instruments:hatpro:hatpro
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===== Introduction ===== | ===== Introduction ===== | ||
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Microwave radiometers are very sensitive receivers designed to measure thermal electromagnetic radiation emitted by material media like the atmosphere. They are usually equipped with multiple receiving channels in order to derive the characteristic emission spectrum of the atmosphere or extraterrestrial objects. Microwave radiometers are utilized in a variety of environmental and engineering applications, | Microwave radiometers are very sensitive receivers designed to measure thermal electromagnetic radiation emitted by material media like the atmosphere. They are usually equipped with multiple receiving channels in order to derive the characteristic emission spectrum of the atmosphere or extraterrestrial objects. Microwave radiometers are utilized in a variety of environmental and engineering applications, | ||
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{{: | {{: | ||
- | [[http:// | + | Fig. 1: [[http:// |
===== History of microwave radiometer measurements ===== | ===== History of microwave radiometer measurements ===== | ||
First developments of microwave radiometer were dedicated to the measurement of radiation of extraterrestrial origin in the 1930s and 1940s [1]. The first operational microwave radiometer was designed by [[https:// | First developments of microwave radiometer were dedicated to the measurement of radiation of extraterrestrial origin in the 1930s and 1940s [1]. The first operational microwave radiometer was designed by [[https:// | ||
- | Soon after satellites were first used for observing the atmosphere, MW radiometers became part of their instrumentation. In 1962 the [[https:// | + | Soon after satellites were first used for observing the atmosphere, MW radiometers became part of their instrumentation. In 1962 the [[https:// |
- | ===== Principle | + | Ground-Based radiometer for the determination |
- | Solids, liquids (e.g. the earth' | + | |
- | The emission and absorption of hydrometeors does not provide characteristic absorption line features as found for atmospheric gases. Liquid hydrometeors (small cloud and rizzle drops) are efficient emitters in the microwave. Besides | + | Here we could keep the graphic from the original article |
+ | https://en.wikipedia.org/ | ||
+ | Fig. 2 | ||
- | Larger rain drops as well as larger frozen hydrometeors | + | ===== Principle of operation ===== |
+ | Solids, liquids | ||
+ | Besides the distinct absorption features of molecular transistion lines, there are also non-resonant contributions by hydrometeors (liquid drops and frozen particles). Liquid water emission increases with frequency, hence, measuring at two frequencies, | ||
- | {{ : | + | Larger rain drops as well as larger frozen hydrometeors |
- | Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; the colored lines are the spectrum obtained | + | |
- | ===== Design ===== | + | {{: |
+ | Fig. 3: Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; the colored lines are the spectrum obtained from a satellite instrument over the ocean measuring at horizontal (blue) and vertical (red) linear polarization. Solid lines indicate simulations for clear-sky (cloud-free) conditions, dotted lines show a clear-sky case with a single layer liquid cloud. The vertical lines indicate typical frequencies used by satellite sensors like the [[https:// | ||
- | The principal components of a microwave radiometer | + | ===== Design and Calibration ===== |
+ | A microwave radiometer | ||
+ | |||
+ | Usually ground-based radiometers are also equipped with environmental sensors (rain, temperature, | ||
- | {{ :instruments:hatpro: | + | {{:stuff:mwr_design.png|Schematic diagram of a microwave radiometer}} \\ |
- | Schematic diagram of a microwave radiometer [8]. | + | Fig. 4: Schematic diagram of a microwave radiometer |
+ | After being received at the antenna the radiofrequency signal is downconverted to the intermediate frequency (IF) with the help of a stable local oscillator signal. After amplification with a Low Noise Amplifier (LNA) and band pass filtering the signal can be detected in full power mode, by splitting or splitting it into multiple frequency bands with a spectrometer. For high-frequency calibrations a Dicke switch is used here. | ||
===== Calibration ===== | ===== Calibration ===== | ||
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The calibration of MWRs sets the basis for accurate measured TB and therefore, for accurate retrieved atmospheric parameters as temperature profiles, integrated water vapor and liquid water path. The simplest version of a calibation is a so-called „hot-cold“ calibration using two reference blackbodies at known, but different, „hot“ and „cold“ temperatures, | The calibration of MWRs sets the basis for accurate measured TB and therefore, for accurate retrieved atmospheric parameters as temperature profiles, integrated water vapor and liquid water path. The simplest version of a calibation is a so-called „hot-cold“ calibration using two reference blackbodies at known, but different, „hot“ and „cold“ temperatures, | ||
The temperatures of the calibration targets should be chosen such that they span the full measurement range. Ground-based radiometers usually use an ambient temperature target as „hot“ reference. As a cold target one can use either a liquid nitrogen cooled blackbody (77 K) [e.g. Ulaby] or a zenith clear sky TB that was obtained indirectly from radiative transfer theory [Paper Westwater]. Satellites use a heated target as „hot“ reference and the cosmic background radiation as „cold“ reference. To increase the accuracy and stabiltity of MWR calibrations further calibration targets, such as internal noise sources, can be used. | The temperatures of the calibration targets should be chosen such that they span the full measurement range. Ground-based radiometers usually use an ambient temperature target as „hot“ reference. As a cold target one can use either a liquid nitrogen cooled blackbody (77 K) [e.g. Ulaby] or a zenith clear sky TB that was obtained indirectly from radiative transfer theory [Paper Westwater]. Satellites use a heated target as „hot“ reference and the cosmic background radiation as „cold“ reference. To increase the accuracy and stabiltity of MWR calibrations further calibration targets, such as internal noise sources, can be used. | ||
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[2] Thermal Microwave Radiation: Applications for Remote Sensing, C. Matzler, 2006, The Institution of Engineering and Technology, London, Chapter 1. | [2] Thermal Microwave Radiation: Applications for Remote Sensing, C. Matzler, 2006, The Institution of Engineering and Technology, London, Chapter 1. | ||
- | [3] http://cetemps.aquila.infn.it/ | + | [3] Westwater, Edgeworth Rupert, 1970: Ground-Based Determination of Temperature Profiles by Microwaves. PH.D. Thesis, UNIVERSITY OF COLORADO AT BOULDER, Source: Dissertation Abstracts International, |
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[4] Passive Microwave Remote Sensing of the Earth, Physical Foundations, | [4] Passive Microwave Remote Sensing of the Earth, Physical Foundations, | ||
- | - Cimini et al., 2009 | + | [5] http:// |
- | - Klein and Gasiewski, 2000 | + | |
- | | + | [6] Westwater, E.R., C. Mätzler, S. Crewell (2004) A review of surface-based microwave |
- | | + | |
- | | + | [7] Westwater, E. R., S. Crewell, C. Mätzler, and D. Cimini, 2006: Principles of Surface-based |
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- | | + | [8] Final report of the COST action EG-Climet, |
+ | |||
+ | [9] Czekala et al. (2001), Discrimination of cloud and rain liquid water path by groundbased polarized microwave radiometry, | ||
+ | |||
+ | [10] Bennartz, R., and P. Bauer (2003), Sensitivity of microwave radiances at 85–183 GHz to precipitating ice particles, Radio Sci., 38(4), 8075, doi: | ||
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+ | [11] Kneifel et al. (2010), Snow scattering signals in ground-based passive microwave radiometer measurements, | ||
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instruments/hatpro/hatpro.txt · Last modified: 2021/01/22 22:17 by 127.0.0.1