3.1 Instrument Description
Principle of Operation
The radar
antenna
beam illuminates the ground to the
right side of the satellite. Due to the satellite
motion and the along-track (azimuth) beamwidth of
the antenna, each target element only stays
inside the illumination beam for a short time. As
part of the on-ground processing, the complex echo
signals received during this time are added
coherently. This process is equivalent to a long
antenna (so called Synthetic Aperture)
illuminating the target. This synthetic aperture
is equal to the distance the satellite travelled
during the integration time.
The along-track
(equivalent to the azimuth in the ground
processing) resolution obtainable with
the SAR principle is half the physical
antenna length. The achieved resolution can
be traded off against other image quality parameters
(such as the radiometric
resolution). The across-track or range resolution is a
function of the transmitted radar bandwidth.
Pulse
compression
techniques are used to improve the
performance taking into account the
instrument peak power capability. The fact
that the end-to-end system works
coherently means that both the amplitude and the phase relationships
between the complex transmitted and received
signals are maintained throughout
the instruments and the processing chain. .
This facilitates aperture synthesis, as well
as multi-pass radar interferometry,
using pairs of images taken over the same
area at different times.
General Description
The Advanced Synthetic Aperture Radar
(ASAR) was built upon the experience gained with
the ERS-1/2 Active Microwave Instrument
(AMI) to continue and extend Earth observation
with SAR. ASAR is a high-resolution, wide-swath imaging radar instrument
that can be used for site-specific investigations as
well as land, sea, ice, and ocean monitoring and surveillance.
Compared to ERS AMI, which is a single-channel,
fixed-geometry instrument, the ASAR instrument
provides a number of technological improvements.
Significant advances have been made in both system
flexibility and the scientific value of its data
sets, employing a number of new technological
developments that allow extended performance. The
most challenging advancement has been the
replacement of the centralised high-power
amplifier, combined with the wave guide slot passive
radiator array of the AMI, by an active phased-array
antenna system using distributed elements.
Transmit/Receive (T/R) modules are arranged across
the antenna such that, by adjusting the gain and
phase of individual modules, the transmit and
receive beams may be steered and shaped, allowing
the selection of different swaths and providing a swath
coverage of over 400-km wide using ScanSAR techniques.
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| Figure 3.3 ASAR operational mode swaths |
ASAR is also equipped with programmable digital
waveform generation, which will allow optimisation
of the product radiometric quality. Another
improvement, compared to ERS, is an 8 bit
Analog-to-Digital Converter (ADC) associated with a
Flexible Block Adaptive Quantiser (FBAQ) to be
used in 8/4 and 8/2 compression ratio that will
allow the collection of a larger dynamic range of
input signals within the data rate constraints.
The two pictures below show the ASAR Flight Model
(FM) Antenna. The first (figure3.4 ) shows the antenna
during radiation testing at ASTRIUM Portsmouth and
the second (figure3.5 ) shows it after a
deployment test.
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| Figure 3.4 ASAR FM antenna during radiation testing (Image by courtesy of ASTRIUM Ltd) |
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| Figure 3.5 ASAR FM antenna after deployment test (Image by courtesy of ASTRIUM Ltd) |
The ASAR instrument comprises two functional groups:
the Antenna Subassembly (ASA) and the Central
Electronics Subassembly (CESA), with subsystems
as shown in the functional block diagram below (figure3.6 ).
The active antenna contains 20 Tiles with 16
Subarrays each equipped with a Transmit/Receive
(T/R) module. The instrument is driven by the Control Subsystem (CSS), which
provides the command and control interface to the
spacecraft, manages the distribution of the
operational parameters (such as transmit pulse
characteristics and antenna beam-set), and
generates the instrument operation timeline.
The transmit pulse characteristics are set in the
Data Subsystem (DSS) the output of which is an
up-chirp pulse centred on the IF carrier (124
MHz). In the RF Subsystem (RF S/S) the pulse is
up-converted to the RF frequency (5.331 GHz) and
amplified. The signal is then passed to the Tile
Subsystem (TSS) through a waveguide distribution
network (RFPF) and subsequently, within the tile, to
each individual T/R module using a microstrip
corporate feed. The T/R modules apply phase and gain
characteristic according to the pre-selected beam
settings transferred from the Control Sub-System and
stored in the Tile Control Interface Unit (TCIU).
In receive the RF-echo signal follows the reciprocal
path down to the Data Sub-System where the raw
science data are generated and provided to the
spacecraft interface.
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| Figure 3.6 ASAR instrument synoptic diagram |
The ASA 3.1.1.1. and CESA 3.1.1.2. subsystems, which are
described in the "Payload Description,
Position on the Platform" 3.1.1. section
to follow, are comprised of the following primary components:
ASA Subsystem
- Antenna Services Subsystem (ASS)
- Tile Subsystem (TSS)
- Antenna Power Switching and Monitoring Subsystem (APSM)
CESA Subsystem
- Data Subsystem
- Radio Frequency subsystem (RFSS)
- Control Subsystem (CSS)
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| Figure 3.7 Power Conditioning Unit (PCU) |
The ENVISAT ASAR system will benefit from the
on-board availability of a Solid State Recorder,
allowing up to 10 minutes recording of ASAR high
rate modes (100 Mbit/s) at any point around the
orbit. Finally, the orbit determination system on
the DORIS instrument will allow for accurate
geolocation of all ASAR products produced in near
real-time (NRT) or off-line.
The main new technical features are:
- Instrument enhancements that include a digital
chirp generator (programmable from 200 kHz to 16
MHz) and an improved linear dynamic range.
- Flexible swath positioning; offering
the choice between several image swath
positions at various distances from the
subsatellite track, with different incidence angles.
- Dual polarisation; offering horizontal (HH)
& vertical (VV) or cross polarisation (HH&HV or
VV&VH) operation.
- Wide swath coverage; 405 km swath with 150 m or
1 km resolution.
- Enhanced Wave Mode with imagettes acquired at
100 km intervals along-track.
- Extended operating time at high-resolution (30
minutes of operation; 10 minutes in eclipse).
- Global SAR coverage; possible using the solid
state recorder or data relay satellite.
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EO Data Access
