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The main objective of the Microwave Radiometer is the measurement of atmospheric humidity as supplementary information for tropospheric path correction of the Radar Altimeter signal, which is influenced both by the integrated atmospheric water vapour content and by liquid water. In addition, MWR measurement data are useful for the determination of
surface emissivity and soil moisture over land, for surface energy budget, investigations to support atmospheric studies and for ice characterization.

The MWR instrument on-board ENVISAT-1 is a derivative of the radiometers used on the ERS-1 and ERS-2 satellites. It is a dual channel, nadir pointing Dicke type radiometer, operating at frequencies of 23.8 GHz and 36.5 GHz.

In order to eliminate Earth’s irradiation, differential measurements at two frequencies have to be performed. The optimal frequency setting is achieved by using one frequency at the maximum and one at minimum attenuation. The frequencies 23.8 GHz ad 36.5 GHz are the result of a trade off between instrument (reflector) size required to cover a horizontal area on the Earth surface comparable to the RA-2 beam and the maximum sensitivity to water vapour change in the atmosphere.

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With one feed horn for each frequency, the MWR points via an offset reflector at an angle close to nadir. The instrument configuration is chosen such that the 23.8 GHz channel is pointing in the forward direction, the 36.5 GHz channel in the backward direction with a footprint of about 20 km diameter for each beam.

The MWR instrument design principle is based on the design for the ERS-1 instrument as shown in the functional block diagram. In nominal Dicke operation, the measured antenna temperatures are continuously compared with an internal reference load at a known temperature. In instrument internal calibration mode, the receiver is either connected to a sky horn pointing to cold space (cold reference) or to a load at ambient temperature (hot reference). The calibration range thuscovered is 3 K to 300 K with a reference temperature
accuracy of better than 0.1 K. Instrument internal calibration is performed every few minutes.

The microwave radiation is received by an offset feed parabolic reflector antenna, routed through a Dicke switch assembly to a down converter which translates the K- and Ka-band signals to a suitable IF range. The IF signal is passed through a band limited amplifier to a synchronous detector.

 

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The detected signal is integrated, sampled and the resulting level converted to a corresponding frequency which is measured. These measurement data, reference load temperatures and ancillary data are routed to the MWR/DORIS ICU for further transmition.

Retrieval of antenna and brightness temperature values from measurement data is accomplished by ground processing. This considers ground calibration data, antenna characteristics and in-orbit characterization data.

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For ENVISAT-1 the design of the MWR had to be modified in some areas compared to its ERS 1/2 predecessor to comply with the different platform and mission requirements. The structure is a new design using CFRP technology having the old deployable antenna replaced by a non-deployable one, fully integrated into the instrument structure.

The design of the Central Electronics has been adapted to the revised internal and external interface requirements and to the change of components. The Local Oscillators feature a completely new design where the Gunn Oscillators have been replaced by Dielectric Resonator Oscillators.

The MWR is developed under leadership of Alenia Aerospazio.