Instruments

AIRS is a high spectral resolution spectrometer with 2378 bands in the thermal infrared (3.7 - 15.4 µm) and 4 bands in the visible (0.4 - 1.0 µm). These ranges have been specifically selected to allow determination of atmospheric temperature with an accuracy of 1°C in layers 1 km thick, and humidity with an accuracy of 20% in layers 2 km thick in the troposphere. In the cross-track direction, a ±49.5 degree swath centered on the nadir is scanned in 2 seconds, followed by a rapid scan in 2/3 second taking routine calibration related data that consist of four independent Cold Space Views, one view of the Onboard Blackbody Calibrator, one view of the Onboard Spectral Reference Source, and one view of a photometric calibrator for the VIS/NIR photometer. Each scan line contains 90 IR footprints, with a resolution of 13.5 km at nadir and 41km x 21.4 km at the scan extremes from nominal 705.3 km orbit. The Vis/NIR spatial resolution is approximately 2.3 km at nadir.

The primary spectral calibration of the AIRS spectrometer is based on the cross-correlation between spectral features observed in the upwelling radiance spectrum with precalculated spectra. And additional spectral reference source is provided to aid pre-launch testing in the thermal vacuum chamber during spacecraft integration and for quality monitoring in orbit.

AIRS will make measurements of the Earth's atmosphere and surface that will allow scientists to improve weather prediction and to observe changes in Earth's climate. Inaugurating a new generation of operational atmospheric monitors, AIRS, the Advanced Microwave Sounder Unit (AMSU) and the Humidity Sounder for Brazil (HSB) are scheduled to fly together on the second platform of NASA's Earth Observing System (EOS Aqua).

Table of Contents:

• 1 Overview
• 2 Layout, Design, and Measurement Geometry
• 3 Manufacturer
• 4 Calibration
• 5 References
• 6 Glossary of Terms
• 7 List of Acronyms
• 8 Document Information

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1. Overview:

Sensor/Instrument Long Name, Sensor/Instrument Acronym:

Instrument: Atmospheric Infrared Sounder (AIRS)
Sensors: Infrared Spectrometer, Visible light/Near-infrared Photometer

Introduction:

AIR is a complex instrument, especially with regard to the downlink telemetry of science and engineering data. There are 2378 spectral channels, i e., one channel refers to a spectral resolution element, with two samples per channel. There are 4482 total detectors on the IR Focal Plane Assembly (FPA), with hybrid PV/PC:HgCdTe detector types. There are 2,104 IR spectral channels, with dual PV detectors, and 274 channels in the critical 13.6-15.4 µm region, with single PC detectors. The dual detectors are summed (or selected) on board prior to downlink. After digitizing and formatting, each AIRS science data packet contains 2666 detector samples that include both IR and VIS/NIR.

The heart of the AIRS system is the multi-aperture, echelle grating spectrometer and corresponding multiple detector arrays. The IR Spectrometer Assembly includes a pupil imaging (i.e. detectors are located at a pupil stop of the spectrometer optics as opposed to the detectors being at a field stop) telescope which views the Earth and calibrator assemblies through a rotating scan mirror in the scan head assembly. While in-flight calibration measurements are made once per scan line (every 2.667 seconds), data from ten or more scan lines are combined by the ground calibration software to update calibration coefficients.

Views of the flight blackbody and the cold space view provide a two-point radiometric calibration for gain measurement and for background signal an electrical offset correction. The radiometric accuracy of the AIRS instrument depends directly on the accuracy of its flight calibration source and the quality of the cold space view.

Each scan line contains 90 IR footprints. The VIS/NIR Sensor Assembly shares the scan mirror with the spectrometer so that each IR footprint is overlayed with a grid of visible detector pixels in four wavelength channels. The four channels provide spectral coverage from 0.4 µm to 1.0 µm.

The VIS/NIR channels have nominally six times the spatial resolution of the IR Sensor Assembly. The VIS/NIR channels are essential to account for the effect of prevalent low level clouds. In these regions of the atmosphere, neither infrared nor microwave channels are capable of providing the required sensitivity. The photometric reference source is provided for calibration of the VIS/NIR channels.

Swath:1650 km

Spatial resolution:IR:13.5 km and Vis/NIR: 2.3 km horizontal at nadir, 1km vertical

Mass:177 kg

Duty cycle: 100%

Power: 220 W

Data rate: 1.27 Mbps

Thermal control: IR detectors: active cooler at 60 K, Passive radiator at 150 K and Electronics at ambient

Thermal operating range: 20-25 degrees C

Field of View: ± 49.5 degrees cross-track

Instrument Instantaneous Field of View: 1.1 degrees circular

Pointing requirements (platform+instrument, 3s):

Control:360 arcsec

Knowledge:180 arcsec

Stability:360/60 sec

Jitter:TBD

Physical Size:116.5 x 80 x 95.3 cm (stowed), 116.5 x 158.7 x 95.3 cm (deployed)

Mission Objectives:

Together with Advanced Microwave Sounding Unit (AMSU) and Humidity Sounder for Brazil (HSB), AIRS measures temperature at an accuracy of 1°C in layers 1 km thick and humidity at an accuracy of 20% in layers 2 km thick in the troposphere (the lower part of the atmosphere) in order to allow meteorologists to improve and extend weather predictions from the current five-day forecasts to over a week into the future and to observe changes in Earth's climate.

