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3.4 MWR Instrument Characteristics and Performance

3.4.1 Pre-flight Characteristics and expected performance

3.4.1.1 Radiometric Receiving System

Key performances which have to be considered as the receiver design driving elements are the Noise Equivalent Temperature, the input impedance matching (VSWR), the frequency response, gain linearity, gain stability over temperature and ageing, Analog to Digital Converter quantisation noise.

 

Further design driving requirements are imposed to receivers as flight space hardware, in particular for minimum power consumption, mass, dimensions, wide temperature ranges under vacuum conditions, high reliability, exposure to radiation, electro-magnetic and radio frequency compatibility with the instrument and the spacecraft environment, mechanical vibrations and shocks, use of proven and already qualified (or at least qualifiable) technologies for space applications.

 

In the following major emphasis will be put on the trade-offs and design choices driven by radiometric performance, with some basic considerations regarding the construction, environment and technological aspects, as far as performance are concerned.

 

MWR receivers are based on the "Dicke Radiometer" concept, thus providing the improved short term gain stability but degraded (1/2 according to theory) radiometric sensitivity performance when compared to Total power radiometer concept.

 

The detected output voltage is linear with respect to the input mean noise power collected from the antenna, i.e. the detector diode is operated within the quadratic region of its characteristics.

 

The receiver architectures used is the superheterodyne.

 

The superheterodyne offers the inherent advantage of flexibility, as by proper selection of the Local Oscillator (LO) frequency, the band defining filtering, as well as most of the needed amplification, can be allocated in a convenient Intermediate Frequency (IF) band, where adequate components and technologies can be selected to satisfy the requirements.

 

The key elements to be considered in the trade-offs are the filtering, amplification, and the stability aspects.

The filtering of a radiometric chain is a key element. Supposing and ideal quadratic detector characteristics, the digitalised output voltage can be expressed, in a simplified form, as:

 

Vod = k (Trec + TA) Bn Geq 3.17

 

where k is the Boltzmann constant, Trec is the receiver noise temperature, TA is the antenna noise temperature, G is the receiver end to end transfer function (expressed as output voltage to input Antenna noise power, Volts/Watts), Bn is the noise equivalent bandwidth.

 

This means that all the energy collected in Bn will be detected as brightness temperature coming from the Earth scene being observed, and therefore the Bn shall be as much as possible closed to the allocated bandwidth for the radiometric measurement of that channel (the filter shape shall be as much as possible rectangular).

 

MWR requirements impose a pre-detector rejection of at least 40 dB in a band which is 1.7 times the RF band (pass band 200 MHz, 40 dB stop-band 340 MHz), which means that the energy collected outside the 1.7 band is 1/10000 with respect to the in-band energy.

 

Also the Bn variations over temperature and life will be seen as gain variations, and therefore the selected filter shall be very stable.

 

The best suited filters are of the Elliptical and Chebyshev type, where excellent out of band attenuation characteristics are achievable at frequencies of 1.7 times the pass band frequency, with acceptable pass band ripple and order number (acceptable filter complexity).

 

In case a direct detection topology would have been chosen for each of the MWR channels, fractional bandwidths in the range 0.84 % (at 23.8 GHz) to 0.55 % (at 36.5 GHz) would result.

 

In these conditions to achieve the required rejection a rather complex cavity filter would have been developed, with significant size and losses. An additional constraint is represented by the required center frequency stability over temperature and life, which leads to the development of an extremely stable Invar filter or similar.

 

Selecting the current superheterodyne concept, the IF frequency is selected in order to achieve a confortable fractional bandwidth, where a lumped elements filter can be used with excellent performance and higher level of miniaturisation.

 

Frequency stability is higher for the superheterodyne concept, due to the lower frequency (as it is a percentage of the centre frequency) and by proper selection of the Local Oscillator performance.

 

MWR receiver is a superheterodyne Double Side Band (DSB) with low pass filtering function at low frequency range (DC to 340 MHz).

 

The DSB concept also improves the receiver Noise Figure, at the expense of considering a wider RF bandwidth with respect to the IF one.

 

The required pre-detection band-width is about 200 MHz, and therefore by extending the RF band to 400 MHz and by choosing a LO frequency equal to the centre frequency (23.8 GHz and 36.5 GHz respectively), a base band of 0 · 200 MHz results at the mixer output: the energy in this 200 MHz base band includes the energy of the full 400 MHz RF band, which means a noise power 3 dB higher (as the "negative" frequency side-band folds-up to the positive one).

 

Consequently, from the radiometric point of view, the mixer insertion losses, and hence the receiver Noise Figure, is improved by a 3 dB factor. This characteristic of the DSB architecture makes it very attractive when compared to a Single Side Band (SSB) one or to a direct detection one.

 

The disadvantage is, of course, the double RF bandwidth to be considered, which will degrade the mixer input VSWR, the Noise Figure and the in band ripple.

