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Microwave Radiometers 
The Advanced Microwave Radiometer (AMR) Electronic assembly, delivered to the NASA/CNES Ocean Surface Topography Mission for launch in 2009. The instrument measures total water vapor along the path viewed by the radar altimeter.
The Advanced Microwave Radiometer (AMR) Electronic assembly, delivered to the NASA/CNES Ocean Surface Topography Mission for launch in 2009. The instrument measures total water vapor along the path viewed by the radar altimeter.
AMR Electronic assembly.  Detailed view of gallium-arsenide high electron mobility transistors monolithic microwave integrated circuit low-noise amplifiers (GaAs HEMT MMIC LNA).
AMR Electronic assembly. Detailed view of gallium-arsenide high electron mobility transistors monolithic microwave integrated circuit low-noise amplifiers (GaAs HEMT MMIC LNA).
JUNO MWR footprints along the orbit track.
JUNO MWR footprints along the orbit track.

Microwave Radiometers for ground-based, suborbital, and space flight observations have a long history of development. Development of the Radiometers, as well as the Imagers and Spectrometers, requires a multi-faceted team with expertise that includes instrument system design and system engineering, algorithm development, microwave analog and digital circuit design, fabrication, packaging design, instrument integration and test, field operations and data analysis. Current examples of radiometers are

  • Advanced Water Vapor Radiometer (AWVR), a very stable ground-based microwave radiometer for measurements of the water vapor contribution to the atmospheric refractive index at microwave frequencies. A key application is measurement of "path delay" in Deep Space Network communication links.
  • Microwave Temperature Profiler (MTP), a "forward looking" airborne instrument that has flown on numerous aircraft, for measurements of atmospheric temperature profiles. MTP is a very important data provider for studies of chemistry and dynamics in the upper troposphere and lower stratosphere.
  • Passive-Active L/S band system (PALS), an airborne instrument for measurement of ocean surface salinity, a precursor and demonstrator for the ESSP Aquarius mission. PALS includes both a passive radiometer and a radar scatterometer in one integrated system.
  • High Altitude MMIC Sounding Radiometer (HAMSR), a "nadir looking" airborne instrument for profiling temperature and humidity.
  • Advanced Microwave Radiometer (AMR) onboard Jason 2 measures water vapor content in the atmosphere so that we can determine how it impacts radar signal propagation. Its measurements also can be used directly for studying other atmospheric phenomena, particularly rain. The AMR is a passive receiver that collects radiation reflected by the oceans at frequencies of 18.7, 23.8, and 34 GHz. Radiation measured by the radiometer depends on surface winds, ocean temperature, salinity, foam, absorption by water vapor and clouds, and various other factors. To determine atmospheric water vapor content accurately, we need to eliminate sea surface and cloud contributions from the signal received by the radiometer. That is why the AMR uses different frequencies, each of which is more sensitive than the others to one of these contributions. The main 23.8-GHz frequency is used to measure water vapor; the 34-GHz channel provides the correction for non-rainbearing clouds; and the 18.7-GHz channel is highly sensitive to wind-driven variations in the sea surface. By combining measurements acquired at each of these frequencies, we can extract the water vapor signal.

Scientists and engineers are implementing new instrument designs for future applications. These designs will be driven by strict requirements on radiometric stability over a wide range of frequencies.

  • The Juno MWR (Microwave Radiometer) will be built for the Juno mission to Jupiter, expected to launch in 2012. MWR is presently in the preliminary design phase. The Juno MWR will be the second microwave instrument to explore the planets since the first observations from Mariner 2 of Venus in 1962, which confirmed that the high temperature inferred from radio measurements indeed reflected surface and deep atmospheric conditions rather than a hot ionosphere. The Jet Propulsion Laboratory builds the MWR.

    The primary goal of the Juno Microwave Radiometer is to probe the deep atmosphere of Jupiter at radio wavelengths ranging from 1.3 cm to 50 cm using six separate radiometers to measure the planet's thermal emissions. The MWR experiment will provide answers to two key questions: How did Jupiter form? and How deep is the atmospheric circulation that was measured from the Galileo Probe down to 22 bars of pressure, and at the cloud top level from imaging data returned by other missions?

    The first question will be addressed by the determination of the water abundance in the deep atmosphere. The MWR will obtain measurements of ammonia and water in the Jupiter atmosphere, which are the principle absorbers in the microwave region, by scanning Jupiter along the orbital track as the spacecraft spins. These observations will allow scientists to determine whether the water abundance on Jupiter is three times that of the sun or nine times that of the sun.

    The Juno MWR avoids the synchrotron emission from Jupiter's magnetosphere by using shorter wavelengths and achieves high accuracy to measure water abundance in the deep atmosphere by using "relative limb darkening," a parameter that depends on the emission angle of the radiation. The vertical profile of water abundance is obtained by using multiple frequencies, much like the retrieval of temperature profiles on earth with multi-spectral infrared measurements from orbiting weather satellites.

    The MWR uses three antennae mounted on the spacecraft body, which sweep across the planet as the spacecraft spins to measure the radiation at six different wavelengths along the orbital track. Successive orbits will map the planet longitudinally. The six different wavelengths observed by the MWR, combined with the emission angle dependence will provide a good idea of the atmospheric temperature profile to ~ the 200 bar pressure level on Jupiter. The latitudinal dependence of the temperature profile and depth will enable inference of the circulation of Jupiter's deep atmosphere to a much greater depth than that obtained by the Galileo probe. Dr. Mike Janssen (JPL) is the Lead Co-Investigator for the MWR Instrument Team.


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