Note. The accuracy of temperature and humidity profiles is equivalent to radiosonde accuracy. This was accomplished by advances in IR detector array and cryogenic cooler technology.

Key Variables:

AQUA, AIRS, AMSU, HSB, Spectrometer, Photometer, Infrared, Near-Infrared, Visible, Temperature, Humidity, Multispectral infrared sounder

Scanning or Data Collection Concept/Principles of Operation:

The AIRS instrument incorporates numerous advances in infrared sensing technology to achieve a high level of measurement sensitivity, precision, and accuracy.  The heart of the instrument is a cooled (155 K) array grating spectrometer operating over the range of 3.7 - 15.4 µm at a spectral resolution (l/Dl) of 1200. The spectrometric approach uses a grating to disperse infrared energy across arrays of high sensitivity HgCdTe detectors operating at 58 K.  The concept requires no moving parts for spectral encoding and provides 2378 spectral samples, all measured simultaneously in time and space. Simultaneity of measurement is an essential requirement for accurate temperature retrievals under partly cloudy conditions.

Spatial coverage and calibration targets are provided by the scan head assembly, containing a cross track rotary scan mirror and calibrators. The scan mirror has two speed regimes: During the first 2 seconds it rotates at 49.5 degree/second, generating a scan line with 90 ground footprints, each with a 1.1 degree diameter FOV. During the remaining 0.667 seconds the scan mirror finishes the remaining 261 degrees of a full revolution. Routine calibration related data are taken during this time. These consist of four independent Cold Space Views, one view into the Onboard Blackbody Calibrator (OBC), one view of the Onboard Spectral Reference Source (OBS), and one view into a photometric calibrator for the VIS/NIR Photometer.

AIRS Scan Geometry Diagram(Courtesy of AIRS Science Team, NASA/JPL)

2. Layout, Design, and Measurement Geometry:

List of Sensors:

IR Spectrometer, VIS/NIS Photometer

Sensor Description:

Sensor	Description/Key Requirements	Temperature(K)
IR Spectrometer	Multi-aperture, non-Littrow echelle array grating spectrometer configuration Two-stage passive radiative cooler with retractable earth shield. IR spectral Coverage: 3.74 - 4.61 µm /2169 - 2674 cm 6.20 - 8.22 µm /1265 - 1629 cm 8.80 - 15.40 µm /649 - 1136 cm Spectral Resolution:l/Dl = 1200 nominal: (900-1400 required) Sensitivity: < 0.20 K from 3.7 to 13.4 µm < 0.35 K from 13.4 to 15.4 µm Channel Knowledge: Frequency Knowledge: 0.01 Dl Wavelength Stability: 0.05 Dl/ 24 hour. On-board spectral calibrator required. In-flight alignment adjust capability. Radiometric Calibration: < ±3% absolute error. On-board two-point calibration every scan.	155K Stability: < 0.7 mK / scan Maximum: 350 K
VIS/NIR Photometer	Four channels, from 0.4 - 1.0 µm: Wavelengths at 50% peak response: Channel 1 0.41 µm - 0.44 µm Channel 2 0.58 µm - 0.68 µm Channel 3 0.71 µm - 0.92 µm Channel 4 0.49 µm - 0.94 µm Contiguous ground coverage at nadir with IR. No redundancy. Data Rate (12-bit ADC): 119 kbps before formatting. Three clocks for sample, start, amd reset.	300 K

Measurement Approach

• High spectral resolution, multispectral infrared sounder

• Operates with AMSU for all-weather capability

• 1K temperature retrieval accuracy

• 0.05 emissivity accuracy

• Array grating spectrometer (3.74 to 15.4 µm), with a spectral resolution of 1,200 (l/Dl)

• Swath: 1,650 km

• Spatial resolution: 13.5 km horizontal at nadir, 1km vertical

Weighting Function:

Click on a wavenumber in the list below to see typical weighting functions for the AIRS 16 micron temperature sounding channels.

Wavenumber
663.86	667.11	667.39	667.67	667.94	668.22	668.50	668.78
669.06	669.34	669.62	669.90	670.17	670.45	670.73	671.01

3. Manufacturer:

British Aerospace Systems, Infrared and Imaging Systems Division(LMIRIS),
in Lexington, Massachusetts.

