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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. ASAR antenna is an active phased array, 320 T/R modules

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Asar instrument calibration

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)

Differences between ASAR and ERS


Asar calibration steps

Internal Monitoring/Calibration

In-Orbit Check-Out

Antenna Pattern Characterization

Absolute Gain Calibration

Calibrated Products

Temperature Profiles

PF-ASAR

processor normalization

antenna pattern

gain drift

ACF

Calibration Pulses

Antenna Model

Transponder Measurements

Rain Forest

MS22

Preflight Data

External Characterization

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: Objectives


Internal calibration pulse diagram

P3 scheme to compensate for phase/gain drifts at T/R module level

P2

Tx Aux

P1A

P1

P1

P1

Cal BFN

Cal

Rx Aux

Internal Calibration Pulse Diagram

T/R Module

(1 of 32x10)

Radiating

Row

Signal BFN

Tx Main

Rx Main


Internal calibration

Row by row measurement in a pre-determined sequence scheme to compensate for phase/gain drifts at T/R module level 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

Internal Calibration


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 Characterization


Antenna pattern preflight characterization
Antenna Pattern: Preflight Characterization two-way antenna beam (0.1dB)

  • 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
Antenna Pattern Characterization: Module Stepping two-way antenna beam (0.1dB)

  • 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

P1 two-way antenna beam (0.1dB)

Ground Receiver

Antenna Pattern: External Characterization

Tx Aux

Radiating

Row

Signal BFN

Tx Main

Rx Main

Cal BFN

Cal

Rx Aux


Antenna pattern external characterization1
Antenna Pattern: External Characterization two-way antenna beam (0.1dB)

  • A series of cw-pulses - sent by each row in turn - is simultaneously sensed by the antenna calibration loop and recorded on ground by the receiver embedded in the calibration transponder

  • Per row subtract amplitude/phase recorded on board from ground receiver measurements => characteristics of the passive part of the antenna minus calibration network

  • Antenna pointing error

  • Measurements will be repeated every 6 months


Antenna pattern characterization rain forest
Antenna Pattern Characterization : Rain Forest two-way antenna beam (0.1dB)

  • 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: Objectives


Gain calibration techniques

Receive AMI-SAR, that is:

Detect

Delay

Transmit

Amplify

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
Gain Calibration: Targets AMI-SAR, that is:

  • 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

  • 4 RADARSAT transponders, 31 MHz frequency offset but

  • 100 MHz bandwidth, latitude range 45 - 74deg,

  • cross-calibration at 5.3 GHz successfully performed,

  • need to be characterized at 5.331 GHz



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

Conclusions


Antenna model

Beam experience gained with ERS

Gain

Beam

Radiation

Pattern

Antenna Model

Antenna Gain

Test Results

Transponder

Calibration

Beam Coefficients

Module Stepping

Internal Cal.

Azimuth

Pattern

Antenna B/F

Near-field data

Rain

Forest

Elevation Pattern

External

Characterisation

Embedded Row

Test Results

Antenna B/F

Test Results


Internal calibration pulse diagram1

P3 experience gained with ERS

P2

Tx Aux

P1A

P1

P1

Cal BFN

Cal

Rx Aux

Internal Calibration Pulse Diagram

T/R Module

(1 of 32x10)

Radiating

Row

Signal BFN

Tx Main

Rx Main


In flight evaluation of performance parameters

Impulse Response Function from transponder experience gained with ERS

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

In-Flight Evaluation of Performance Parameters


Asar performance summary
ASAR Performance Summary experience gained with ERS