3.1.1 Payload Description and Position on the Platform
The Advanced Synthetic Aperture Radar (ASAR),
operating at C-band, ensures continuity with the
Image Mode (SAR) and the Wave Mode (WM) of
the ERS-1/2 sensor. As explained in earlier
sections, it features enhanced capability in
terms of coverage, range of incidence angles,
polarisation, and modes of operation.
The picture below shows the ENVISAT satellite and
the relative position of the instruments on board:
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| Figure 3.8 ENVISAT payload locations |
ASAR consists of a coherent, active phased array
antenna that is mounted with the long axis of
the antenna aligned with the
satellite's flight direction, or Y-axis. (
see figure3.2 "Subsatellite
Track" in the previous section entitled
"ASAR Instrument Description" and Figure3.9 below). The SAR
antenna, with its two-dimensional beam pattern,
will image a strip of ground to the right
side of the flight path which has potentially
unlimited content in the direction of motion,
which is the azimuth direction, but is
bounded in the orthogonal, or range, direction
by the antenna elevation beamwidth. The
objective of the SAR system is to produce a
two-dimensional representation of the scene
reflectivity at high-resolution, with axes
defined in both the range and azimuth directions.
|
| Figure 3.9 ASAR antenna |
A coherent, active phased array
SAR, is mounted with the antenna long axis
aligned with the satellite's flight
direction (i.e., Y-axis)
3.1.1.1 ASA Subsystem
The ASAR active antenna is a 1.3 m x 10 m
phased array. The antenna consists of five
1.3 m x 2 m panels which are folded over for
launch. Each panel is formed by four 0.65 m
x 1 m tiles mounted together. Each tile
consists of 16 linear subarrays of 24
dual-polarised radiating elements. Each
subarray is connected to a T/R module with
independent connection for the two polarisations.
As indicated in the synoptic diagram3.6 shown
in the previous section entitled
"ASAR Instrument
Description 3.1. ," the Antenna
Subassembly (ASA) consists of three
primary components:
It is one of the two main functional
subsystems within the ASAR instrument, with
the CESA subsystem 3.1.1.2. working
along side it. The primary units of the ASA
subsystem are described below.
Antenna Services
Subsystem (ASS)
The antenna is based on a mechanical
structure consisting of five rigid Carbon
Fibre Reinforced Plastic (CFRP) frames and a
RF distribution network consisting of
two similar sets of CFRP waveguides running
in parallel along the five panels (RFPF). In
launch configuration, the five panels
are stowed, folded over a fixed central one,
and are held together by eight Hold-Down and
Release Mechanisms (HRM) up to a preload
of 31 kN. In order to avoid coupling with
the launcher, this guarantees the first
axial vibration mode frequency to be
higher than 42 Hz.
Each HRM consists of a retractable telescopic
tube levered by a secondary mechanism based
on a non-pyrotechnic device (kevlar cable
cut by a redundant thermal knife with a
cutting time of less than 120 sec.) derived
from well-proven solar array hold-down technology.
After release, the panels are sequentially
deployed around four hinge lines by using a
stepper motor, each, with 200:1 reduction
harmonic drive, thus providing a high
motorisation margin (approximately 3 times
the expected resistive torque). The final
latching is performed with the eight
builtin latches to achieve the final antenna
planarity of ± 4 mm in orbit (this
including an apportionment of ± 1.5
mm for the overall thermoelastic effects).
Associated to inter-panel contact points,
the latches ensure waveguide flange
alignment and deployed rigidity of higher
than 2.4 Hz to avoid AOCS disturbances. A
specific unit (DCU) contains the electronics
for driving and controlling the release,
deployment and latching operations.
Each HRM consists of a retractable telescopic
tube, levered by a secondary mechanism based
on a non-pyrotechnic device derived from
well-proven solar array hold-down technology
The ASS is comprised of the following primary components:
· Antenna Mechanical Structure (AMS)
· RF Distribution Network (RFPF)
· Hold-Down and Release Mechanism (HRM)
· Deployment Mechanism (DEM)
· Deployment Control Unit (DCU)
· Antenna Harness
.
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| Figure 3.10 Antenna Mechanical Structure (AMS) |
Tile
Subsystem (TSS)
Each of the twenty tiles is a self-contained
full-operating subsystem which contains 4
power units (PSUs), a local control unit
(TCIU), 2 RF distribution corporate
feeds and 16 subarrays each one fed by a T/R
module. The 16 subarrays are mounted
together on a panel (the Radiating
Panel) that provides the structural and
thermal integrity to the tile.
 full size
|
| Figure 3.11 Tile (Image courtesy of Alcatel-Telecom) |
The corporate feeds (one for signal
distribution and another for calibration)
are 1:16 power dividers made in microstrip
on Duroid 6002. Each of the four Power
Supply Units (PSU) provides power to a group
of four T/R Modules and the TCIU. The
control functions within the Tile are
achieved by a Tile Control Interface Unit
(TCIU) which is carrying out local control
of the T/R Modules including temperature
compensation and beamforming setting, it is
transferring data and interfacing to the
Control Subsystem. There is one TCIU per
Tile using internal duplication for
redundancy purposes. A picture of one of the
ASAR transmit tiles, ready for radiation
testing, is shown in figure3.12 below.
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| Figure 3.12 ASAR Transmit Tile (Image by courtesy of Alcatel) |
Radiating Panel
The subarrays and the modules are mounted on
a supporting plate, thermally and
mechanically decoupled. The subarrays are
formed by 24 l-diameter ring slot
radiating elements, electrically coupled to
a dualpolarisation low-loss dispersion-free
triplate feeding system which provides a
constant phase and amplitude illumination.
The measured radiating characteristics of the
subarrays are showing a gain of 20.5 dB,
loss of 1.5 dB, VSWR of 18 dB and a very
good crosspolarisation better than 35 dB
for both polarisations.
Transmit/Receive Module
|
| Figure 3.13 ASAR Transmit/Receive Module (Image by courtesy of Alcatel) |
The T/R module receive signals passed to it
from the RF subsystem, within the CESA
subsystem. The T/R modules apply phase and
gain changes to the signal in accordance
with the beam forming characteristics which
have been given by the TCIU. For an active
antenna, the amplitude and phase
characteristics of the T/R Modules vary
principally as a function of temperature. To
handle this, fluctuation the instrument
includes a scheme to compensate for drifts
over the temperature range. For this
purpose, the temperature of each T/R module
is monitored and utilised by the TCIU to
compensate for the amplitude and phase
variations. This scheme provides the antenna
with a high degree of stability. The
signal is then power-amplified and passed to
the radiator panel.
Echo signals are received through the same
antenna array passing to the T/R modules,
for low noise amplification and phase and
gain changes, which determine the
receive beam shape. The outputs from each
module are then routed at the RF, via the
corporate feed, and the antenna RF
distribution system, which acts as a
combiner effectively adding signal inputs
coherently and noise inputs incoherently.
Each module consists of two (H/V) transmit
and one common receive chains, which amplify
and control the signal of each individual
subarray, in phase and amplitude. These
functions are implemented using MMIC's,
glued on etched TMM10i circuits, component
interconnections performed by using
parallel-gap welding (Au and Ag ribbons).
The input Phase Shifter (5 bits) is
controlling the Tx/Rx signal, a SP3T switch
selects either Tx (H/V) or Rx chains. On the
Tx chain a Variable Gain Amplifier (VGA)
provides 42 dB of control range, followed by
a medium power driver. A Telettra Power
Amplifier (SSPA of 10 W at 1 dB
compression, 30% efficiency), provides the
final output power. The connection to the
subarrays is made via a
Circulator/Isolator/Passive-Limiter (0.25 dB
insertion loss in Tx, 1 dB in Rx, 35 dB
isolation). On the receive side, a 1.3 dB NF
LNA (CFY67 HEMT) is connected to the
limiter, a SPDT switches the V/H chain to
the common Rx path which includes a third
VGA. For Calibration Purposes, a coupler
(-24 dB) has been implemented at the output
of the module to the antenna. The T/R Module
is commanded by a serial link consisting
of 3 signals: a data signal, a gated control
signal and a strobe signal. The temperature
of each T/R Module is monitored and used
for correction of amplitude and phase setting.
.
|
| Figure 3.14 Supporting Plate |
Antenna Power Switching
and Monitoring Subsystem (APSM)
The APSM forms the heart of the
antenna power distribution controlling the
power input to 80 tile power supplies. Each
tile PSU is independently switched under
full control from the instrument central
electronics. Current monitoring occurs on
two levels. The first, at each tile PSU
interface ensures that hazardous loads are
disabled autonomously within a few hundred
micro-seconds. Secondary protection is
provided through current telemetry back to
the central processor, which checks against
expected limits and commands switch-off
in the event that they are exceeded. The
setting of the APSM autonomous current trip
level required careful attention since it
was simultaneously required to guarantee the
safety of the tile PSU in the event of
anomalous loads, while ensuring adequate
availability. That is, not switching off
under extreme conditions of component
tolerance, temperature or peak ripple current.