4. Calibration:

Specifications:

Tolerance:

< ±3% absolute error.
Temperature Stability : < 0.7 mK / scan

Frequency of Calibration:

Two-point IR radiometric calibration:
4 cold space views (2 before/2 after each scan).
One cavity blackbody source (308 K).
Occurs once per scan, every 2.67 seconds.

Spectral calibration:
Illuminated parylene sheet for IR spectral reference.
Occurs once per scan, every 2.67 seconds.

VIS/NIR photometric calibration:
Tungsten lamp/diffuser (triple redundant).
Measured once every other orbit.

Other Calibration Information:

AIRS Calibration Team Home page - AIRS Instrument Calibration

5. References:

Aumann,H.H., et. al, "AIRS Level1B Algorithm Theoretical Basis Document", November 2000

Aumann,H.H., et. al, "AIRS Visible and Infrared In-Flight Calibration Plan", May 2001

Pagano, T., et. al., "Operational Readiness of the Atmospheric Infrared Sounder on the Earth Observing System Aqua Spacecraft", SPIE 4483-04, August 2001

Weiler, M.H., et. al.,"Spectral Test and Calibration of the Atmospheric Infrared Sounder (AIRS)", SPIE 4483-05, August 2001

Pagano, T., et. al., "Pre-Launch Performance Characteristics of the Atmospheric Infrared Sounder", SPIE 4169-41, September 2000.

Pagano, T., et. al "Scan Angle Dependent Radiometric Modulation due to Polarization for the Atmospheric Infrared Sounder (AIRS)", SPIE 4135-14, August 2000

Gigioli, G., et al., "AIRS instrument polarization response: measurement methodology", SPIE 3759-30, July 1999

Morse, P., et. al., "Development and Test of the Atmospheric Infrared Sounder (AIRS)", SPIE 3759-27, July 1999

Overoye, Ken et. al.,"Test and Calibration of the AIRS Instrument," SPIE 3759-28, July 1999.

Aumann, H.H., and Miller, Chris, "Atmospheric Infrared Sounder (AIRS) on the Earth Observing System", SPIE Vol.2583, 32-343, 1995.

6. Glossary of Terms:

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

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"). Source:EPO

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.

ECHELLE. A diffraction grating made by ruling a plane metallic mirror with lines having a relatively wide spacing.

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

GRATING. A system of close equidistant and parallel lines or bars ruled on a polished surface to produce spectra by diffraction.

HgCdTe DETECTOR. Photoconductive Mercury Cadmium Telluride (HgCdTe) detector.

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.

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.

PHOTOMETER. An instrument for measuring luminous intensity, luminous flux, illumination, or brightness.

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.

SPECTROMETER. Any of various instruments for producing a spectrum and measuring the wavelengths, energies, etc. involved. A simple type, for visible radiation, is a spectroscope equipped with a calibrated scale allowing wavelengths to be read off or calculated. In the X-ray to infrared region of the electromagnetic spectrum, the spectrum is produced by dispersing the radiation with a prism or diffraction gratin (or crystal, in the case of hard X-rays). Some form of photoelectric detector is used, and the spectrum can be obtained as a graphical plot, which shows how the intensity of the radiation varies with wavelength. Such instruments are also called spectrophotometers. Spectrometers also exist for investigating the gamma-ray region and the microwave and radio-wave regions of the spectrum. Instruments for obtaining spectra of particle beams are also called spectrometers.

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).

THERMAL VACUUM CHAMBER. A comprehensive environmental test complex that is specifically designed to simulate the high vacuum and varying thermal conditions that spacecraft encounter.

7. List of Acronyms:

AIRS	Atmospheric Infrared Sounder
AMSU-A	Advanced Microwave Sounding Unit Version A
Dl	delta wavelength
EOS	Earth Observing System
ESADS	Earth Science and Applications Data System (NASA Headquarters)
FOV	Field-of-view
FPA	Focal Plane Assembly
HgCdTe	Mercury-Cadimium Telluride
HSB	Humidity Sounder for Brazil
IR	Infrared
kbps	kilobits per second
Mbps	megabits per second
NIR	Near Infrared
NOAA	National Oceanic and Atmospheric Administration
PC	Photoconductive Detector
PV	Photo Voltaic Detector
VIS	Visible

8. Document Information:

Document Revision Date:

 

 

Thu Oct 31 13:04:55 EST 2002

Fri May 10 11:30:49 EST 2002

Document Review Date:

 

Fri May 10 11:30:49 EST 2002

 

 

 

Document URL:

http://disc.sci.gsfc.nasa.gov/AIRS/documentation

 

 

 

 

 

Document Curator:

 

Sunmi Cho