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• Multiple wavelength: Uses multiple-wavelength lidar, enabling direct discrimination of photosynthetic (vegetative) surfaces from nonphotosynthetic (nonvegetative) surfaces as well as monitoring of vegetation changes
  
  
• Reliable: Employs commercial, off-the-shelf lasers with known reliability
  
  
• Three-dimensional: Differential absorption provides a 3-D view of chlorophyll in plant canopies, enabling researchers to obtain information about surface type as well as volume
  
  
• Accurate: Direct illumination provides unprecedented information for calibration of passively acquired multispectral vegetation data, such as data from the Advanced Very High Resolution Radiometer (AVHRR)
  
  
• Proven: Demonstrates the ability to accurately determine changes in vegetation over time and to discriminate between vegetative and nonvegetative surfaces in NASA testing

Goddard’s technology has great potential for airborne mapping and monitoring of topography and ecosystem changes for applications such as:

• Agriculture
  • Forestry inventory
  • Precision farming
    
    
• Environmental monitoring
  • Biomass measurements
  • Habitat quality assessments
  • Ecosystem damage assessments
    
    
• Mapping
  • Surface topography beneath vegetation
  • 3-D surface details of forested flood plains and wetlands
  • Mixtures of artificial, geological, and living features

How it works

The Normalized Difference Vegetation Index measures the reflectivity and absorption of chlorophyll-containing vegetation. Chlorophyll absorbs visible light for use in photosynthesis, illustrated by blue and red peaks near 0.4 and 0.67 µm (see Figure 1 below). The leaves strongly reflect near-infrared (NIR) light from 0.7 to 1.2 µm. The transition around 0.7 µm is known as the red edge. Less reflected radiation in red wavelengths versus NIR wavelengths indicates healthy and dense vegetation. If the difference between the intensity of the reflected wavelengths is small, then the vegetation is presumed sparse, dead, or absent. Using a wavelength close to the red edge provides an unambiguous signal.

Figure 1. Spectral Reflectance of Vegetation

Goddard’s technology provides a land-surface lidar method that uses spectral reflectance to make these determinations. The process calls for the use of two telecommunications lasers (O-band and C-band), both of which are frequency doubled, providing imaging wavelengths of approximately 665 nm and 775 nm. Light from both lasers is rapidly modulated to provide precise range information without requiring powerful short laser pulses.  Calculating a ratio of the returned signals from these two wavelengths enables vertical resolution of, and differentiation between, vegetative and nonvegetative surfaces. Because the method uses two wavelengths that are absorbed differently by chlorophyll-containing (vegetative) surfaces, changes in the vegetation itself can be determined and monitored.

NASA testing

NASA Goddard researchers tested the lidar process using a low-power demonstration instrument consisting of two semiconductor laser diodes, a 20-cm diameter telescope, and two photon-counting detectors. The instrument was operated horizontally from a rooftop laboratory over several hundred meters to a stand of deciduous tress over a two-month period in Fall 2003 as the tree leaves changed color and fell. The instrument demonstrated dramatic differences in reflectance of the foliage at the varying wavelengths of incident light. Measurements were taken approximately once per week at several locations to average surfaces that may have included bark or leaves. Initial reflectance ratios of 40 to 50 indicated a volume of healthy, green vegetation during the first two weeks of the test period. As time progressed and the vegetation lost chlorophyll content, the reflectance ratios dropped and then settled at a ratio of 2 to 2.5.

Why it is better

Typical imaging and mapping lidar systems use a single-wavelength approach that is capable of detecting altitude differences between the ground and objects above the ground. This difference is used to infer the height of vegetation assumed to be in the field of view, but it does not account for dead trees, rocks, buildings, or other structures. Single-wavelength methods, therefore, cannot readily distinguish between healthy vegetation and nonvegetative surfaces. Precursor lidar techniques were able to discriminate vegetative ranges but could not monitor relative changes to the vegetation itself. Simply adding green light to a conventional NIR lidar system doesn’t provide an unambiguous distinction. Using other wavelengths may make it difficult to distinguish between living vegetation and some common, non-vegetative surfaces that have similar reflectance ratios.   

Contrasted with these methods, Goddard’s technique is a paired-wavelength approach, providing a proven method that enables both determination and tracking of vegetative land surfaces.

If you are interested in more information or want to pursue transfer of this technology(GSC-14439-1), please contact:

Innovative Partnerships Program Office
NASA Goddard Space Flight Center
E-mail: techtransfer@gsfc.nasa.gov