The objective of the INDREX-II campaign was to build up a SAR database over different tropical forest types with the main focus on L- and P-band polarimetric and interferometric data. The initiative to this campaign originated from the Pol-InSAR Workshop held at ESA-ESRIN in 2003. During this workshop the participating scientists identified gaps in the knowledge of the SAR behaviour in tropical forest. The main scientific question was to figure out if L-band radar signal penetrates in the dense tropical forest down to the ground and if the Random-Volume-over-Ground model is applicable in this case for the retrieval of tropical forest height measurements.
|Location of the test sites at Kalimantan (Mawas, Sungai Wain, Meratus, Samboja Lestari) and the two main airports (Balikpapan, Palangkaraya)|
|P-/ X-/ L-Band polarimetrc intensity image of the MAWAS test site with a disturbed burned forest area (red) and a natural tropical forest (green). This is a geocoded image with a spatial resolution of 2m x 2m. Color Composite RGB = P-(HH+HV+VV)/3 – X-VV – L (HH-HV-VV)/3|
|A 15 km long wooden bridge installed for collecting ground measurements in the Mawas Peat forest test site ( Kalimantan, Indonesia, 2004)|
|Interferometric coherence (bottom) and radar backscatter (top) at L-band in HH polarisation over the hilly dense dipterocarp forest of the Sungai Wain test site. The red dots indicate the sample locations taken for the Pol-InSAR unit circle investigations|
|Complex coherences in L-band at different polarisations and for two baselines plotted into the unit circle. Six sample plots over the Sungai Wain test site are shown|
|Tropical (primary) forest height maps obtained by single baseline Pol-InSAR data inversion at L- (left) & P-band (right) in the frame of the INDREX-II experiment in Indonesia|
Eight different forest types were chosen for data acquisition on ground and from the aircraft:
- Mawas Reserve – Undisturbed
- Mawas Reserve – Disturbed
- Sungai Wain Reserve
- Balikpapan Bay - Mangrove area
- Penajam - Oil Palm Plantation
- Samboja Lestari Plantation
- Gunung Meratus Reserve
- Rubber Tree Plantation
For INDREX-II the E-SAR system was shipped separately from Germany to Kalimantan and system integration into the aircraft was performed on site. All requested data could be recorded with high precision in flight navigation and careful radiometric calibration was performed during processing.
More than 200GB raw data in 43 flight hours were acquired in polarimetric and interferometric mode and at different frequencies (X-, C-, L- and P-band). The length of the flight strips varied from 23 to 80km. The INDREX-II data were processed and delivered to ESA in 2005, which is responsible for the distribution to the European scientific community. An overview of INDREX-II data is also available at DLR’s EOWEB.
The open scientific questions posed in the beginning could also be answered by the detailed data analysis. Plotting the complex coherence at different polarisations in a unit circle, the loci of the coherence values are distributed along a line. This is an indication that L-band is penetrating in the dense volume up to the ground. The different coherence loci would cluster to one point or would not be aligned, if this would not be the case. The distribution of the points is due to the polarisation diversity of the ground contribution and to the random nature of the forest canopy in L-band.
In a second step, tropical forest heights were successfully derived from Pol-InSAR in L- and P-band. For both frequencies a similar forest height distribution is obtained. In a first order analysis the Pol-InSAR derived heights correspond very well to the forest heights estimated on ground..
The INDREX-II campaign and the obtained results certainly deliver an important contribution for the future development of SAR satellites in Europe devoted to the monitoring of forested areas and their above-ground biomass. A first demonstration of the Pol-InSAR technique using spaceborne systems is planned with the PALSAR L-band instrument on-board the Japanese ALOS satellite within the Kyoto & Carbon initiative.