The AMSU-A Instrument

AMSU-A is a 15-channel microwave temperature sounder implemented as two independently operated modules. Module 1 (AMSU-A1) has 12 channels in the 50-58 GHz oxygen absorption band which provide the primary temperature sounding capabilities and 1 channel at 89 GHz which provides surface and moisture information. Module 2 (AMSU-A2) has 2 channels: one at 23.8 GHz and one at 31.4 GHz which provide surface and moisture information (total precipitable water and cloud liquid water).

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 are mounted on a scan axis at a 45° tilt angle, so that radiation is reflected from a direction along the scan axis ( a 90° reflection). AMSU-A scans three times as slowly as AIRS (once per 8 seconds) and its footprints are approximately three times as large as those of AIRS (45 km at nadir). This results in three AIRS scans per AMSU-A scans and nine AIRS footprints per AMSU-A footprint.

AMSU-A instrument state

On September 24, 2016 power to the AMSU-A2 instrument on Aqua was lost, and a series of attempts to restore it to normal operation were unsuccessful. AMSU-A1 remains operational, although three of its channels have degraded to the point where they are no longer used. A version of AIRS Level 2 and Level 3 Data Products is available up to the date of AMSU-A2 failure where retrievals used AIRS and AMSU radiances.

  • On 11/16/2004 at 13:21:19 UT all of the AMSU-A2 temperature read-outs except the warm load temperatures showed a sudden and simultaneous increase in noise. Subsequent analyses indicate that failure of a compensation capacitor in the reference voltage amplifier is the most probable cause. This will have a negligible effect on science products because RF shelf temperature enters into the calibration in a small second-order term. At the same time, however, the warm load temperature appeared to undergo a decrease of 0.15 K. Analysis continues to determine whether the warm load temperature offset continued. If so, the DN to EU conversion in the calibration algorithm will require modification.
  • AMSU channel 4 failed 1 October 2007: Radiances useful until mid-2007
  • AMSU channel 5 progressively degraded beginning January 2010: Noise level of 0.5 K in January 2010; Noise level of 1.0 K in February 2011; Noise level of 2.0 K in February 2012 and sharply increased thereafter
  • AMSU channel 7 exhibits abnormal noise levels: Noise level is about 5x NEdT on the average, but varies substantially; The added noise is not random; probable cause is spacecraft transmitter interference; The underlying random noise (NEdT) is within specs; Channel 7 should not be used until this systematic noise can be removed
  • AMSU channel 6 exhibits some of the same noise characteristics as channel 7: Added noise level is a fraction of NEdT; overall level still meets specs; Use channel 6 with confidence
  • AMSU channel 9 radiometer counts exhibit sudden, large change (~0.1%) recovering suddenly or gradually after 1-3 minutes; typically appears once or a few times per day, possibly clustered; no other channels affected: The phenomenon is being characterized, cause as yet unknown; Negligible effect in most cases; use channel 9 with confidence
  • AMSU-A2 with its channels 1 and 2 has not been operational since 24 September 2016-19:46:58 UTC when a fuse in an Aqua quiet bus relay opened and shut off the AMSU-A2 quiet bus power.
  • AMSU channel 14 scene temperature underwent a sudden drop of about 4 K on 21 June 2018-15:50 UTC. The channel recovered, with temperatures increasing by about 4 K on 19 June 2019. The cause is not known. A faulty SAW filter is a possibility.

The HSB Instrument

The Humidity Sounder for Brazil (HSB) is a 4-channel microwave moisture sounder implemented as a single module. Three channels are located near 183 GHz, while the fourth is a window channel at 150 GHz. Physically HSB is identical to AMSU-B, which is operated by NOAA on its most recent POES satellites, but HSB lacks the fifth channel (89 GHz) of AMSU-B. Like AIRS, HSB is a cross-track scanner, and it has only one parabolic scan antenna. Its scan speed as well as its footprints 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.

HSB ceased operation on February 5, 2003 due to a failure in the mirror scan motor electronics.

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 symptoms seen on orbit were replicated on an engineering model.