Temilola Fatoyinbo¹, Rafael F. Rincon¹, Guoqing Sun², K. Jon Ranson¹

¹NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA

²University of Maryland College Park, Lefrak Hall, College Park, MD USA

Figure 1. This figure shows NASA’s P3 aircraft with the EcoSAR instrument and an example of an InSAR image of different ecosystems (forest, wetlands and permafrost). The EcoSAR vegetation height measurements will provide similar, but higher resolution and higher accuracy tree height measurements than Shuttle Radar Topography Mission derived mean tree heights shown above (Fatoyinbo et al, 2008). EcoSAR will also be able to measure permafrost depth and wetland extent. Source: Authors.

1. INTRODUCTION

Climate change constitutes the greatest environmental problem of this century, and is destined to significantly impact all societies. Quantifying the carbon cycle is the most important element in understanding climate change and its consequences, yet it is poorly understood (Le Toan et al, 2004). Forests store 85 percent of terrestrial carbon, yet the amount of carbon contained in the Earth’s forests is not known to even one significant figure, ranging from 385 to 650 petagrams (10¹⁵ grams or 10⁹ tons) carbon (Saugier et al. 2001, Goodale et al. 2002, Houghton et al. 2009). Terrestrial biomass, which is the woody mass per unit area, ecosystem structure including height and density, and extent need to be quantified on a global scale and with meaningful frequency to account for changes from both natural and human-induced disturbances. This can only be done efficiently and uniformly through remote sensing.

In this paper we present a new instrument concept called the EcoSAR. The EcoSAR is an advanced airborne polarimetric and interferometric P-band SAR instrument in development at NASA’s Goddard Space Flight Center through NASA’s Earth Science Technology Office Instrument Incubator Program (ESTO IIP). This instrument will provide two- and three-dimensional fine scale measurements of terrestrial ecosystem structure and biomass. These measurements directly support science requirements for the study of the carbon cycle and its relationship to climate change, as recommended by the National Science Foundation’s Decadal Survey (2007) and highlighted in NASA’s Plan for a Climate-Centric Architecture (2010).

Synthetic Aperture Radar (SAR) illuminates the vegetation with microwave energy in a manner that interacts with vegetation and ground, and senses the entire vegetation volume and density or biomass. P-Band SAR (0.3 – 1 GHz or 30 – 100 centimeter wavelength) is particularly desirable for forest biomass estimates, as it is able to measure forest biomass directly up to 200 megagrams per hectare or Mg/ha, because of the longer wavelengths and deeper penetration of the microwaves into the canopy. Measurements at shorter wavelengths, such as L-band saturate at a maximum of 140 Mg/ha (Mougin, 1999). Many tropical and temperate forests’ biomasses can reach up to 600 Mg/ha, which cannot be measured with current SAR capabilities. To measure tree height using radar data, a technique known as interferometric Synthetic Aperture Radar (InSAR) is used (Graham, 1974). InSAR estimates tree height by using interference patterns between two SAR signals in order to derive forest canopy height. However, InSAR can only be used to measure vegetation or canopy height when the height of the ground beneath the vegetation is also known from a Digital Elevation Model (DEM). These ground DEMs are not available globally, which limits the use of InSAR. With Polarimetric InSAR (PolInSAR), a new methodology, it is possible to separate the signal from the ground and from the forest canopy, and thereby measure canopy height without the use of a ground DEM. To derive biomass, the tree height is then directly used through allometric equations.

Figure 2: EcoSAR’s Digital Beam-forming capability will permit measurement of ecosystem structure with high accuracy. Source: Authors.

2. TECHNOLOGY

The EcoSAR system will employ digital beam-forming architecture, a digital waveform generator and receiver system, and advanced dual-polarization array antennas on the NASA P3 aircraft (see Fig. 1). The end result will be a unique, and highly reconfigurable system. The design leverages the L-band Digital beam-forming SAR (DBSAR) architecture that has already been developed at NASA’s Goddard Space Flight Center and which uses advanced digital beam forming SAR techniques for surface imaging and biomass applications (Rincon et al 2011). The architecture of the EcoSAR instrument will provide considerable measurement flexibility such as cross track scanning over a wide range of angles, the synthesis of multiple beams in post processing, the simultaneous measurement over both sides of the flight track, and the ability to vary the incidence angle of the measurement (Fig.2). The beams can be reprocessed with different beam widths and side lobe levels to control swath widths and minimize side lobe contamination.

The EcoSAR instrument will operate at P-band center frequency of 435 MHz (69 cm wavelength) and feature a fully programmable bandwidth. The operational mode with spatial resolutions between 5 to 25 meters (6 MHz -30 MHz) will be used in frequency-restricted areas, and a high-resolution science mode with resolution up to 0.75 meters (up to 200 MHz) will be used in remote areas. The EcoSAR will be a fully polarimetric instrument and will perform interferometry using two antennas at 25-meter baseline.

3. SCIENCE

The primary objective of the EcoSAR instrument is to estimate forest biomass and forest canopy height, which both represent a critical gap in current science measurements. More specifically, the EcoSAR will be capable of providing estimates of canopy height at 1 meter accuracy and estimates of aboveground woody biomass to an accuracy of 20 percent. The EcoSAR will also be able to map areas of forest extent and areas of disturbance. In addition to quantifying biomass, ecosystem structure, extent disturbance and recovery, our EcoSAR instrument will also be able to perform mapping of ice sheets, ice dynamics and thickness, permafrost depth, flux and storage of water.

4. CONCLUSIONS

The EcoSAR will provide highly adaptive, dual polarization interferometric SAR measurements that will greatly improve the retrieval of biomass and ecosystem structure. In addition to contributing to a better understanding of the carbon cycle, the improved carbon estimates we will acquire with EcoSAR will be able to support international carbon emission reduction initiatives. For example, the United Nations ‰ÛÏReduced Emissions from Deforestation and Degradation‰Û (REDD) initiative is a mechanism intended to cut greenhouse gas emissions associated with forest degradation and clearing. The United Nations REDD agreement ‰ÛÏRequests ‰Û_ Scientific and Technological Advice ‰Û_ to develop, as necessary, modalities for [measuring, reporting and verifying] anthropogenic forest-related emissions by sources and removals by sinks, forest carbon stocks, and forest area changes ‰Û_‰Û (UNFCCC LCA Agreement on REDD, 2009). To estimate and mitigate the effects of future climate change, and make REDD projects viable, the rate and extent of present forest change and carbon storage must be quantified. The EcoSAR, InSAR and PolSAR data will quantify vegetation structure, land cover and volume, necessary for determining forest degradation, deforestation and reforestation. In this way, the EcoSAR will provide not only cutting edge technological developments but also have a direct societal impact.

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