Dr. Jacques Louet

ESA

The Netherlands

CALIBRATION

PRACTICALEXPERIENCEWITH ERS-1

/_ _ _/

N94-15907

• Introduction

• Radiometric Calibration

• Geometric Calibration

° Phase Calibration

• Polarimetric Calibration

PI_IEOEDtNG PAGE BLANK NOT F)LMED 299

Introduction

Basic SAR measurement parameters are :

• Radar backscattering

• Target position

• Target speed

• Polarisation

SAR calibration facilitates quantativemeasurements needed to derive geophysi-cal parameters of the area under observa-tion from basic SAR measurements (e.g.soil moisture, biomass,ocean wave en-ergy,ocean currents, ice type, ice flow, ..... )

Inversion Model

Sensor

Calibration

Direct Model

Geophysical Parameter

Data

Calibration

Geophysical

Calibration©

GeophysicalParameterEstimate

-9m

Geophysical

Validation

__ 30_.

Radiometric Calibration

Radiometric calibration is relating the SARoutput in terms of digital number to _o.

Design for stability

Sensor calibration

pre-flight characterisation

internal calibration

external calibration

• Data calibration

302-

Characterisation parameters for ERS-1

Transmitter Parameters

Frequency

Pulse power

Pulse duration

Chirp bandwidth

Antenna Parameters

Relative azimuth pattern

Absolute elevation pattern

Receiver Parameters

Sensitivity

Bandwidth

Power transf er function

Calibration Parameters

Internal calibration stability

3o_

Instantaneous Amplitude

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Internal Calibration

The objective of internal calibration is moni-toring system parameter changes withtime, such as

Output, power

Receiver grain

Linear distortion (amplitude & phase)

Non-linear distortion ( amplitude &phase)

Noise level

Level of spurious signals

Internal calibration methods include

Calibration using transmit pulse sample(ERS-1 method)

Reference signals : noise, cw

Transmit power measurements

309

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External Calibration

The objective of external calibration is to

measure and monitor system componentsoutside the internal calibration loop.

External calibration uses made-made ornatural targets.

External calibration can be used to deter-mine the radar image calibration factoreither in combination or without referenceto internal calibration.

-2

AMI Transponder Specifications

Parameter Wind Mode Image/Wave

Radar Cross Section _ 87.5dBm 2 65.0dBm 2

Adjustment Range +0,-5dB +0,-5dB

Calibration Accuracy +.5dB +.5dB

Cross-calibration +.2dB +.2dB

Accuracy

Stabilityt'Over 3 Years) '2_.ldB +.ldB

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antennaerrors

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Transponder Calibration Accuracy

Antenna Gain Error

SAR SCATT

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Electronic Gain Error

Antenna Coupling Error

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Single Transponder Observation Error

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Atmospheric Loss Uncertainty

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I Transponder Calibration Accuracy

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Representative SAR Calibration Budgets

SAR Radiometric Accuracy

A Absolute Calibration Error

B Cross Swath Calibration Error

C Radiometric Stability Drift

I ]Nominal Calibration Accuracy

D Radiometric Stability Drift

Nominal- Observation

E Across Swath Characterisation Error

F Across Dynamic Range Characterisation

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(a) Dominate_ by t_der calibration

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Antenna Gain Characterisation

Single Transponder Observation Error

Number of

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Calibration of other areas

The application of the calibration factor toother areas requires knowledge of geome-try (range & antenna angle) and stabilityover time.

High resolution information about the an-tenna diagram can be obtained from im-agery of extended uniform targets such asthe tropical forest or ice shelves.

No absolute information is required forthese 'targets of opportunity'. Stabilitv overtime and a smooth dependence of G" onincidence angle is necessary.

328-

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Data calibration

Data calibration include the following steps

Correct raw data for a number of systemparameters

Linear distortion (amplitude & phase)

Non-linear distortion

Detector imperfections (DC-biases,amplitude & phase imbalance)

time variations as measured by inter-nal calibration

noise bias

Application of calibration factor to theentire image, taking into account ge-ometry and antenna angle.

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Geometric calibration

Geometric calibration can be achieved fol-

lowing the same principles as for radiomet-ric calibration.

Design for stability

Characterise SAR system in terms ofdelay, phase error and frequency offset

Calibrate using point target with accu-rately known position

Geometric image calibration is carried outin the data calibration stage by correctionfor kn#wn system biases and applying ge-omet_ information (platform position & atti'tude and terrain models).

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Phase calibration

During the formation of an intensity imagethe phase information in the radar data islost. For applications in interferometry andpolarimetry complex images are formed

which provide two data points per pixel,amplitude & phase or real & imaginaryparts of the complex signal.

Careful and elaborate phase characterisa-tion of both the SAR sensor and the SAR

processor are required. Time stability iscritical for the above applications.

335

Polarimetric calibration

L

Polarimetric calibration can be treated es-sentiai|y as a multi-channei extension ofthe methods discussed above. PolarimetricSAR differ from single channel SAR in thefollowi .....ng ways. _

There are two orthogonal transmit chan-nels to COnsider and four receive chan,nets.

Ra_ometric calibration can be sepa-rated into absolute calibration and rela-tive between channel calibration

Geometric calibration can be Separatedinto absolute calibration and relative be-tween channel calibration

Phase calibration between channels isessential

Crosstalk has to be taken into account

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336