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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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:// | 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 ===== | + | ===== Design |
+ | 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, | ||
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+ | Usually ground-based radiometers are also equipped with environmental sensors (rain, temperature, | ||
- | The principal components of a microwave radiometer often follow a similar design and can be grouped into: antenna system, microwave radio-thermal receiver, recording and storage devices and a final processing unit. Usually ground-based radiometers are also equipped with environmental sensors (rain, temperature, | + | {{:stuff:mwr_design.png|Schematic diagram of a microwave radiometer}} \\ |
- | + | Fig. 4: Schematic | |
- | {{:instruments: | + | 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. |
- | Fig. 2Schematic | + | |
===== 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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instruments/hatpro/hatpro.txt · Last modified: 2021/01/22 22:17 by 127.0.0.1