Above-Ground Biomass Retrieval over Tropical Forests: A Novel GNSS-R Approach with CyGNSS

Hugo Carreno-Luengo,Michele Crosetto,Guido Luzi

doi:10.3390/rs12091368

Abstract

An assessment of the National Aeronautics and Space Administration NASA’s Cyclone Global Navigation Satellite System (CyGNSS) mission for biomass studies is presented in this work on rain, coniferous, dry, and moist tropical forests. The main objective is to investigate the capability of Global Navigation Satellite Systems Reflectometry (GNSS-R) for biomass retrieval over dense forest canopies from a space-borne platform. The potential advantage of CyGNSS, as compared to monostatic Synthetic Aperture Radar (SAR) missions, relies on the increasing signal attenuation by the vegetation cover, which gradually reduces the coherent scattering component σ coh , 0 . This term can only be collected in a bistatic radar geometry. This point motivates the study of the relationship between several observables derived from Delay Doppler Maps (DDMs) with Above-Ground Biomass (AGB). This assessment is performed at different elevation angles θ e as a function of Canopy Height (CH). The selected biomass products are obtained from data collected by the Geoscience Laser Altimeter System (GLAS) instrument on-board the Ice, Cloud, and land Elevation Satellite (ICESat-1). An analysis based on the first derivative of the experimentally derived polynomial fitting functions shows that the sensitivity requirements of the Trailing Edge TE and the reflectivity Γ reduce with increasing biomass up to ~ 350 and ~ 250 ton/ha over the Congo and Amazon rainforests, respectively. The empirical relationship between TE and Γ with AGB is further evaluated at optimum angular ranges using Soil Moisture Active Passive (SMAP)-derived Vegetation Optical Depth ( VOD ), and the Polarization Index ( PI ). Additionally, the potential influence of Soil Moisture Content (SMC) is investigated over forests with low AGB.

Highlights

Tropical forests are a key terrestrial ecosystem that play a leading role in the global carbon cycle
These types of measurements are only sensitive to the upper canopy layers and they suffer from weather conditions and clouds
This study is based on measurable biomass parameters (AGB, Canopy Height (CH))

Summary

Introduction

Tropical forests are a key terrestrial ecosystem that play a leading role in the global carbon cycle. The analysis was performed over different types of forests, such as Congo rain (Figure 3a–d, Figure 4a–d, and Figure 5a–d), Amazon rain (Figure 3e–h, Figure 4e–h, and Figure 5e–h), coniferous (Figure 6a–d, Figure 7a–d, and Figure 8a–d), dry (Figure 6e–h, Figure 7e–h, and Figure 8e–h), and moist (Figure 6i–l, Figure 7i–l, and Figure 8i–l) tropical forests. These target areas were selected so as to cover a wide variety of forests on a pantropical scale. TThhee ssccaatttteerriinngg ooff GGNNSSSS ssiiggnnaallss normal iiss ssttrroonngg direction oovveerr aann aarreeaa to the Earth’s asaruroorufuanncdde.tthThereannnosommmiitnitnaealdlsGsppeNecScuuSlalsarirgpnpoaoinlisntatr θeee,Ri,i i==ghθ teeH,,ssa=n=dθ eCe·ir.θc ueel=a=r9P900o◦°laaarttizttehhdee (nRoHrmCaPl),dairlethctoiuonghtowthitehEaarctehr’tsasinurdfaecger.eTeroanf sdmepitoteladrGizNatSioSns.igRneafllseactreedRGigNhtSHS asnigdnCalisrcaurlearRPigohlatriaznedd

Methods

Results

Conclusion