N), launched May 20, 2005, AMSU-B was replaced by a similar instrument, the Microwave Humidity Sounder (MHS).
Versions of AMSU-A also fly on the NASA Aqua Earth science satellite and the EUMETSAT MetOp series. On these spacecraft, AMSU-B is replaced by similar microwave humidity sounders: HSB for Aqua and MHS for MetOp.
The AMSU was an improvement of the Microwave Sounding Unit (MSU), incorporating capabilities of the Stratospheric Sounding Unit (SSU), both of which had flown on TIROS-N in 1978 and continued on the NOAA-6 through NOAA-14 satellites. The next generation in the family is the Advanced Technology Microwave Sounder (ATMS), first flown on Suomi-NPP in 2011 and now standard equipment on the JPSS series of satellites, the first of which, NOAA-20, launched in 2017.
AMSUs were manufactured by Aerojet Corporation in Azusa, California. This organization is now part of Northrop Grumman Electronic Systems.
It is preferred that AMSUs always situated on polar-orbiting satellites in Sun-synchronous orbits, however NOAA-15 to NOAA-19 all had drifting LTANs. In the case of Metops and EOS Aqua this results in their crossing the equator at the same two local solar times every orbit. For example, EOS Aqua crosses the equator in daylight heading north (ascending) at 1:30 pm solar time and in darkness heading south (descending) at 1:30 am solar time.
The AMSU instruments scan continuously in a "whisk broom" mode. During about 6 seconds of each 8-second observation cycle, AMSU-A makes 30 observations at 3.3° steps from −48° to +48°. It then makes observations of a warm calibration target and of cold space before it returns to its original position for the start of the next scan. In these 8 seconds the subsatellite point moves about 45 km, so the next scan will be 45 km further along the track. AMSU-B meanwhile makes 3 scans of 90 observations each, with a spacing of 1.1°.
During any given 24-hour period there are approximately 14 orbits. Almost the entire globe is observed in either daylight or nighttime mode, many in both. Polar regions are observed nearly every 100 minutes.
The AMSU has two sub-instruments, AMSU-A and AMSU-B. AMSU-A has 15 channels between 23.8 and 89 GHz, and is used primarily for measuring atmospheric temperatures (known as "temperature sounding"). It has a ground resolution near nadir of 45 km. AMSU-B, with five channels between 89 and 183.3 GHz, has a spatial resolution near nadir of 15 km and is primarily intended for moisture sounding. Spot size of both sub-instruments becomes larger and more elongated toward the edges of the swath. When the two instruments are used together, there are roughly 9 AMSU-B fields-of-view in a 3x3 array corresponding to each AMSU-A field-of-view. This reflects the higher spatial variability of water vapor compared to temperature. HIRS/3 infrared sounders with the same spatial resolution as AMSU-B are also included on NOAA 15–17 satellites and are used together with AMSU-A and AMSU-B. Together the three instruments form ATOVS, the Advanced TIROS Operational Vertical Sounder.
The Aqua and MetOp AMSU-A instruments are 15-channel microwave sounders designed primarily to obtain temperature profiles in the upper atmosphere (especially the stratosphere) and to provide a cloud-filtering capability for tropospheric temperature observations. The EOS AMSU-A is part of a closely coupled triplet of instruments that include the AIRS and HSB. The MetOp AMSU-A similarly works with HIRS, IASI, and MHS. MHS and HSB are variants on AMSU-B.
Table 1 Radiometric characteristics of the AMSU-A (from 3,5).
