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ASAR instrument calibration

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 · December 2001

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ASAR INSTRUMENT CALIBRATION

C H Buck, J-L Suchail, R Torres, M Zink

ESA-ESTEC ENVISAT Project, PO Box 299,

2200 AG Noordwijk, The Netherlands

e-mail: 

[e-mail]

ABSTRACT

This paper presents the approach to be taken during the commissioning phase for the in-flight calibration of the

ENVISAT-1 ASAR instrument. The philosophy presented is a logical progression from the experience gained during

calibration of the ERS SARs. The ASAR has a comprehensive internal calibration loop, which is described distinctly

from the external calibration and characterization.

The antenna patterns of the various beams have been fully measured during on-ground flight-model testing and have

been used for initial performance predictions. In the first instance these patterns will also be used by the ground

processor although in-flight characterization of the mainbeams will be made over the South American rainforest. As for

ERS, radiometric calibration will be achieved using three fixed and one transportable precision calibration

transponders situated in the Netherlands. These transponders are also capable of recording the azimuth beam patterns

and supporting the external characterization mode of ASAR. The techniques and the performance objectives are also

reviewed.

INTRODUCTION

Unlike the passive ERS AMI-SAR, ASAR has an active antenna made up of 320 transmit/receive modules (TRMs).

Each TRM has two transmit chains, one for horizontal and one for vertical polarization, and one receive chain. The

three chains are independently programmable in amplitude and phase to provide the required elevation beam patterns.

Any instabilities in the gain and phase characteristics of the TRMs will distort the beam patterns and can potentially

contribute to radiometric errors in the SAR image. For these reasons the ASAR calibration scheme is different to that

used for ERS. Figure 1 shows an overview of the various steps required to calibrate the ASAR.

The calibration activities start with the in-orbit checkout, during which the general instrument functionality will be

verified. System temperature variations are the major factor causing gain changes. Therefore one important part of the

in-orbit checkout is the verification of in-flight temperature profiles. For the most sensitive part, the T/R-modules, we

expect one stable temperature per operating mode. This stability has to be verified and the actual temperature has to be