Humidity Sounder for Brazil (HSB) Instrument Guide
The AIRS/AMSU/HSB suite of instruments is flying on board of Aqua satellite, and thus their "overpass" pattern is this of Aqua.
The Humidity Sounder for Brazil (HSB) is primarily a humidity sounder providing supplementary water vapor and liquid data to be used in the cloud clearing process. HSB is a near identical copy of AMSU-B. Due to budget constraints, it implements only four passive moisture sounding channels of the five AMSU-B channels. One so-called window channels (at 150 GHz) measures a part of the water vapor absorption line. It samples ninety 1.1 ° scenes per 2.67-second crosstrack scan. Due to the higher spatial resolution (which equals that of AIRS) and a higher scan rate, the measurement density is 2.4 times that of AMSU-A (20% less than for AMSU-B).
The HSB is the object of a scientific and technical cooperation agreement between NASA and AEB (Agencia Espacial Brasileira), Brazilian Space Agency. The HSB instrument will be supplied by INPE to fly in the year 2002 the Aqua spacecraft, together with the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU-A), constituting an advanced sounding system to aid multi-disciplinary studies and systematically monitor the changes on the global system. The instrument is being developed by Matra-Marconi Space (Bristol, UK), with participation of the Brazilian company Equatorial Sistemas.
The HSB instrument ceased operation on Feburary 5, 2003 due to a mirror scan motor failure.
Table of Contents:
- Instrument: Humidity Sounder for Brazil (HSB)
- The HSB is a direct descendant of what was originally intended to form a part of he AMSU system. AMSU-A was to be the temperature sounder and AMSU-B was to be the moisture sounder. HSB is a near identical copy of AMSU-B. The performance of AMSU-B has been disappointing, due to its unanticipated susceptibility to interference from the spacecraft transmitters. However, the RF shielding of HSB has been modified and enhanced, and HSB is not expected to exhibit this susceptibility.
The HSB is 4 moist sounding channel version of AMSU-B, designed to detect radiances in the range of 150 to 183.31 GHz. One so-called window channel (at 150 GHz) measures a part of the water vapor continum, while three are grouped around the 183-GHz water vapor absorption line. Like AMSU-B, it samples ninety 1.1 ° scenes per 2.67-second crosstrack scan. Due to the higher spatial resolution (which equals that of AIRS) and a higher scan rate, the measurement density is 2.4 times that of AMSU-A (20 % less than for AMSU-B). HSB is very similar to AMSU-A, except that is contains only one antenna/receiver system.
This microwave humidity sounder is part of a sounding system that acts in a synergic way, and will provide humidity and profile much more accurately than that of sounders currently in the market. It will also have the capacity of detecting precipitation under the clouds. The horizontal resolution will be 50km for temperature and 15km for humidity, both in the subsatellite point. The temperature error will be around 1 K to 1.5 K and 5 to 15 percent for humidity.
Spatial resolution:13.5 km horizontal at nadir
Duty cycle: 100%
Power: 56 W
Data rate: 4.2kbps
Thermal control: None (ambient)
Thermal operating range: 0-40 degrees C
Field of View: ± 49.5 degrees cross-track
Instrument Instantaneous Field of View: 1.1 degrees circular
Pointing requirements (platform+instrument, 3s):
Stability:74 arcsec sec
Physical Size:70 x 65 x 46 cm
- The science objective for the HSB instrument is to provide the atmospheric information needed to correct infrared measurements (e.g., from AIRS) for the effects of clouds and to determine global humidity profiles in 2-km thick layers in the lower atmosphere (troposphere).
- AQUA, AIRS, AMSU-A, AMSU-B, HSB, Crosstrack Scanner, Microwave, Humidity
- HSB footprints have the same spatial resolution as AIRS IR footprints. Like AMSU-B,HSB samples ninety 1.1 ° Earth scenes, 4 space-look footprints, and 4 calibration target footprints per 2.67-second crosstrack scan. The 150-GHz detector is used to obtain high-resolution measurements of precipitation, snow cover, and sea ice. The remaining detectors are used to improve the accuracy of measurements of atmospheric humidity and total precipitable water vapor.
- HSB is a copy of AMSU-B and implemented as a single module. There is only one antenna. It has a half power beamwidth of 1.1 °, i.e. one-third of the AMSU-A beamwidth and nominally equal to that of AIRS. The diameter of the reflector aperture is 21.9 cm (8.6"). The shape of the "beam" is also similar to that of AMSU-A: it is nearly gaussian near the center, it receives near 98% of its energy within the main beam - which is 2.75 ° wide (2.5 times the half-power width). HSB uses a continuously scanning motor (i.e. not a stepper motor). The radiation is sampled "on the fly", approximately every 18ms. The sample cells, defined by the half-power contours, are therefore motion smeared and overlap each other. The effective, motion smeared, beam width in the scan direction is approximately 1.4 °.
Unlike AMSU-A, there is more than a single feedhorn, however. The single antenna beam is split into three paths with dichroic plates and directed into three feedhorns. One feedhorn is used for the 89-GHz signal, one is used for the 150-GHz signal, and one is used for the 183-GHz signal. The latter is followed by a triplexer which allows three 183-GHz channels to be separated out. Note that for HSB the 98-GHz feedhorn and associated receiver components are absent.
Table. Channel Characteristics of HSB
|Channel No.||Center Frequency (GHz)||Bandwidth (GHz)||Function |
|1*|| || || |
|2||150.0 ||4000||Water Vapor |
|3||183.31 ± 1.0 ||2x500||Water Vapor |
|4||183.31 ± 3.0 ||2x1000||Water Vapor |
|5||183.31 ± 7.0 ||2x2000||Water Vapor |
*Channel 1 (89 GHz) has been deleted for the HSB
- HSB Weighting function diagram
- Matra Marconi Space, Limited (MMS), in the United Kingdom under a contract with the Brazilian National Institute for Space Research, INPE (Insitituto Nacional de pesquisas Espaciais)
|Channel No.||Measured NEDT [K]|
(Obtained during calibration measurements)
|Required NEDT [K]|
NEDT : Noise-equivalent delta-T
- Instrument is automatically calibrated during every scan cycle, 2.67 seconds, by measuring radiation from two calibration targets - the cosmic background radiation enaminating from space (Cold space view) and an internal blackbody calibration target (Blackbody view, typically at 10-15 °C). 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. There it drifts slowly while 4 measurements are taken. The second source, blackbody, is viewed immediatly 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 drifts slowly while 4 measurements are taken.
- 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
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.
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).
|AEB ||Agencia Espacial Brasileira|
|AIRS ||Atmospheric Infrared Sounder|
|AMSU-A ||Advanced Microwave Sounding Unit Version A|
|EOS||Earth Observing System|
|HSB||Humidity Sounder for Brazil|
|INPE ||Instituto Nacional de Pesquisas Espaciais|
|kbps||kilobits per second|
|Mbps||megabits per second|
|NASA||National Aeronautics and Space Administration|
|NOAA||National Oceanic and Atmospheric Administration|
|TOVS||TIROS Operational Vertical Sounder|
- Tue Mar 19 15:30:49 EDT 2002
- Tue Mar 19 15:30:49 EDT 2002
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- Sunmi Cho