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Applications of Low-Frequency SAR Executive Summary November 2004

This report details the work performed within the ALFS (Applications of Low-Frequency SAR) project. The project investigated the needs for forest biomass and soil moisture mapping, and the potential of a spaceborne P-band (435MHz) synthetic aperture radar (SAR) to meet those needs. Through a detailed study of published results, investigation of archived data from test-sites, backscatter modelling, and simulation of SAR system performance (including the important effects of ionospheric propagation), the ability of a P-band SAR satellite to meet user requirements has been evaluated. Forest applications have been investigated using AIRSAR data from the boreal forest and tropical forests in the Amazon basin and Indonesia. For boreal forests, the main application envisaged is the mapping of broad biomass classes. The results show that even a spaceborne SAR should be able to provide reliable maps of biomass classes. In some cases the biomass may be retrieved more accurately, but in general variations of tree number density preclude this, unless additional information is available to complement the P-band backscatter measurements. For tropical forests, the P-band backscatter can be used directly to differentiate between areas of high and low biomass. Moreover, the P-band data shows variations in high biomass forest that are related to the structure and flooding condition, some of which are not visible in L-band SAR images. A P-band SAR satellite is therefore expected to be the only method for mapping these variations, which can be related to forest structural types and give an indication of biomass. For soil moisture mapping, the test-sites studied show some correlation between the copolarized backscatter ratio (HH/VV) and the soil moisture content. However, the low backscatter levels is easily disturbed by interference from strong scatterers, and while airborne SAR data is difficult to interpret, spaceborne SAR will be even harder. The possibility to map temporal changes of moisture is one possible application of P-band data, that has not studied in this project. The studies of the technical challenges involved in P-band SAR indicate that they can be overcome sufficiently well to provide data useful for forestry applications. The studies indicate that full polarimetric measurements should be provided to meet the needs for the applications.