| Channel Number | Frequency (GHz) | Polarization (at nadir) | Number of Bands | Instrument Sensitivity | Primary Function |
| 1 | 23.8 | vertical | 1 | 0.30 | Water Vapor Burden |
| 2 | 31.4 | vertical | 1 | 0.30 | Water Vapor Burden |
| 3 | 50.3 | vertical | 1 | 0.40 | Water Vapor Burden |
| 4 | 52.8 | vertical | 1 | 0.25 | Water Vapor Burden |
| 5 | 53.596 ± 0.115 | horizontal | 2 | 0.25 | Tropospheric Temperature |
| 6 | 54.4 | horizontal | 1 | 0.25 | Tropospheric Temperature |
| 7 | 54.94 | vertical | 1 | 0.25 | Tropospheric Temperature |
| 8 | 55.5 | horizontal | 1 | 0.25 | Tropospheric Temperature |
| 9 | 57.290 | horizontal | 1 | 0.25 | Stratospheric Temperature |
| 10 | 57.290 ± 0.217 | horizontal | 2 | 0.40 | Stratospheric Temperature |
| 11 | 57.290 ± 0.3222 ± 0.048 | horizontal | 4 | 0.40 | Stratospheric Temperature |
| 12 | 57.290 ± 0.3222 ± 0.022 | horizontal | 4 | 0.60 | Stratospheric Temperature |
| 13 | 57.290 ± 0.3222 ± 0.010 | horizontal | 4 | 0.80 | Stratospheric Temperature |
| 14 | 57.290 ± 0.3222 ± 0.0045 | horizontal | 4 | 1.20 | Stratospheric Temperature |
| 15 | 89.0 | vertical | 1 | 0.50 | Cloud Top/Snow |
Table 2 Radiometric characteristics of the AMSU-B (from 4).
| Channel Number | Frequency (GHz) | Polarization (at nadir) | Number of bands | Instrument Sensitivity |
| 16 | 89.9 ± 0.9 | vertical | 2 | 0.37 |
| 17 | 150 ± 0.9 | vertical | 2 | 0.84 |
| 18 | 183.31 ± 1.00 | vertical | 2 | 1.06 |
| 19 | 183.31 ± 3.00 | vertical | 2 | 0.70 |
| 20 | 183.31 ± 7.00 | vertical | 2 | 0.60 |
Aqua is a NASA scientific research satellite in orbit around the Earth, studying the precipitation, evaporation, and cycling of water. It is the second major component of the Earth Observing System (EOS) preceded by Terra and followed by Aura.
The Advanced Very-High-Resolution Radiometer (AVHRR) instrument is a space-borne sensor that measures the reflectance of the Earth in five spectral bands that are relatively wide by today's standards. AVHRR instruments are or have been carried by the National Oceanic and Atmospheric Administration (NOAA) family of polar orbiting platforms (POES) and European MetOp satellites. The instrument scans several channels; two are centered on the red (0.6 micrometres) and near-infrared (0.9 micrometres) regions, a third one is located around 3.5 micrometres, and another two the thermal radiation emitted by the planet, around 11 and 12 micrometres.
NOAA-19, known as NOAA-N' before launch, is the last of the American National Oceanic and Atmospheric Administration (NOAA) series of weather satellites. NOAA-19 was launched on 6 February 2009. NOAA-19 is in an afternoon Sun-synchronous orbit and is intended to replace NOAA-18 as the prime afternoon spacecraft.
NOAA-17, also known as NOAA-M before launch, was an operational, polar orbiting, weather satellite series operated by the National Environmental Satellite Service (NESS) of the National Oceanic and Atmospheric Administration (NOAA). NOAA-17 also continued the series of Advanced TIROS-N (ATN) spacecraft begun with the launch of NOAA-8 (NOAA-E) in 1983 but with additional new and improved instrumentation over the NOAA A-L series and a new launch vehicle.
NOAA-16, also known as NOAA-L before launch, was an operational, polar orbiting, weather satellite series operated by the National Environmental Satellite Service (NESS) of the National Oceanic and Atmospheric Administration (NOAA). NOAA-16 continued the series of Advanced TIROS-N (ATN) spacecraft that began with the launch of NOAA-8 (NOAA-E) in 1983; but it had additional new and improved instrumentation over the NOAA A-K series and a new launch vehicle. It was launched on 21 September 2000 and, following an unknown anomaly, it was decommissioned on 9 June 2014. In November of 2015 it broke up in orbit, creating more than 200 pieces of debris.
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The atmospheric infrared sounder (AIRS) is one of six instruments flying on board NASA's Aqua satellite, launched on May 4, 2002. The instrument is designed to support climate research and improve weather forecasting.
