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Advanced Microwave Sounding Unit-A (AMSU-A) Instrument Guide

Summary:

The AIRS/AMSU/HSB suite of instruments is flying on board of Aqua satellite, and thus their "overpass" pattern is this of Aqua.

The Advanced Microwave Sounding Unit-A (AMSU-A) is a multi-channel microwave temperature/humidity sounder that measures global atmospheric temperature profiles and provides information on atmospheric water in all of it's forms (with the exception of small ice particles, which are transparent at microwave frequencies). Information from AMSU-A in the presence of clouds is used to correct the infrared measurements for the effects of clouds.

The AMSU-A instrument consists of two independent modules (AMSU-A1 and AMSU-A2), with each module having separate spacecraft interfaces. AMSU-A1 module uses two antenna-radiometer systems (A1-1 and A1-2) to provide twelve channels in the 50 to 60 GHz oxygen band for retrieving the atmospheric temperature profile from the Earth's surface to about 42 kilometers (or 2 mb). The AMSU-A1 module also contains a channel at 89 GHz, while AMSU-A2 has two channels at 23.8 and 31.4 GHz to identify precipitation and correct for surface emissivity, atmospheric liquid water, and water vapor effects. These window channels are also used to derive rain rate, sea ice concentration, and snow cover for example.

The instrument is a direct descendant of the NOAA Microwave Sounding Unit (MSU). Although the basic measurement and instrument concepts are the same, the capabilities of AMSU-A exceed significantly those of MSU. The first AMSU-A instrument was launched, as part of the NOAA Advanced TOVS (ATOVS) system, on NOAA-K (now NOAA-15) in May 1998 providing operational heritage for the AIRS/AMSU-A/HSB mission. The AMSU-A instrument will also fly on the NOAA-L and -M satellites prior to the EOS AQUA launch.

 

Table of Contents:

 


1. Overview:


Sensor/Instrument Long Name, Sensor/Instrument Acronym:

Instrument: AMSU-A: Advanced Microwave Sounding Unit-A

Introduction:

AMSU-A is primarily a temperature sounder that provides atmospheric information in the presence of clouds, which can be used to correct the infrared measurements for the effects of clouds. This is possible because microwave radiation passes, to a varying degree, through clouds - in contrast with visible and infrared radiation, which are stopped by all but the most tenuous clouds. This cloud clearing technique has been demonstrated to work well for scenes which are partially cloudy - at up to 75-80% cloud cover, and is used routinely by NOAA as part of the operational processing of Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) data.

The AMSU-A instrument consists of two independent modules (AMSU-A1 and AMSU-A2), with each module having separate spacecraft interfaces. Like AIRS, AMSU-A is a crosstrack scanner. AMSU-A1 has two antenna/receiver systems and AMSU-A2 has one for processing the microwave channels. The three receiving antennas are parabolic focusing reflectors than rotate continuously, completing one revolution in 8 seconds. The 8-second scan cycle is divided into three segments. In the first segment the Earth is viewed at 30 different angles, symmetric around the nadir direction, in a step-and-stare sequence. Each of the 30 Earth views (scene stations) takes about 0.2 seconds, for a total of approximately 6 seconds. The second segment is a rapid scan covering a cold space view and an internal (warm) blackbody calibration target. Finally, each antenna returns to the starting position to start a new scan cycle. (There is also a stare mode, where the antenna is permanently pointed to the nearest-nadir direction, but that is only used for special purposes - such as for spatial calibration using coastline crossings.)

The capabilities of AMSU-A exceed significantly those of the NOAA Microwave Sounding Unit (MSU). While MSU has only 4 channels (in the 50-GHz oxygen band for temperature sounding) and samples eleven 7.5 ° scenes per 26.5-second crosstrack scan, AMSU-A has 12 temperature sounding channels as well as 3 moisture channels and samples thirty 3.3 ° scenes per 8-second crosstrack scan. The size of an AMSU-A "footprint" at nadir is therefore less than half the size of and MSU footprint. In addition, the AMSU-A is co-aligned with the AIRS instrument onboard the Aqua platform so that successive blocks of 3 x 3 AIRS pixels are contained within the AMSU-A footprint.

