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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


2.11.4 External Characterisation

ASAR can be put into External Characterisation Mode while flying over a calibration transponder. This involves sending a series of pulses from each of the 32 rows in turn followed by each of the 10 columns in turn.
These pulses are detected both by the internal calibration loop and the receiver embedded in the transponder. Comparison of these data allows characterising the passive part of the antenna and the calibration network. The baseline is to repeat measurement every six months. Characterisation of the Antenna Beam Pattern

For the characterisation of the antenna beam pattern, images of the Amazon Rain Forest are used. This is because the rai forest it is a stable, large-scale, isotropic distributed target with a relatively high backscatter and a well-understood relationship between backscatter and incidence angle.

In order to determine the two-way beam pattern, an uncorrected rain forest image is averaged in the azimuth direction. In the final processed image, the inverted beam pattern is applied and hence the effect of the pattern on the backscatter is removed.

Alternative distributed targets at different latitudes are being investigated. Promising results have been found from ERS data over Lake Vostok in Antarctica. Antenna pattern estimates at different latitudes could be used to verify the round-orbit performance of the ASAR. Gain Calibration

The purpose of the ASAR gain calibration is to provide the users of ASAR data with the possibility to determine the absolute level of backscatter from any target, point (s) or distributed (s0). For ASAR this is achieved in fundamentally the same way as for ERS, namely by providing an Absolute gain Calibration Factor (ACF) in the header of the (processed) product. Since ASAR, however, has a total of eight beams and five different modes and up to four polarisations more ACFs will need to be determined for ASAR than for the ERS single beam, single polarisation with two modes.

The method to be used to determine the ACFs is to image a target of known radar cross-section, integrate the power in its Impulse Response Function (IRF) corrected for the associated background (clutter) power and hence calculate the correction (the ACF) which must be applied to the image values in order to arrive at the same cross-section for that target. For this purpose, precision calibration transponders are deployed in the Netherlands. The radar cross-section of these transponders is 65dBm2 and is known to within ±0.13dB and they are stable to 0.08dB Ref. [2.14 ] . It is necessary to use active radar calibrators (transponders) as opposed to passive ones (e.g. corner reflectors) since the ratio of signal to clutter determines the accuracy to which the calibration can be made.

Once the ACF for a particular configuration has been calculated it will be possible to make a direct comparison with the on-ground measurements of the end-to-end system gain carried out during FM testing. Global Monitoring Mode

This is a special case since the spatial resolution of 10001000m makes the normal use of the transponders unfeasible. For a reasonable calibration to be made (3s value of ± 0.5dB), a signal to clutter ratio of better than 30dB is required. If the clutter at the calibration sites typically has a sigma nought of -6dB then this would require a transponder RCS greater than 84dBm2. This would inevitably saturate the receiver invalidating the calibration. As a result, it is necessary to come up with an alternative scenario for calibrating this mode.

The first option (baseline) is using the other modes (namely Wide Swath and Image) to calibrate GM mode by means of the Amazonian rain forest. Since the s0 of the rain forest is stable to within 0.3dB it will be possible to use the s0 value obtained from a previous (or subsequent) pass in WS or IM to calibrate GM mode. In addition, other relatively stable distributed targets may be used such as the ice caps and specific desert regions (Gibson, Gobi etc).

The second option will allow direct calibration using a special global monitoring mode setting and a modified calibration transponder. The intention is to use the ASAR's digital chirp generator to offset the centre frequency by 5MHz. This is possible since the chirp bandwidth in GM mode is only around 1MHz. In the calibration transponder, the received signal is shifted back by 5MHz allowing it to be received by the ASAR. As the clutter return will all be outside the range of the reduced bandwidth filter in GM mode, only the transponder response will be seen in the processed image against a background of noise. The result of this operation is to provide a transponder signal to clutter ratio of between 25 and 30dB allowing for reliable calibration of Global Monitoring Mode.