Image preprocessing defines the quality of data input for image analysis and image interpretation. The methodologies carried out in preprocessing remotely sensed data must be implemented with a certain degree of reliability and accuracy, producing consistent images. Preprocessing satellite images at a regional or national scale presents challenges on automation and processing efficiency. Unlike with optical satellite images, the availability of tools and software for the preprocessing of synthetic aperture radar (SAR) images are very limited. A module was developed using the ENVI software application programming interface (API) based on the Interactive Data Language (IDL) to implement several preprocessing tasks designed for the Advanced Land Observing System (ALOS) Phased Array L-band Synthetic Aperture Radar (PALSAR) 25-meter mosaic datasets, which are freely distributed by the Japan Aerospace Agency (JAXA). These preprocessing tasks include: (1) ENVI file association; (2) Speckle filtering; (3) Normalisation; (4) Calculation of band ratio images; (5) Pixel-based mosaicking, and (6) Image-to-Image registration. The module is designed for batch processing the PALSAR global mosaic datasets, and potentially for future ALOS-2/PALSAR-2 mosaics, generated through the ALOS Kyoto & Carbon (K&C) Initiative.