Swath:1690 km

Spatial Resolution:40 km horizontal at nadir

Mass:49 kg (A1), 42kg (A2)

Duty cycle: 100%

Power: 77 W (A1), 24 W (A2)

Data rate: 1.5 kbps (A1), 0.5 kbps (A2)

Thermal control: None (ambient)

Thermal operating range: 0-20 degrees C

Field of View: ± 49.5 degrees cross-track

Instrument Instantaneous Field of View: 3.3 degrees circular

Pointing requirements (platform+instrument, 3s):

Control:720 arcsec

Knowledge:360 arcsec

Stability:360 arcsec/sec

Jitter:360 arcsec/sec

Physical size:72 x 34 x 59 cm (A1), 73 x 61 x 86 cm (A2)


Mission Objectives:

The science objective for the AMSU-A is to measure the temperature profile of the atmosphere from 0 to 40 km altitude and provide atmospheric water vapor/precipitation estimates. AMSU-A measures radiant energy with 15 channels between 23 and 89 GHz, specifically, the V,W,K, and Ka frequency bands. AMSU-A1 has 12 channels in the 50-58 GHz oxygen absorption band which provide he primary temperature sounding capabilities and 1 channel at 89 GHz which provides surface and moisture information. AMSU-A2 has 2 channels, one at 23.8 GHz and one at 31.4 GHz, which provide surface and moisture information.

In combination with measurements from the AIRS instrument, AMSU-A provides means to independently account for clouds in the AIRS Field of View, resulting in more accurate temperature and humidity retrievals.

Key Variables:

AQUA, AIRS, HSB, AMSU-A, AMSU-B, Radiometer, Crosstrack Scanner, Microwave, Temperature, Humidity

Scanning or Data Collection Concept/Principles of Operation:

Refer to NOAA KLM User's Guide Appendix J.3 - AMSU Scan and FOV information
for detailed information on this topic.

2. Sensor/Instrument Layout, Design, and Measurement Geometry:


List of Sensors:

AMSU-A1, AMSU-A2

Sensor Description:

Hardware for the two lowest frequencies is located in one module (AMSU-A2) and that for the remaining thirteen frequencies in the second module (AMSU-A1). This arrangement puts the two lower atmospheric moisture viewing channels into one module and the oxygen absorption channels into a second common module to ensure commonality of viewing angle independent of any module and/or spacecraft misalignment due to structural or thermal distortions. The AMSU-A1's concept of multiplexing thirteen frequencies in this second module is provided by a two-antenna system. This multiplexing approach provides minimum front-end RF loss and a constant 3.3 degree antenna beam width with greater than 95 percent beam efficiency.
 
Table Channel Characteristics and Specifications of AMSU-A
Chan. # Channel Frequency (MHz) # bands Nominal Bandwidth (MHz) Nominal Beamwidth (degrees) NEDT (K) (Spec.) Polarization at nadir (See Note 1) Function Instrument Component
1 23,800 1 270 3.3 0.30 V Water Vapor Burden A2
2 31,400 1 180 3.3 0.30 V Surface Temperature A2
3 50,300 1 180 3.3 0.40 V Surface Temperature A1-2
4 52,800 1 400 3.3 0.25 V Surface Temperature A1-2
5 53596115 2 170 3.3 0.25 H Tropospheric Temp A1-2
6 54,400 1 400 3.3 0.25 H Tropospheric Temp A1-1
7 54,940 1 400 3.3 0.25 V Tropospheric Temp A1-1
8 55,500 1 330 3.3 0.25 H Tropospheric Temp A1-2
9 f0=57,290.344 1 330 3.3 0.25 H Stratospheric Temp A1-1
10 f0217 2 78 3.3 0.40 H Stratospheric Temp A1-1
11 f0322.248 4 36 3.3 0.40 H Stratospheric Temp A1-1
12 f0322.222 4 16 3.3 0.60 H Stratospheric Temp A1-1
13 f0322.210 4 8 3.3 0.80 H Stratospheric Temp A1-1
14 f0322.24.5 4 3 3.3 1.20 H Stratospheric Temp A1-1
15 89,000 1 <6,000 3.3 0.50 V Cloud Top/Snow A1-1
Notes:

1. H indicates horizontal and V indicates vertical polarization.


Weighting Function:

AMSU-A Weighting function diagram

3. Manufacturer of Sensor/Instrument:

Aerojet Corporation in Azusa, California.

4. Calibration:


Specifications:


Tolerance:

The accuracy of the warm calibration load brightness temperature is better than ± 0.2K.
Beam pointing accuracy is within ± 0.2 degrees.

