instruments:hatpro:hatpro
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instruments:hatpro:hatpro [2016/05/24 16:44] – stefan | instruments:hatpro:hatpro [2016/06/11 21:30] – susanne | ||
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===== Introduction ===== | ===== Introduction ===== | ||
+ | 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, | ||
- | The atmosphere in the [[https:// | + | Using the [[https:// |
For weather and climate monitoring, microwave radiometers are operated from space [1] [2] as well as from the ground [3]. As [[https:// | For weather and climate monitoring, microwave radiometers are operated from space [1] [2] as well as from the ground [3]. As [[https:// | ||
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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:// | ||
- | 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 | + | Here we could keep the graphic from the original article |
+ | https:// | ||
+ | Fig. 2 | ||
===== Principle of operation ===== | ===== Principle of operation ===== | ||
- | Solids, liquids (e.g. the earth' | + | Solids, liquids (e.g. the earth' |
+ | 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, | ||
- | In addition to gaseous absorption, scattering, absorption, and emission | + | Larger rain drops as well as larger frozen hydrometeors (snow, graupel, hail) also scatter microwave radiation especially at higher frequencies (>90 GHz). These scattering effects can be used to distinguish between rain and cloud water content exploitinh polarized measurements [5] but also to constrain the columnar amount of snow and ice particles from space [6] and from the ground [7]. |
+ | {{: | ||
+ | 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:// | ||
+ | ===== Design and Calibration ===== | ||
+ | A microwave radiometer consits of an antenna system, microwave radiofrequency components (frontend) and a backend for signal processing at intermediate frequencies (Fig. 5). The atmospheric signal is very weak and the signal needs to be amplified by around 80 dB. Therefore often heteorodyne techniques are used to convert the signal down to lower frequencies that allow the ise of commercial amplifiers and signal processing. Increasingly low noise amplifiers become available at higher frequencies, | ||
+ | |||
+ | Usually ground-based radiometers are also equipped with environmental sensors (rain, temperature, | ||
- | {{ :instruments:hatpro: | + | {{:stuff:mwr_design.png|Schematic diagram of a microwave radiometer}} \\ |
- | Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; | + | Fig. 4: Schematic diagram of a microwave radiometer using the [[https:// |
- | + | After being received at the antenna | |
- | ===== Design ===== | + | |
- | + | ||
- | The principal components | + | |
- | + | ||
- | {{ : | + | |
- | Schematic diagram of a microwave radiometer [8]. | + | |
===== Calibration ===== | ===== Calibration ===== | ||
+ | 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. | ||
===== | ===== | ||
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- | Temperature profiles can be obtained by measuring along the oxygen absorption complex at 60 GHz. The emission at any altitude is proportional to local temperature and density of oxygen. Unlike water vapor, oxygen is relatively homogeneously distributed within the atmosphere and around the globe. Thanks to the relatively well known vertical profile of oxygen concentration, | ||
The retrieval of physical quantities (e.g. temperature or water vapor profiles) is not straight-forward and comprehensive retrieval algorithms (using inversion techniques like [[https:// | The retrieval of physical quantities (e.g. temperature or water vapor profiles) is not straight-forward and comprehensive retrieval algorithms (using inversion techniques like [[https:// | ||
- | + | Temperature profiles are obtained by measuring along the oxygen absorption complex | |
- | + | ||
- | Along with measurements | + | |
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| {{ : | | {{ : | ||
- | Time series | + | Time series |
===== MWRnet ===== | ===== MWRnet ===== | ||
- | [[http:// | + | [[http:// |
{{ : | {{ : | ||
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===== References ===== | ===== References ===== | ||
+ | [1] Microwave Remote Sensing—Active and Passive”. By F. T. Ulaby. R. K. Moore and A. K. Fung. (Reading, Massachusetts: | ||
+ | |||
+ | [2] Thermal Microwave Radiation: Applications for Remote Sensing, C. Matzler, 2006, The Institution of Engineering and Technology, London, Chapter 1. | ||
+ | |||
+ | [3] http:// | ||
+ | |||
+ | [4] Passive Microwave Remote Sensing of the Earth, Physical Foundations, | ||
+ | |||
+ | [5] Czekala et al. (2001), Discrimination of cloud and rain liquid water path by groundbased polarized microwave radiometry, Geophy. Res. Lett., DOI: 10.1029/ | ||
+ | |||
+ | [6] Bennartz, R., and P. Bauer (2003), Sensitivity of microwave radiances at 85–183 GHz to precipitating ice particles, Radio Sci., 38(4), 8075, doi: | ||
+ | |||
+ | [7| Kneifel et al. (2010), Snow scattering signals in ground-based passive microwave radiometer measurements, | ||
+ | |||
- | - http:// | ||
- | - http:// | ||
- | - http:// | ||
- | - Thermal Microwave Radiation: Applications for Remote Sensing, C. Matzler, 2006, The Institution of Engineering and Technology, London, Chapter 1. | ||
- | - Eugene A. Sharkov, “Passive Microwave Remote Sensing of the Earth”, Physical Foundations, | ||
- | - Cimini et al., 2009 | ||
- | - Klein and Gasiewski, 2000 | ||
- | - Eugene A. Sharkov, “Passive Microwave Remote Sensing of the Earth”, Physical Foundations, | ||
- | - http:// | ||
instruments/hatpro/hatpro.txt · Last modified: 2021/01/22 22:17 by 127.0.0.1