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ASAR Data Formats Products
Geolocation Grid ADSRs
Doppler Centroid parameters
Chirp parameters
Antenna Elevation pattern
ASAR external characterization data
ASAR external calibration data
Level 0 SPH
Level 0 MDSR
SPH for auxiliary data with N=1 DSDs
Wave Mode Geolocation ADS
ASAR Wave Mode Products Base SPH
Slant Range to Ground Range conversion parameters
Measurement Data Set containing spectra. 1 MDSR per spectra.
Ocean Wave Spectra
Map Projection parameters
ASAR Image Products SPH
Measurement Data Set 1
Auxilliary Products
ASA_XCH_AX: ASAR External characterization data
ASA_XCA_AX: ASAR External calibration data
ASA_INS_AX: ASAR Instrument characterization
ASA_CON_AX: ASAR Processor Configuration
Browse Products
ASA_WS__BP: ASAR Wide Swath Browse Image
ASA_IM__BP: ASAR Image Mode Browse Image
ASA_GM__BP: ASAR Global Monitoring Mode Browse Image
ASA_AP__BP: ASAR Alternating Polarization Browse Image
Level 0 Products
ASA_WV__0P: ASAR Wave Mode Level 0
ASA_WS__0P: ASAR Wide Swath Mode Level 0
ASA_MS__0P: ASAR Level 0 Module Stepping Mode
ASA_IM__0P: ASAR Image Mode Level 0
ASA_GM__0P: ASAR Global Monitoring Mode Level 0
ASA_EC__0P: ASAR Level 0 External Characterization
ASA_APV_0P: ASAR Alternating Polarization Level 0 (Cross polar V)
ASA_APH_0P: ASAR Alternating Polarization Level 0 (Cross polar H)
ASA_APC_0P: ASAR Alternating Polarization Level 0 (Copolar)
Level 1 Products
ASA_IMS_1P: ASAR Image Mode Single Look Complex
ASA_IMP_1P: ASAR Image Mode Precision Image
ASA_IMM_1P: ASAR Image Mode Medium Resolution Image
ASA_IMG_1P: ASAR Image Mode Ellipsoid Geocoded Image
ASA_GM1_1P: ASAR Global Monitoring Mode Image
ASA_APS_1P: ASAR Alternating Polarization Mode Single Look Complex
ASA_APP_1P: ASAR Alternating Polarization Mode Precision Image
ASA_APM_1P: ASAR Alternating Polarization Medium Resolution Image product
ASA_WSS_1P: Wide Swath Mode SLC Image
ASA_WVS_1P: ASAR Wave Mode Imagette Cross Spectra
ASA_WSM_1P: ASAR Wide Swath Medium Resolution Image
ASA_APG_1P: ASAR Alternating Polarization Ellipsoid Geocoded Image
Level 2 Products
ASA_WVW_2P: ASAR Wave Mode Wave Spectra
ASAR Glossary Terms
Sea Ice Glossary
Land Glossary
Oceans Glossary
Geometry Glossary
ASAR Instrument Glossary
Acronyms and Abbreviations
ASAR Frequently Asked Questions
The ASAR Instrument
Instrument Characteristics and Performance
Inflight Performance Verification
Preflight Characteristics and Expected Performance
Instrument Description
Internal Data Flow
ASAR Instrument Functionality
Payload Description and Position on the Platform
ASAR Products and Algorithms
Auxiliary Products
Common Auxiliary Data Sets
Auxiliary Data Sets for Level 1B Processing
Summary of Auxiliary Data Sets
Instrument-specific Topics
Level 2 Product and Algorithms
Level 2 Product
ASAR Level 2 Algorithms
Level 1B Products
ASAR Level 0 Products
Level 0 Instrument Source Packet Description
Product Evolution History
Definitions and Conventions
Organisation of Products
ASAR Data Handling Cookbook
Hints and Algorithms for Higher Level Processing
Hints and Algorithms for Data Use
ASAR Characterisation and Calibration
The Derivation of Backscattering Coefficients and RCSs in ASAR Products
External Characterisation
Internal Calibration
Pre-flight Characterisation Measurements
ASAR Latency Throughput and Data Volume
Data Volume
Products and Algorithms Introduction
Child Products
The ASAR User Guide
Image Gallery
Further Reading
How to Use ASAR Data
Software Tools
How to Choose ASAR Data
Special Features of ASAR
Geophysical Coverage
Principles of Measurement
Scientific Background
Geophysical Measurements
ASAR Product Handbook
ASAR instrument characterization data
Wave Mode processing parameters
ASAR processor configuration data
Main Processing parameters
ASA_WVI_1P: ASAR Wave Mode SLC Imagette and Imagette Cross Spectra
Product Terms
RADAR and SAR Glossary
Level 1B Products
Summary of Applications vs Products
Site Map
Frequently asked questions
Terms of use
Contact us


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:

ENVISAT Payload Locations
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.

ASAR Antenna
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) 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 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


Hold-Down and Release Mechanisms (HRM)
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.

Tile (Image courtesy of Alcatel-Telecom)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.

ASAR Transmit Tile (Image by courtesy of Alcatel)
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

ASAR Transmit/Receive (T/R) Module (Image by courtesy of Alcatel)
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.


Supporting Plate
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. 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 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. 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.

Data Subsystemfull size
Figure 3.15 Data Subsystem

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). 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.

RF Subsystemfull size
Figure 3.16 RF Subsystem

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.

Calibration/switch equipmentfull size
Figure 3.17 Calibration/switch equipment 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. 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.

Figure 3.18 Power Conditioning Unit (PCU)