Frequency of Calibration:

AMSU-A is automatically calibrated during every scan cycle, 8 seconds, by measuring radiation from two calibration targets - the cosmic background radiation emanating from space (Cold space view) and an internal blackbody calibration target (Blackbody view, typically at 283 - 288K). The first source is viewed immediately after the earth has been scanned. The antenna is quickly moved to point in a direction between the earth's limb and the spacecraft's horizon, where it pauses while 2 measurements are taken. The second source, blackbody, is viewed immediately after the space calibration view. The antenna is again quickly moved to point in the zenith direction, where the blackbody target is located. Again, the antenna pauses while 2 measurements are taken.

Other Calibration Information:

One of AMSU-A Subsystems is Antenna/Drive/Calibration Subsystem. It consists of a conical corrugated horn-fed shrouded reflector, multiplexer, closed-loop antenna scan drive assembly and closed path calibration assembly. The shrouded reflector is rotated once every scan line (8 sec) for:

  • each of 30 earth viewing scene observations,
  • a view of the cosmic background (~2.73K), and
  • a view of a warm calibration load (~300K).

During the rotation cycle, the shroud prevents solar reflections from interacting with the warm load and also ensures maximum coupling of the source radiation to the antenna feed. A complete end-to-end in-flight calibration is achieved in a through-the-antenna method, which provides maximum in-flight calibration accuracy. This through-the-antenna calibration system allows most system losses and spectral characteristics to be calibrated, since the calibration measurements involve the same optical and electrical signal paths as earth scene measurements. (The only exception is that the internal calibration target appears in the antenna near field and can reflect leakage emission from the antenna itself. That effect is taken into account in the calibration processing, however.) This approach has a significant advantage over calibration systems using switched internal noise sources injected into the signal path after the antenna, at the cost of some significant weight gain since the internal calibration target is fairly massive.


For more information on AMSU-A calibration algorithm, post-launch calibration and evaluation,
see NOAA KLM User's Guide Section 7.3.

5. References:

Arturo Revilla, Roger A.Davidson, and Susan C. Murphy, "EOS-PM1 Spacecraft Advanced Microwave Sounding Unit-A (AMSU-A), ESDIS Core System (ECS) Preliminary Instrument Flight Operations Understanding (IFUO)", JPL D-12815, January 15,1997

Tsan Mo, "Prelaunch Calibration of the Advanced Microwave Sounding Unit-A for NOAA-K", IEEE Trans. Microwave Theory and Techniques, vol.44, pp.1460-1469, 1996

Tsan Mo, "AMSU-A Antenna Pattern Corrections", IEEE Trans. Geoscience and Remote Sensing, 1997

Bjorn Lambrigtsen, "AIRS Level1B Algorithm Theoretical Basis Document, Part 3: Microwave Instruments", November 10, 2000

Geoffrey Goodrum, Katherine B. Kidwell and Wayne Winston, "NOAA KLM User's Guide" , September, 2000


6. Glossary of Terms:

ANTENNA. A device used for radiating or receiving electromagnetic waves (especially microwaves and radio waves).

BEAM WIDTH. The angle, measured in a horizontal plane, between the directions at which the intensity of an electromagnetic beam, such as radar or radio beam, is one-half its maximum value.

CALIBRATION. 1) The activities involved in adjusting an instrument to be intrinsically accurate, either before or after launch (i.e., "instrument calibration). 2) The process of collecting instrument characterization information (scale, offset, nonlinearity, operational, and environmental effects), using either laboratory standards, field standards, or modeling, which is used to interpret instrument measurements (i.e., "data calibration").

CROSS TRACK SCANNER.A sensor that uses a mirror system that moves from side to side in the range or across track dimension to obtain optical data. Diagram

DETECTOR. A device in a radiometer that senses the presence and intensity of radiation. The incoming radiation is usually modified by filters or other optical components that restrict the radiation to a specific spectral band. The information can either be transmitted immediately or recorded for transmittal at a later time.

FIELD OF VIEW The area or solid angle which can be viewed through an optical instrument.
 

INFRARED RADIATION. Electromagnetic radiation lying in the wavelength interval from 0.7 µm to 1000 µm. (Near Infrared: 0.7 - 2 µm, Thermal Infrared:3 - 25 µm) Its lower limit is bounded by visible radiation, and its upper limit by microwave radiation. Most of the energy emitted by the Earth and its atmosphere is at infrared wavelengths. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The tri-atomic gases, such as water vapor, carbon dioxide, and ozone, absorb infrared radiation and play important roles in the propagation of infrared radiation in the atmosphere.

