Summary:
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
(AMSU) and the Humidity Sounder
Brazil (HSB)
are scheduled to fly together on the second platform of NASA's Earth Observing System
(EOS AQUA).
Table of Contents:
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- Instrument: Atmospheric Infrared Sounder (AIRS)
Sensors: Infrared Spectrometer, Visible light/Near-infrared Photometer
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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)
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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.
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- AQUA, AIRS, AMSU, HSB, Spectrometer, Photometer, Infrared, Near-Infrared, Visible, Temperature, Humidity, Multispectral infrared sounder
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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)
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- IR Spectrometer, VIS/NIS Photometer
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| 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
AIRS IR Spectral Coverage Chart
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.
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155K
Stability:
< 0.7 mK / scan
Maximum: 350 K
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| 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
AIRS Vis/NearIR Spectral Coverage Chart
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.
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300 K |
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High spectral resolution, multispectral infrared sounder
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Operates with AMSU for all-weather capability
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1K temperature retrieval accuracy
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0.05 emissivity accuracy
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Array grating spectrometer (3.74 to 15.4 µm), with a spectral resolution of 1,200 (l/Dl)
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Swath: 1,650 km
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Spatial resolution: 13.5 km horizontal at nadir, 1km vertical
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Click on a wavenumber in the list below to see typical weighting functions
for the AIRS 15 micron temperature sounding channels.
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- British Aerospace Systems, Infrared and Imaging Systems Division(LMIRIS),
in Lexington, Massachusetts.
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- < ±3% absolute error.
Temperature Stability : < 0.7 mK / scan
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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.
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- AIRS On-Board Calibrators
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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.
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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.
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| 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 |
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Document Revision Date:Mon Apr 28 13:46:51 EDT 2003
- Fri May 10 11:30:49 EST 2002
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- Fri May 10 11:30:49 EST 2002
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- NASA GES DISC
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- http://disc.sci.gsfc.nasa.gov/AIRS/documentation/airs_instrument_guide.shtml
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Atmos Composition