 

A more detailed description of each receiver component (Antenna, RF Front-End, IF module, and Analog module) follows in the following paragraphs See Antenna 3.4.1.2. to See Intermediate Frequency (IF) and Video.

 

Two directional couplers have been implemented at the input of the Measurement Antenna paths, in order to allow injection of RF signals in the receiver channels for RFC and functional tests.

These couplers are embedded in the Measurement feed-horns, as described in the following section See Measurement Feed-Horns.

 

From a functional and development philosophy view point, the following subsystems have been identified.

3.4.1.2 Antenna

Antenna subsystem includes a reflector, the measurement feed-horns (one for each frequency channel), and the Sky-Horn.

 

The measurement antenna retains the concept of a single 60 cm chord offset parabolic antenna with a the reflector in aluminium structure.

 

The feed-horns are configured so as to optimize the antenna pattern as well as to provide high beam efficiency performance. The tight HPBW allows high spatial resolution on Earth surface.

 

The measurement feed-horns and the Sky-Horn have been breadboarded and test measurements have exhibited performances in accordance with specifications.

 

A summary of the main characteristics is reported in the following paragraphs.

3.4.1.2.1 Measurement Feed-Horns

The measurent feed-horns are two, namely FH-24 for 23.8 GHz channel and FH-37 for the 36.5 GHz one.

These feeds have an internal geometry constituted of stepped circular sections, which, near the feeds throat, guarantee with their diameter and length an adequate return loss behaviour of the feed, an simultaneously the excitation of only one fundamental mode TE11.

The other stepped circular sections near the feed aperture excite higher order modes, mainly the TM11 mode.

The internal feed geometry near the aperture provides a symmetric primary pattern with an adequate combination in phase and amplitude of the two TE11 and TM11 modes.

Each feed is connected to a circular to rectangular waveguide transition to extract the vertical polarisation. The two feeds are placed in adequate positions near the parabola focus to satisfy (and not exceed) at secondary pattern level the required antenna boresight depointing of ± 3° and ellipticity of 0.4°.

The last section of the rectangular waveguide allocates a directional coupler, to allow, with the feed integrated on the instrument, the injection of RF signals within radiometric receivers for RFC and functional tests at instrument level.

3.4.1.2.2 Measurement Antenna

The measurent Antenna optic is the same as per ERS-1 and ERS-2 design.

 

The reflector is an offset paraboloid with the following geometry:

 

Projected diameter: 600 mm

Focal Length: 350 mm

Clearance: 50 mm

Offset angle: 46.965°

Half illumination angle: 38.793°

 

The feed-horn positions with respect to the parabola focus have been chosen to optimise the beam pointing and ellipticity requirements for the secondary patterns.

3.4.1.2.3 Sky-Horn Antenna

The Sky-Horn is a feed able to receive simultaneously the Cold Sky radiation in the two frequency band of interest for MWR.

 

The ratio between the two frequencies is 1.534, and, to achieve good radiative performances in such wide band, a corrugated feed design has been necessary.

 

The frequency ratio of 1.534 imposes a careful design and optimisation to provide a corrugated feed which simultaneously presents resonances in the two bands.

 

ALS have taken into account the necessity to have a compact feed design with a narrow pattern.

For these reasons a scalar horn has been selected, with a 20° flare angle and an aperture diameter of about 80 mm.

Considering the narrow Sky-Horn pattern the Cold Sky radiation is received with the minimum interference from potential spurious hot sources.

 

An orthomode Tee transducer (OMT) has been implemented to separate the two frequency bands and extract the Cold Sky reference signal for each of the MWR receiver channels.

3.4.1.3 Radio Frequency Front End

The RF Front - End Assembly (RFFE) is composed by two receivers, one for 23.8 GHz and the other for 36.5 GHz channels. Each channel contains the following subassemblies:

 

Switching and Load Subassembly (SA)

Mixer-Amplifier Subassembly (MA)

Local Oscillator Subassembly (LO)

 

All these unit is mounted on a common baseplate and is interconnected by a dedicated harness.

 

On the RFFE is implemented an heating system (HS) to maintain the temperature in a reduced range, in order to improve the stability of the Dicke and Hot reference loads, and to optimise the Isolation characteristics of the ferrite circulators used in the Switching Assembly.

 

The RFFE is integrated to the CEU in terms of DC power lines, switch control lines, IF coaxial lines and thermistors monitor lines.

The latter are connected to thermistors which are used to measure the reference temperatures inside the RFFE which is necessary for the instrument MWR calibration algorithm.

 

The RFFE is also interfaced to the feeds (Main Horn and Sky Horn) of the Antenna unit.

 

A cold redundancy is foreseen per each Local Oscillator assembly.

The operating LO is selected activating at CEU level the proper power supply line.

 

A functional block diagram of the RFFE is given in figure below.