The Humidity Sounder for Brazil (HSB) was an instrument launched on NASA's Earth Observing System satellite Aqua launched in May 2002. It was a four-channel passive microwave radiometer, with one channel at 150 GHz and three channels at 183 GHz. It was very similar in design to the AMSU-B instrument, except it lacked the 89 GHz surface sounding channel. It was intended to study profiles of atmospheric water vapor and provide improved input data to the cloud-clearing algorithms in the Unified AIRS Retrieval Suite, but the scan mirror motor failed on February 5, 2003. It worked with the Atmospheric Infrared Sounder and AMSU-A to form the AIRS Sounding Suite.
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NOAA-13, also known as NOAA-I before launch, was an American weather satellite operated by the National Oceanic and Atmospheric Administration (NOAA). NOAA-I continued the operational, polar orbiting, meteorological satellite series operated by the National Environmental Satellite System (NESS) of the National Oceanic and Atmospheric Administration (NOAA). NOAA-I continued the series (fifth) of Advanced TIROS-N (ATN) spacecraft begun with the launch of NOAA-8 (NOAA-E) in 1983. NOAA-I was in an afternoon equator-crossing orbit and was intended to replace the NOAA-11 (NOAA-H) as the prime afternoon (14:00) spacecraft.
NOAA-7, known as NOAA-C before launch, was an American operational weather satellite for use in the National Operational Environmental Satellite System (NOESS) and for the support of the Global Atmospheric Research Program (GARP) during 1978-1984. The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment. An earlier launch, NOAA-B, was scheduled to become NOAA-7, however NOAA-B failed to reach its required orbit.
NOAA-6, known as NOAA-A before launch, was an American operational weather satellite for use in the National Operational Environmental Satellite System (NOESS) and for the support of the Global Atmospheric Research Program (GARP) during 1978–1984. The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment.
NOAA B was an American operational weather satellite for use in the National Operational Environmental Satellite System (NOESS) and for the support of the Global Atmospheric Research Program (GARP) during 1978-1984. The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment.
NOAA-8, known as NOAA-E before launch, was an American weather satellite operated by the National Oceanic and Atmospheric Administration (NOAA) for use in the National Environmental Satellite Data and Information Service (NESDIS). It was first of the Advanced TIROS-N series of satellites. The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment.
NOAA-10, known as NOAA-G before launch, was an American weather satellite operated by the National Oceanic and Atmospheric Administration (NOAA) for use in the National Environmental Satellite Data and Information Service (NESDIS). It was the third of the Advanced TIROS-N series of satellites. The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment.
NOAA-11, known as NOAA-H before launch, was an American weather satellite operated by the National Oceanic and Atmospheric Administration (NOAA) for use in the National Operational Environmental Satellite System (NOESS) and for support of the Global Atmospheric Research Program (GARP) during 1978–1984. It was the fourth of the Advanced TIROS-N series of satellites. The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment.
NOAA-12, also known as NOAA-D before launch, was an American weather satellite operated by National Oceanic and Atmospheric Administration (NOAA), an operational meteorological satellite for use in the National Environmental Satellite, Data, and Information Service (NESDIS). The satellite design provided an economical and stable Sun-synchronous platform for advanced operational instruments to measure the atmosphere of Earth, its surface and cloud cover, and the near-space environment.
NOAA-14, also known as NOAA-J before launch, was an American weather satellite operated by the National Oceanic and Atmospheric Administration (NOAA). NOAA-14 continued the third-generation operational, Polar Orbiting Environmental Satellite (POES) series operated by the National Environmental Satellite Service (NESS) of the National Oceanic and Atmospheric Administration (NOAA). NOAA-14 continued the series of Advanced TIROS-N (ATN) spacecraft begun with the launch of NOAA-8 (NOAA-E) in 1983.
The Advanced Technology Microwave Sounder (ATMS) is a 22-channel scanning microwave radiometer for observation of the Earth's atmosphere and surface. It is the successor to the Advanced Microwave Sounding Unit (AMSU) on NOAA weather satellites. ATMS units have been flown on the Suomi NPP and on the Joint Polar Satellite System.