ASAR INSTRUMENT CALIBRATION

• C.H. Buck, J.-L. Suchail, R. Torres, M. Zink
• ESA - ESTEC
• ENVISAT Project
• ERS - ENVISAT Symposium
• Gothenburg, 16.-20. October 2000

Differences between ASAR and ERS

• ASAR antenna is an active phased array, 320 T/R modules
  
• Each T/R module has two transmit chains (H/V) and one receive chain, each chain is independently programmable in amplitude and phase => 32 transmit/receive beams
  
• alternating polarization mode, ScanSAR operation
  
• Digital chirp generator
  
• Block adaptive quantiser (BAQ)

ASAR Calibration Steps

Internal Calibration: Objectives

• ASAR is equipped with an automatic temperature compensation scheme to compensate for phase/gain drifts at T/R module level
  
• Direct monitoring of any residual gain drifts for later correction
  
• It is desirable to have continuous measurements even during data acquisition
  
  
• Use of special calibration signals (cal pulses) and additional hardware (cal network)

Internal Calibration Pulse Diagram

Internal Calibration

• Row by row measurement in a pre-determined sequence campaign:
  • Monitoring of gain drifts
  • Elevation beam pattern calculation
  • Replica reconstruction
  
  
• Corrections applied in the ground processor
• update rate 5 - 35 s (mode dependent)
  
• Does not include the passive part of the antenna and the calibration network

Antenna Pattern Characterization

• Radiometric performance requires accurate knowledge of the two-way antenna beam (0.1dB)
  
• Transmit and receive antenna patterns have been measured on ground for all eight beams and both polarizations
  
• In-flight, the patterns will be re-determined using several techniques:
  • Module Stepping (T/R module characteristics only)
  • External Characterization (transmit only)
  • Rain Forest or other suitable distributed targets (two-way, mainlobe only)

Antenna Pattern: Preflight Characterization

• All antenna beams have been characterised as part of the on-ground test campaign:
  • Main beam measured to an accuracy of 0.1 dB (to be checked with Rain Forest overpasses)
  • Full sidelobe structure compared with required template (to be checked by processing module stepping and external characterisation data)
• On-ground characterisation data to be used by the Ground Processor initially
• Capability to re-synthesise the pattern validated during FM test campaign

Antenna Pattern Characterization: Module Stepping

• A dedicated Module Stepping Mode has been implemented to gather all data from all T/R Modules automatically:
  • Sampling performed in less than 1 second for all 320 Modules using calibration pulse P1
• Data to be downloaded and processed on ground
• Data to be compared wrt reference Database from on-ground tests
• Results of the analysis will provide information of:
  • T/R Module gain and phase drift and temperature behaviour
  • T/R Failures
• Outcome used to:
  • Implement corrections by updating T/R Module coefficients
  • Re-synthesis of antenna beams if required

Antenna Pattern: External Characterization

Antenna Pattern Characterization : Rain Forest

• Stable and isotropic target with relatively high backscatter
  
• Uncorrected image over the rain forest is averaged in the azimuth direction to produce the two-way mainlobe pattern
  
• Suitability of other distribut-ed targets at different latitude is under investigation

Gain Calibration: Objectives

• The ASAR Gain Calibration uses the same technique as the ERS AMI-SAR, that is:
  
• The overall Absolute gain Calibration Factor (ACF) will be given by a single number per beam per polarisation (14 ACFs for Image Mode)
  
• Any user interested in the absolute level of the backscatter from a target (sigma 0) will make use of the ACF

Gain Calibration: Techniques

• The power in the IRF is integrated and the associated background power is measured in order to determine the ACF
  
• A comparison with on ground measurements of the end-to-end system gain can then be made
  
• Narrow swath modes: ERS methodology
  
• ScanSAR modes: use of narrow swath calibration + dedicated calibration processing (support studies on calibration processing)

Gain Calibration: Targets

• 3 fixed and 1 mobile ASAR precision transponders in the Netherlands: calibration mode, external characterization mode, receiver mode, decoupling of background contribution via frequency offset for GM calibration
• one experimental transponder at ESTEC with option for pulse repetition, delay and coding

Transponder Visibility over The Netherlands

Conclusions

• ASAR calibration plan is the logical progression from the experience gained with ERS
• Active phased array antenna requires a comprehensive internal monitoring network
• Antenna pattern characterization includes preflight data, measurements in special operating modes and rain forest acquisitions
• Absolute gain calibration is based on high precision ASAR transponders deployed in the Netherlands
• RADARSAT transponders will support verification of round-orbit calibration performance
• Image, Wave and Wide Swath modes calibrated within six months

Slide 18

Antenna Model

Internal Calibration Pulse Diagram

In-Flight Evaluation of Performance Parameters

• Impulse Response Function from transponder
  • Spatial resolution defined as half-width of response
  • Side-lobe levels
  • Point-target ambiguity levels
  • Radiometric Accuracy
  • Radiometric Stability
  • Localization Accuracy
• Noise Equivalent Sigma Nought
  • Delimit the minimum sigma nought which can be measured
  • Determined from the apparent backscatter obtained over still water such as a lake in an image

ASAR Performance Summary

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry