You can download the manual of G-Portal here.
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Conversion Tool for Earth Observation Data

Earth Observation Data Conversion Tool Spacecraft/Sensor(Products): GCOM-C, GPM/DPR, GSMaP, GCOM-W, JASMES Platform: Windows HDF5/HDF4/Binary format can be converted to GeoTIFF or KMZ format using this tool. Please refer to the following manual for more information.

Format Conversion Tool Operation Manual for Users Appendix A Conversion target products Appendix B Colorbar specification

GCOM-C is a command line tool that converts to GeoTIFF. - CUI Version Tool: Details are here

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HDF View Spacecraft/Sensor: Not Evaluated Platform: Windows, Mac, Linux, Solaris HDF5/HDF4 format data can be read. For more information, please refer to the following document. HDF View User's Manual (PDF).

Quantum GIS Spacecraft/Sensor not Evaluated Platform Windows, Mac, Linux GeoTIFF Format data can be read. Please refer to this document for further information. Quantum GIS User's Guide, PDF

Panoply Spacecraft/Sensor: Not Evaluated Platform: Windows, Mac, Linux NetCDF and HDF data can be browsed. The following URL provides more information.