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    24-Jul-2014
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1.1.6 Peculiarities of MWR

The MWR design concept derives from the experimental radiometers embarked on ERS-1 satellite. It is a two channels passive Dicke microwave radiometer, operating at 23.8 GHz and 36.5 GHz, and devoted to measure the amount of water content in the atmosphere beneath the satellite's track (Nadir pointing).

Its output products are of prime importance for products of Radar Altimeter (RA-2) Instrument, part of the Envisat-1 payload, providing correction of atmospheric propagation data. A secondary objective is the direct evaluation of brightness temperature to characterise polar ice, land surface properties.

Elements of the experimental radiometer of ERS mission have been used as input, but substantial new design has been done to improve instrument performance and its calibration accuracy, and to upgrade the design to current technology. In particular the Calibration and characterisation approach, the radio frequency design and electromagnetic compatibility aspects, the thermal and structural design, have been completely re-addressed.

The following key elements are modified/improved in the MWR of ENVISAT-1:

 

  • Instrument Supporting Structure

The structure subsystem is fully re-designed due to ENVISAT-1 requirements and to the new satellite configuration. The new structure provides optimum stiffness and stability performance, reduced mass, and includes also the Reflector support interface.

 

  • Instrument Thermal Control

Instrument thermal design is completely revised and an active thermal control is included to optimise the performance reducing temperature excursions on the Radio Frequency section.

 

  • Antenna

The reflector design is the same as for ERS, except for its supporting interfaces which are new design. Feed-horns design is optimised to improve some performance and to optimise the beam efficiency performance. which was not specified on ERS. The antenna does not need deployment like on ERS, it is already in place, thus reducing the risk of misdeployment or mispointing.

 

  • RF Front End

From functional point of view the ERS architecture is maintained. The overall RF Front End design is reviewed from mechanical view point, for thermal control hardware implementation, for EMC design, and for reliability and product assurance aspects.

 

  • Centralised Electronic Unit (CEU)

From functional architecture point of view a similar architecture to the ERS one is adopted. The CEU is significantly re-designed due to new system requirements either in terms of interfaces and performance.

 

  • Ground Support Equipment

This area is really new design area, given the completely new calibration, verification and validation approach. Also the development of a set of stimuli equipment for radiometric tests and calibration has been necessary to allow the completion of these activities (waveguide cryogenic loads, blackbody targets, thermal vacuum calibration targets, etc.).

 

The driving performances of the Instrument are summarised in the following table:

 

Table 1.2

Performance

Value

Radiometric Sensitivity

0.4 K

Radiometric Stability

0.4 K

Dynamic Range

3K to 335K

Non-linearity

0.35 K

Radiometric Accuracy (after calibration)

1 K, with Tant = 300K

< 3K with Tant = 85 to 330K

On board settable Intercalibration period

38.4, 76.8, 153.7, 307.4 sec

Noise Figure

4.8 dB incl. Antenna

Frequency accuracy 36.5 and 23.8 GHz

< ± 3.0 MHz

Antenna Radiation Efficiency

97 %

Antenna Main Beam Efficiency

95.26% worst case (23.8 GHz)

Antenna Side Lobes Level (in 3° half angle)

24 dB

31 dB

Antenna Half Power beamwidth (3 dB)

1.5 °

Instrument Mass

24 Kg

Operational Power

18 Watts

 

In the following figure the apparent radiometric temperature contributions are shown:

 

 


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry