ESTIMATION OF FOREST BIOMASS FROM ESTAR IMAGE DATA 
R.H. Lang”, D.M. Le Vine”” and C. Utku" 
* George Washington University, Dept of ECE, Phillips Hall, Washington, DC 20052 USA (lang, cuxu)@gwu.edu 
** NASA Goddard Space Flight Center, Microwave Sensor Branch, Greenbelt, MD 20771 USA david.m.levine(@nasa.20\ 
  
KEY WORDS: Remote Sensing, Forestry, Hydrology, Radiometry, Soil, Vegetation, Modeling, Passive 
ABSTRACT: 
O 
Image data from L-band airborne radiometer measurements over conifer forest stands in southern Virginia USA have shown a strong 
sensitivity to biomass and a weaker sensitivity to soil moisture. The data, was obtained by the horizontally polarized synthetic 
aperture radiometer, ESTAR, deployed aboard a NASA P3 aircraft. The data was acquired in July, August and November of 1999, 
The imaged region, which is owned by International Paper Company, consists of even aged loblolly pine stands of approximately 
one to two kilometers on a side. The stands range in biomass from 20 to 200 tons/hectare and are located on mostly flat ground. 
To understand the dependence of brightness temperature on biomass and soil moisture, a passive discrete scatterer model of the 
forest has been employed. Soil moisture and tree geometry measurements, made at the time of the over-flights, provide input data for 
the forest model. The model results agree with the measurements to within experimental error. The results of the measurements and 
the modeling indicate that L-band brightness temperature is sensitive to forest biomass. However, for stands of smaller biomass, the 
emission from the ground has an increasing role. 
I. INTRODUCTION 
In 1999 ESTAR, an L-band radiometer was flown over conifer 
forest sites located near Waverly, Virginia USA. The forest 
stands, which were owned by the International Paper Company, 
were of various ages. The purpose of the flights was to 
determine the effect of varying biomass and soil moisture 
conditions on the response of the L-band radiometer. 
The retrieval of forest biomass from space has been the subject 
of investigation for over a decade. Investigators have found that 
L-band (1.413 GHz) is a good compromise between the ability 
to penetrate canopies and drawbacks such as antenna size and 
the effects of the ionosphere. Radar backscatter at L-band has 
been shown to be sensitive to the level of biomass however, the 
signal saturates rapidly as the biomass increases (Dobson et al, 
1992, Ranson & Sun, 1994 and Imhoff, 1995). Ferrazzoli and 
Guerriero (1996) have made a model investigation of the 
sensitivity of brightness temperature to variations in biomass. 
Their calculations indicated that passive sensing at L-band is 
more sensitive to biomass than active sensing techniques. Lang 
et al (2000), Macelloni et al (2001) and Lang et al (2001) 
reported on passive L-band measurements of biomass which 
demonstrated their increased sensitivity to biomass. 
In the present paper the modeling work that was done to verify 
the passive measurements at the Waverly site in Virginia will 
be discussed. Of the many stands that were imaged by the 
ESTAR radiometer, two were chosen for careful study. Their 
forest statistics and ground moisture have been measured. This 
data will be used in a passive model to estimate the brightness 
temperature. The effects of biomass and soil moisture will be 
examined by using the model. 
2. DESCRIPTION OF EXPERIMENT 
ESTAR is an L-band radiometer, which uses a real aperture to 
obtain resolution along track and a synthetic aperture to obtain 
resolution in the cross track direction (Le Vine, 1999). The 
radiometer operates at 1.413 GHz and is horizontally polarized. 
The instrument is mounted in the bomb-bay door of a P3 
aircraft as shown in Figure 1. Four flights were made over the 
Waverly test site in 1999. Flights were made on July 7, 
    
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Figure 1. ESTAR radiometer on board P3 aircraft 
August 27, November 15 and November 30. Moisture 
conditions and temperatures varied for each of the days. The P3 
flew two flight paths in the form of a cross whose intersection 
overlapped a portion of the test sites. The plane flew at an 
altitude of 1.5 km (1500 ft) or 3.0 km (3000 ft) which resulted 
in swath widths of 1 km and 2 km respectively. 
The Waverly site is owned by the International Paper Company 
and consists of plantation stands of loblolly pine growing on 
mostly flat ground. The stands range in age from 2 to 30 years 
old with some selected stands being even older. The stands 
were typically 1 to 2 km on a side; large enough to contain 
several ESTAR resolution cells. Six stands were chosen for 
detailed investigation. To help show the location and extent of 
each stand, a Landsat image of the region is presented in Figure 
2 with the stands 1-6 labeled on the image. 
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