3.4 MWR Instrument Characteristics and Performance
3.4.1 Pre-flight Characteristics and expected performance
18.104.22.168 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
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
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
k (Trec + TA) Bn G||eq 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
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 22.214.171.124. 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.
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
126.96.36.199.1 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
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.
188.8.131.52.2 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°
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.
184.108.40.206.3 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.
220.127.116.11 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
Switching and Load
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
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
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.
|Figure 3.2 RFFE Functional Block Diagram
18.104.22.168 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.
22.214.171.124 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.
Every 38.4 sec
Every 76.8 sec
Every 153.6 sec
Every 307.2 sec
126.96.36.199 Instrument Data Characteristics and
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.).
188.8.131.52 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
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.
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.
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.
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.
improvement, e.g. linearity, temperature
stability, short term gain stability, video
section, IF section, etc.
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).
modifications, to meet the ENVISAT-1
launch loads and the new instrument thermal
control concept (category A).
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.).