Full text: Remote sensing for resources development and environmental management (Volume 1)

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
121 
Measurements of the backscatter and attenuation properties 
of forest stands at X-, C- and L-band 
D.H.Hoekman 
Wageningen Agricultural University, Netherlands 
ABSTRACT: The new airborne multi-band scatterometer (DUTSCAT) promises to be a useful tool for research 
in the field of active microwave remote sensing. An evaluation of the use of this system for research 
in forestry is given. Besides accurate o° values the system can acquire information on the vertical dis 
tribution of backscattering. Through inversion of the multi-level model the parameter a 0 can be divided • 
in contributions from a number of layers (3 or 4). An experiment with big corner reflectors placed on the 
forest floor was conducted in an effort to gain more insight into the attenuating properties of the forest 
canopy. The measurements of attenuation properties together with the division of O 0 in contributions from 
several layers simplifies the model-making effort considerably. 
KEYWORDS: Active microwave remote sensing, forest canopy attenuation, sources of scattering, modelling. 
I . INTRODUCTION 
The research activities of the working group ROVE- 
Forestry concentrate on modelling radar backscatte 
ring of forests and the identification of poten 
tial applications of radar remote sensing in fores 
try. 
Until recently experimenting in the Netherlands by 
ROVE-Forestry was limited to the use of an X-band, 
HH-polarized SLAR. 
The introduction of an airborne multi-band scatte 
rometer (DUTSCAT) in 1986 promises to be a major 
break-through in research possibilities. Some results 
in L- and C-band recently obtained with experi 
mental (single band) versions of the new system 
will be discussed in this paper. 
Both sensors (SLAR and scatterometer) are primarily 
used for the acquisition of accurate values of O 0 
or Y • Other physical parameters like the canopy 
attenuation or the vertical distribution of sources 
of backscattering in the forest canopy can be mea 
sured too. The physical parameter Y is the only 
parameter SLR systems normally supply and this para 
meter together with the appropriate ground truth 
data would in principle suffice to study the poten 
tial applications of interest. Therefore it seems 
peculiar, at first sight, that the use of the re 
search instruments, the X-band SLAR and DUTSCAT, has 
not been limited to measurements of Y . The reason 
to measure other physical properties of the forest, 
as far as possible, is that these data can simplify 
the model-making effort considerably. 
Applying radar remote sensing in general asks for 
methods to interpret the acquired radar imagery. For 
some applications the spatial information (patterns, 
textures) is relevant, for other applications the 
temporal information (changes). For most applica 
tions however relevant information is contained in 
the absolute value of y .To extract this informa 
tion backscatter models are essential. The modelling 
can be done in various ways. For specific applica 
tions one can start from the assumption that rela 
tions between y values and object (forest) parame 
ters can follow empirically constructed expressions 
well enough. Modelling radar data on a physical 
basis however is more complicated but is to be pre 
ferred since these models are likely to have a broa 
der validity and can be used to extrapolate thus 
gained insights to other situations or to explain 
observed phenomena still unknown or unexpected. 
In general models with a physical basis describe 
the radar return y as a sum of direct contributions 
of different types of scatterers in the object's 
volume and interactions (e.g. multiple scattering) 
between these scatterers. In these models the inci 
dent power and the received backscattered power of 
scatterers in lower layers is attenuated by scat 
terers in layers, above. A direct acquisition of data 
on attenuation properties or a method to indicate the 
sources of scattering directly would be, along with 
Y measurements, of great importance in model 
research. 
2. SENSOR DESCRIPTION 
.The X-band SLAR is internally calibrated and has 
digital recording facilities for radar and flight 
data. Pre-processing software has been developed 
to accurately correct the raw radar data both 
geometrically and radiometrically. The resulting 
imagery indicates relative Y values, the final 
pixel size is 7.5 meter square. The specifications 
of the SLAR are listed in table 1. 
Table 1. Specifications of Dutch digital SLAR. 
Frequency 
Antenna 
Transmitted power 
Pulse length 
PRF 
Polarization 
(Two-way) antenna beamwidth 
Sample frequency 
Dynamic range 
Pixel size 
Geometric resolution 
in range 
in azimuth 
Independent samples 
9.4 GHz 
2.5 m slotted wave guide 
25 kW 
50 ns 
200 Hz 
HH 
10 mrad 
50 MHz (3 m) , 12 bits 
70 dB 
7.5 m x 7.5 m 
7.5 m 
10 m/km 
~ 15 per pixel 
The airborne scatterometer DUTSCAT (Attema & Snoeij 
1985) is sideways looking and of the coherent pulse 
type. The parabolic dish antenna has a two-way 
beam width of 13 degrees at L-band and 3 degrees 
at C-band, is mechanically steerable during flight 
and can be pointed between 0 degrees and 80 degrees 
incidence angle. The range resolution of 15 meters
	        
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