Keywords : Forest Fire, Burnt Scars, Synthetic Aperture Radar, ERS, JERS

Abstract
Synthetic aperture radar (SAR) is capable to acquire images in the presence of cloud cover. Hence, it may be a good tool for fire damage studies in tropical region. In this paper, we report on the use of ERS C-band data and JERS L-band data to study selected burnt area near the town of Banjarmasin, South Kalimantan. The area was extensively cleared with fire for development during the 1997 fire episode. Three ERS images and three JERS images which were acquired during the periods of before-fire, fire-burning and after-fire respectively are used for the study. Radar backscatter signals of both sensors are investigated and compared to classify the vegetation types at the area. Corresponding SPOT images in the same period are used as the ground truths to check the validity of the SAR imagery.

Introduction
In many tropical regions, land/forest fires have been a big challenge in environment management. In 1997 and 1998, it was estimated that several million hectares of land was burnt by the fires in Sumatra and Borneo islands [1]. Due to the hugh extent of land burnt, remote sensing serves as an effective tool which provides imagery useful in fire damage analysis, forest fires management and fire prevention. Burnt scars can be delineated from their spectral signatures in optical remote sensing images. In addition, the types of landcover on fire can also be determined from the spectral and contextual features observed in the images. However, one major limitation of optical remote sensing lies in the inability of optical radiations in penetrating clouds and thick haze.

To solve this problem, we investigate the feasibility of using different frequency and polarization Synthetic Aperture Radar (SAR) data to study the forest fire burnt scars. In this case, ERS (C-band, VV polarization, 23 degree incidence angle) and JERS (L-band, HH polarization, 35 degree incidence angle) are used. Three images of an area in Central Kalimantan, Indonesia affected by fires were acquired by each of the two satellites at different dates that correspond to the periods of before, during and after-the fires of 1997. Several SPOT images acquired during the same periods are also available. The optical imagery from SPOT satellites are used as the "ground truths" for the study.

SAR data have been used in forest mapping and monitoring. In particular, tropical forests are known to have a constant backscattering coefficient between -7 and -6 dB in C-band. However, when the forest has been disturbed (eg. by fires), the backscattering coefficient is expected to vary. If multitemporal SAR data of an area of interest are acquired, then land clearings and the areas affected by fires can be detected by an observed change in the backscattered intensity.

Study Area
The study area is located in the southern part of Central Kalimantan where one million ha of peatland has been earmarked for conversion into a rice-production area in June 1995. The construction of irrigation channels began in April 1996. The land conversion project is popularly known as the "Mega-Rice Project". The specific site studied is located at the west bank of the Barito river, about 100 km north of the town of Banjarmasin South Kalimantan. This area is known to be severely affected by fires during the dry season of 1997, from observations using NOAA-AVHRR and SPOT imagery [1]. The area was burnt extensively for about two to three months during the 1997 fire episode which started in July 1997. The location map of the study area is shown in Fig. 1. The cumulative hot spot image over the Mega-Rice area derived from NOAA-14 imagery from June to Nov 1997 is shown in Fig. 2. This image shows that much of the peatswamp area was affected by fires. The burnt areas delineated from SPOT quicklook images acquired from August to December 1997 [1] are shown in Fig. 4. A study of this area using interferometric SAR to delineate burnt areas and change in vegetation cover has also been reported [2, 3].

Fig. 1: Location of the study area.

Fig. 2: Cumulative hot spot image derived from NOAA-14 imagery (June to Nov 1997).	Fig. 3: Burnt areas delineated from SPOT quicklook images (Aug to Dec 1997)

Images Used
The fire in the study area started in July 97 and ended in December 97. We select three images from each of the ERS and JERS satellites during three different periods. The periods are classified as before-fire (before July 1997), during fire (July to November, 1997) and after-fire (after November 1997). The ERS SAR images were acquired and processed by the Centre for Remote Imaging, Sensing and Processing (CRISP), Singapore. The JERS SAR images were provided by NASDA. The acquisition dates of these images are shown in Table 1.

Table 1 : Acquisition Dates of Images Used in the Study.

Satellite	JERS	ERS	SPOT
Date	02 February 1997	07 November 1996	06 June 1997
	24 October 1997	23 October 1997	08 September 1997
	07 December 1997	27 November 1997	20 November 1997