employed that seek to minimize their influence, (Izzawati et al., 
2006; Woodhouse et al., 2006; Balzter et al., 2007a). 
The aim of this paper to assess the impact of incidence angle 
changes on the ability of airborne short-wavelength (X-band) 
InSAR data to retrieve accurate vegetation canopy height 
estimates for shrub, deciduous, coniferous, mixed forest, and 
wetland vegetation. This will be achieved using Intermap 
Technologies NEXTMap multi-pass and single-pass X-band, 
horizontal transmit and receive polarization (X-HH) InSAR 
data. This assessment was performed over three sites in the 
United States. In particular, the nature and extent of vegetation 
canopy height underestimation at different incidence angles and 
the conditions under which they are most likely to occur for the 
single-pass InSAR configuration were investigated. Section 2 
introduces the concept of SAR viewing geometry and scattering 
phase centre heights from InSAR data. Section 3 gives a brief 
description of the data and study sites addressed in this analysis, 
followed by Section 4, which provides the methodology utilized 
to access the vertical accuracy of the NEXTMap X-HH InSAR 
data over three incidence angles in flat terrain (e.g. slopes less 
than 10). In Section 5, the test results are presented. Section 6 
concludes the paper with some discussions of future work. 
2. INSAR INCIDENCE ANGLE AND SCATTERING 
PHASE CENTRE HEIGHT BACKGROUND 
The side-looking SAR sensor configuration defines an 
incidence angle range (0) that is determined by how far from 
nadir (H — Figure 2) the SAR beam points out to the side. The 
incidence angle range will change from the near-range (NR) to 
mid-range (MR) to far-range (FR) across the swath (Figure 1). 
This type of viewing geometry can lead to geometric distortions 
in InSAR elevation data. Furthermore, changes of incidence 
angle modify the relative contribution from structural vegetation 
canopy elements (leaves, twigs, branches, tree trunk) and 
ground surfaces. Due to the different flying altitude, NR — FR 
incidence angle changes are more pronounced in airborne data, 
than in spaceborne data. In the case of the airborne X-band 
InSAR (e.g. NEXTMap) sensor used in this study, the incidence 
angle ranges from 35 in NR to 55 in FR, centred on 45 (MR). 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August - 01 September 2012, Melbourne, Australia 
  
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Figure 1. Scanning configuration of right-looking rectangular 
SAR antenna, modified after Olmsted (1993), to show antenna 
length (L), antenna width (D), and pulse duration (T). 
The X-HH InSAR DSM minus an accurate digital terrain model 
(DTM) determines the scattering phase centre height (ho: 
which is an average of all vertically distributed scattering 
elements within a SAR resolution cell (Figure 2). Note, at X- 
  
band there is penetration into the vegetation canopy (red line, 
Figure 3). X-HH derived hg, is at or very near bare ground 
(Mercer, 2001; blue dashed line, Figure 3) in barren areas; 
whereas in forest canopies the location of hy, depends on the 
penetration depth of microwaves into the canopy. This depth 
depends on wavelength, incidence angle, size and density 
distribution of the scattering elements, geometric arrangement 
of the scatterers, canopy moisture condition, surface roughness, 
and moisture content of the ground layer (Andersen et al., 2006; 
Izzawati et al., 2006; Woodhouse et al., 2006). Vegetation 
canopy heights given by X-HH InSAR hg, data are typically 
located in the upper portion of the vegetation canopy. 
  
  
  
  
  
Figure 2. DSM (yellow line) minus DTM (red line) is the hg. 
  
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Figure 3. Relative position of X-HH InSAR hg, for a dense 
forest, bare ground, and true canopy height (red, blue and white 
dashed lines, respectively. 
Incidence angle variations across swath will change the relative 
signal contribution from features on the ground (Woodhouse et 
al., 2006). For example, steep incidence angles (e.g. 6 = 35) 
permit more exposure of the lower portion of a vegetation 
canopy such that there is greater signal interaction with trunks 
and lower vegetation leading to greater volume scattering if 
there is understory, or if little to no understory, greater double 
bounce, and greater ground scattering contributions resulting in 
a lower scattering phase centre height (hg, Figure 4 left — steep 
incidence angle). The opposite effect occurs in the FR (Figure 4 
— right shallow incidence angle). 
  
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Figure 4. Incidence angle effects on hy, retrieval for a forest 
across one flight line strip of data from NR (steep incidence 
angle) to FR (shallow incidence angle). 
It is anticipated that incidence angle range of a single 
interferometric data-take (e.g. one flight line strip of data) will 
    
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