INSTANTANEOUS FIELD OF VIEW (IFOV) The field of a scanner with the scan motion stopped. When expressed in degrees or radians, this is the smallest plane angle over which an instrument is sensitive to radiation. When expressed in linear or area units such as meters or hectares, it is an altitude dependent measure of the ground resolution of the scanner.

INSTRUMENT. An integrated collection of hardware containing one or more sensors and associated controls designed to produce data on an environment. Source: ESADS.

MICROWAVE. A comparatively short electromagnetic wave; especially : one between about 1 millimeter and 1 meter in wavelength.

NADIR. Direction toward the center of the Earth. Opposite of zenith. e.g., A satellite measurement taken from a point on the earth's surface directly below the spacecraft.

RADIOMETER. An instrument for demonstrating the transformation of radiant energy into mechanical work, consisting of an exhausted glass vessel containing vanes that revolve about an axis when exposed to light. A radiometer consists of a set of vanes, each shiny on one side and blackened on the other, that are mounted in an evacuated vessel. When exposed to light, the vanes spin. The blackened vanes retreat from the light source. The black surface is warmer than the shiny surface and gas molecules will recoil faster from the hot surface. The slight difference in molecule recoil is what causes the device to spin. Light has qualities of both waves and particles. Radiometer demonstrates the particle-like nature of light: when the photons of light strike the surface of the radiometer they transfer their energy and cause the sails to spin.

SENSOR. Device that produces an output (usually electrical) in response to stimulus such as incident radiation. Sensors aboard satellites obtain information about features and objects on Earth by detecting radiation reflected or emitted in different bands of the electromagnetic spectrum. Analyzing the transmitted data provides valuable scientific information about Earth.

Weather satellites commonly carry radiometers, which measure radiation from snow, ice, clouds, and bodies of water. Spaceborne radars are used for Earth observations, bouncing radar waves off land and ocean surfaces to study sea-surface conditions, ice thickness, and land surface features. A wind scatterometer is a special type of radar designed to measure ocean surface winds indirectly by bouncing signals off the water and measuring them from various angles. Infrared (IR) detectors measure heat generated by Earth features in the IR band of the spectrum.

Photographic reconnaissance sensors in their simplest form are large telescope-camera systems used to view objects on Earth's surface. The bigger the lens, the smaller the object that can be detected. Camera-telescope systems now incorporate all sorts of sophisticated electronics to produce better images, but even these systems need cloudless skies, excellent lighting, and good color contrast between objects and their surroundings to detect objects the size of a basketball. Some of the satellites produce film images that must be returned to Earth, but a more convenient method is to record the image as a series of digital code numbers, then reconstruct the image from the electronic code using a computer at a ground station.

SOUNDER. An instrument that measures atmospheric profiles (e.g. temperature, pressure, moisture, etc.). Measurements can either be taken in the horizontal plane by nadir-viewing sounders, or in the vertical plane by limb sounders. Limb sounders begin scanning at the limb (the horizon).


7. List of Acronyms:

AIRSAtmospheric Infrared Sounder
AMSU-AAdvanced Microwave Sounding Unit Version A
EOSEarth Observing System
FOVField-of-view
HSBHumidity Sounder for Brazil
IRInfrared
kbpskilobits per second
Mbpsmegabits per second
MUXMultiplexer
NEDTNoise Equivalent Temperature Difference
NIRNear Infrared
NOAANational Oceanic and Atmospheric Administration
PLLOPhase-Locked Loop Oscillators
PRTPlatinum Resistance Thermometers
RFRadio Frequency
TOVSTelevision Infrared Observation Satellite (TIROS) Operational Vertical Sounder
VISVisible

8. Document Information:


Document Revision Date:

Thu Feb 28 12:15:10 EDT 2002

Document Review Date:

Thu Feb 28 12:15:10 EDT 2002

Document ID:

...(currently leave this blank)

Document Curator:

Sunmi Cho

Document URL:

http://disc.sci.gsfc.nasa.gov/AIRS/documentation/amsu_instrument_guide.shtml
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Last updated: Feb 03, 2014 01:19 PM ET