3.1.1.2 CESA Subsystem
As indicated in the synoptic diagram3.6 shown
in the previous section entitled
"ASAR Instrument Description," the
Central Electronics Subassembly (CESA)
consists of three subsystems:
- Data Subsystem
- Radio Frequency (RF) Subsystem
- Control Subsystem (CSS)
It is one of the two main functional
subsystems within the ASAR instrument, with
the ASA subsystem 3.1.1.1. working
along side it.
The CESA is in charge of generating the
transmitted chirp, converting the echo
signal into measurement data, as well as
controlling and monitoring the whole instrument.
Compared to ERS-1 and ERS-2, which used
Surface Acoustic Wave devices for analog
chirp generation and On Board Range
Compression, ASAR uses digital
technologies for on-board chirp generation
and data reduction. A fundamental advantage
of using digital chirp generation is the
inherent flexibility of such a design which
allows for chirp versatility in terms of
pulse duration and bandwidth, thus
accommodating efficiently the various
requirements associated with the high number
of available operational modes and swaths of
the instrument.
In order to optimise raw data transfer, the
data equipment also contains science memory,
where the echo samples are temporarily
stored before their transmission to the
on-board recorders.
3.1.1.2.1 Data Subsystem
At reception, the echo signal is first
filtered and down-converted in the Radio
Frequency (RF) Sub-System, then
demodulated into the I & Q
components of the carrier. These two
signals are then both digitised into
8-bit samples. If required, it is then
possible to perform digital
decimation of the samples, in order to
reduce the data stream, such as in
Global Monitoring Mode where the
transmit bandwidth is low. Following
this decimation, a Flexible Block
Adaptive Quantiser (FBAQ) compression
scheme is applied to the echo samples.
The transmit pulse characteristics are
set within the data subsystem by
coefficients in a digital chirp
generator, which supplies in-phase
(I) and quadrature (Q) components. The
output of the data subsystem is a
composite up-chirp centred at the IF carrier.
The signal is then passed on
to the RF Subsystem where the coherent
RF/IF conversion of the RF echo signals
is performed in the downconverter.
I/Q detection of the Intermediate
Frequency (IF) echo signal is
accomplished in the demodulator of the
data subsystem. The resulting
baseband I/Q signals are further
processed in the data subsystem, which
performs filtering, digitalization, and
compression of this data. After
buffering and packetizing, the echo data
is transmitted to the measurement data
Interface (I/F).
3.1.1.2.2 Radio Frequency (RF) Subsystem
When the signal is passed on to the RF
Subsystem, it is mixed with the local
oscillator frequency to generate the RF
signal centred on 5.331 GHz. The
upconverted signal is routed via the
calibration/switch equipment to the
antenna signal feed waveguide.
The upconverted signal is
then routed, via the calibration/switch
equipment, shown in figure3.17 below, to the
antenna signal feed waveguide. At the
antenna the signal is distributed by
the RF panel feed waveguide network to
the tile subsystems within the ASA Subsystem.
 full size
|
| Figure 3.17 Calibration/switch equipment |
3.1.1.2.3 Control Subsystem (CSS)
The instrument is driven by the Control
Subsystem (CSS), which provides the
command and control interface to the
spacecraft, maintains the database,
manages the distribution of the
operation parameters, and generates the
time-lining of the instrument.
The ASAR instrument is controlled by its
instrument control equipment (ICE),
which provides the command and control
interface to the satellite.
Macrocommands are transferred from the
payload management computer to the ICE
where they are expanded and queued. The
ICE maintains and manages a database
of operation parameters, such as
transmit pulse and beam characteristics,
for each swath of each mode as well
as timing characteristics, such as pulse
repetition frequencies and window
timings. The ICE downloads parameters
from the database during transition
to the operation mode and provides the
operational control of the ASAR
equipment, including the control of
power and telemetry monitoring.
3.1.1.2.4 Power Conditioning Unit (PCU)
Shown in figure3.18 below, provides
a regulated supply to the data
subsystem and the RF subsystem, as well
as auxiliary power to the antenna power
switching and monitoring unit.
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| Figure 3.18 Power Conditioning Unit (PCU) |
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EO Data Access
