ESA Earth Home Missions Data Products Resources Applications
EO Data Access
How to Apply
How to Access
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 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.

imagefull size
Figure 3.2 Subsatellite Track

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.

ASAR Operational Mode Swaths
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.

ASAR FM Antenna During Radiation Testing (Image by courtesy of ASTRIUM Ltd)
Figure 3.4 ASAR FM antenna during radiation testing (Image by courtesy of ASTRIUM Ltd)

ASAR FM Antenna After Deployment Test (Image by courtesy of ASTRIUM Ltd)
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.

ASAR Instrument Synoptic Diagram
Figure 3.6 ASAR instrument synoptic diagram

The ASA and CESA 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)

Power Conditioning Unit (PCU)
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.