 

Extensive trade-off was performed to select the best solution for the RF input filter included in the Switching assembly. The solution adopted is the most performing in terms of low insertion losses, with the objective of improve Instrument sensitivity, with slightly degraded out of band rejection characteristics.

 

 

3.4.1.4 Intermediate Frequency (IF) and Video

The combination of Intermediate Frequency Module and the Analog Board is used to process the downconverted radiometric signals. Two identical units are required, one for channel 1 (23.8 GHz) and the other one for channel 2 (36.5 GHz).

 

IF Module (historically named Amplifilter) and Analog Module are allocated within the Centralised Electronic Unit (CEU) assembly.

3.4.1.5 Instrument Operational Mode

The MWR is a standalone instrument: it operates as an instrument composite by the common ICU together with DORIS.

The only operating mode of the instrument is the MWR-ON mode. When MWR is in the MWR-ON mode it shall measure in two frequency band, operating continuously in the whole orbit, the thermally emitted microwave radiation coming from the antenna look direction.

During the MWR-ON several calibration status are actionable by the telecommands: they establish the intercalibration periods (functional modes) i.e. how many main antenna measurements can be performed before performing again hot load and sky horn calibration. The possible intercalibration periods (calibration status) are reported in the following table.

 

 

Table 3.3

CALIBRATION PERIOD

OPERATION

Every 38.4 sec

Nominal

Every 76.8 sec

Nominal

Every 153.6 sec

Nominal

Every 307.2 sec

Nominal

3.4.1.6 Instrument Data Characteristics and Data Rate

The MWR is a LBR Instrument ( Data Rate is <10 Mbit/sec ).

The overall MWR system breaks down into two propagation paths from the observed scene (one for each measurement channel) to the instrument, the source segment (MWR instrument and platform interface) an up/down link to the ground segment and the ground segment itself.

The ground segment includes some co-processing of data with the Radar Altimeter (RA-2) instrument data.

The Data rate of MWR scientific data is 0.427 Kbps (64 bit every 0.150 sec)

As in Nominal conditions it will operate continuously, the 100 minutes orbit total amount of data is 2.56 Mbits only for scientific data.

Considering the expected ICU packaging and the data rate of the MWR dedicated channel the global data rate is 1332 bytes every 24 sec sent to ICU to High Speed Multiplexer, i.e. 0.375 Mbytes for the whole orbit taking into account the whole data (headers, packet controls, etc.).

3.4.1.7 Instrument Improvements

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 configuration (different launch loads, different position, non deployable antenna, etc.). The structure is developed in CFRP (Carbon Fibre Reinforced Plastic) to provide optimum stiffness and stability performance, while reducing mass, and include also the Reflector support interface, thus being a key element for the Antenna and instrument performance optimisation (pointing, beam efficiency, etc.).

 

  • Instrument Thermal Control

Thermal design is significantly different due to the class A definition of the instrument, which is now mounted externally to the PEB and completely de-coupled from PPF structure (radiative coupling to cold space only). Instrument thermal design is completely revised and an active thermal control is included to optimise the performance reducing temperature excursions on the RF 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 (return loss, side lobes level, sky horn OMT isolation, etc.) and to optimise the beam efficiency performance, which was not specified on ERS. Pointing and thermal distortions are re-considered given the new structure approach.

 

  • 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. Technological improvements/upgrade are extensively implemented for the:

 

Switching and Loads Sub-assembly new design to reduce insertion losses, improve isolation and return losses, improve Dicke and Reference Loads characteristics and thermal stability.

Local Oscillators to implement higher reliability, higher efficiency and higher frequency stability Dielectric Resonator Oscillator technology, which is qualified in the frame of this programme. The unit is completely re-designed (fundamental 23.8 GHz Oscillator, active doubler for 36.5 GHz oscillator, single supply operation).

Mixer amplifier to improve Noise figure and stability performance and to implement state of the out beam lead schottky diodes waveguide mixer and Low Noise hybrid amplifier.

 

  • 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, in particular:

 

Technological upgrade, new components/functions are implemented.

The RF Front End evolution requires significant improvement of the power supply performance, with excellent stability at End Of Life and very low noise.

The CEU power supply is a completely new design, also from architecture point of view, given the new requirements.

Higher accuracy for instrument temperature measurement, required by the instrument radiometric performance optimisation and calibration.

Receiver performance improvement, e.g. linearity, temperature stability, short term gain stability, video section, IF section, etc.

Failure propagation avoidance requirements for implementation of switches/heaters redundancy protections implementation.

New functions requiring additional commands and telemetry data.

Reliability figure required (N/A or ERS).

EMC requirements (N/A or ERS).

Mechanical/Thermal design modifications, to meet the ENVISAT-1 launch loads and the new instrument thermal control concept (category A).

 

  • Ground Support Equipment

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


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