The HSB Instrument

image of EOS Aqua Humidity Sounder for Brazil (HSB) before installation

The Humidity Sounder for Brazil (HSB) is a 4-channel microwave moisture sounder. HSB has four channels. Three channels span the strongly opaque water vapor absorption line at 183 GHz and provide data on the atmosphere's humidity level. The fourth channel at 150 GHz enables deeper penetration through the atmosphere to the Earth's surface. Physically HSB is almost identical to AMSU-B, but HSB lacks the fifth channel (89 GHz) of AMSU-B. Like AIRS, HSB has a cross-track scanner and has only one parabolic scan antenna The scan speed and footprint size is similar to AIRS (three scans per 8 seconds and about 15 km at nadir, respectively). There is therefore one HSB footprint per AIRS footprint. For information on AMSU-B, see here.

The U.K.-based Matra Marconi Space (becoming Astrium in 2000) was the prime contractor for HSB, A number of British and Brazilian subcontractors also contributed.

HSB instrument state

The HSB instrument failed on February 5, 2003. An anomaly investigation team concluded the most likely failure cause was a bad connection or solder joint in the motor drive electronics commutation circuit. The investigation team replicated the symptoms seen on orbit on an engineering model.

The AMSU-A Instrument

AMSU-A, part of the AIRS Project Instrument Suite, is a microwave temperature sounder implemented as two independently operated modules. AMSU-A also measures surface and moisture information. AMSU-A is one of a series of similar instruments. Aerojet (now part of Northrop Grumman) built AMSU-A.

  • EOS Aqua AMSU-A1, wrapped for transport
  • EOS Aqua AMSU-A1, ready for installation on EOS Aqua
  • Photo of an AMSU-A2 unit built for NOAA. The EOS Aqua AMSU-A2 unit is missing the compensator motor shown here

Wavelengths, Channels, and Measurements

AMSU-A has a total of 15 channels:

  • Module 2 (AMSU-A2) has 2 channels (23.8 GHz and 31.4 GHz, numbered 1-2) providing surface and moisture information (total precipitable water and cloud liquid water).
  • Module 1 (AMSU-A1) has 12 channels (numbered 3-14) in the 50-58 GHz oxygen absorption band, providing the primary temperature sounding capability.
  • Module 1 also has 1 channel at 89 GHz (numbered 15) providing surface and moisture information

See Spectral Coverage and What is Sounding? for more information.

Channel Frequency

Channel Frequency
Polarization at Nadir
1 23,800 270 V
2 31,400 180 V
3 50,300 180 V
4 52,800 400 V
5 53596±115 170 H
6 54,400 400 H
7 54,940 400 V
8 55,500 330 H
9 f0 = 57,290.344 330 H
10 f0 ± 217 78 H
11 f0 ± 322.2 ± 48 36 H
12 f0 ± 322.2 ± 22 16 H
13 f0 ± 322.2 ± 10 8 H
14 f0 ±3 22.2 ± 4.5 3 H
15 89,000 < 6,000 ​ V


Like AIRS, AMSU-A is a cross-track scanner. The three receiving antennas, two for AMSU-A1 and one for AMSU-A2, are parabolic focusing reflectors that rotate to scan.

AMSU-A scans once per 8 seconds, three times more slowly than AIRS. The footprints are approximately three times as large in diameter as those of AIRS (45 km at nadir). This results in three AIRS scans per AMSU-A scan and nine AIRS footprints per AMSU-A footprint. See Footprints and Earth Scan for more information.

AMSU-A instrument state

The AMSU-A status (as of 2019):

  • channels 3, 8, 10-13, and 15 are working well
  • channels 1, 2, 4, and 5 are no longer usable
  • channels 6, 7, 9, and 14 function but have noise issues.