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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
  
order, using systems such as Optech, TopEye, FLI-MAP, 
TopoSys, TerraPoint and Leica. 
Spaceborne LIDARS are also in operation, the most important of 
these is the Geoscience Laser Altimeter System (GLAS) on 
ICESat. GLAS produces a series of approximately 70 m 
diameter spots that are separated by nearly 170 m along track. 
2.2 HSAR 
Synthetic Aperture radar determines the amount of scattered 
energy returned to the antenna, its range and position along 
track (azimuth). SAR can operate in a number of frequencies 
shown in table 1. 
  
  
  
  
  
Band | Wavelength | Frequency 
X 3cm 9.6GHz 
C 5.3cm 5.6GHz 
T, 24cm 1.3GHz 
P 68cm 0.3GHz 
  
  
  
  
  
Table 1. Typical wave length and frequency for SAR bands. 
Two SAR images can be combined to use the technique of 
interferometric SAR (IfSAR) to generate digital elevation 
models. The principle of IfSAR is shown in figure 2. 
A2 cs 
  
  
  
  
Figure 2. Geometry of single-pass Interferometric SAR 
The two antennae are shown at Al and A2. H represents the 
altitude above the reference ellipsoid, h indicates the 
topography of the Earth's surface. The baseline, ie. the 
separation between antenna 1 and 2 , is given by B. The slant 
range (look direction of the antenna) to the target is given by p, 
the look angle at target location is represented by 0 and the 
angle of the baseline with respect to the horizontal is given by a. 
Assuming 0 is known, the elevation of the targeted point on the 
Earth's surface can be calculated from: 
h-H - pcosÓ 
(p áp) = p «B^ -2pBcos(90 - 0 * a) 
Where ôp is the slant range difference 
Op — Ad 2m 
Where À is the wavelength of the radar pulses and ¢ the phase 
difference between the two returns. The phase difference can be 
measured only as a variable with 2m period. Therefore, phase 
unwrapping needs to be applied in order to resolve the absolute 
modulo-2z ambiguity, i.e. to determine the integer portion of q. 
91 
The two scenes may be obtained from a repeat pass, usually 
from satellites, where the images are acquired from two passes 
of the sensor in very similar orbits. Single pass data is acquired 
from an aircraft or spacecraft on which there are two antenna 
separated by a known base length. The suitability of a pair of 
images for generating IfSAR DEMs is measured by the 
coherence between them. Poor coherence is caused if the 
returned radar signals are different on the two images, phase 
unwrapping cannot then be carried out. Coherence is usually 
good on single pass images but can be poor on repeat pass, 
especially if there is a significant time difference between 
images. Errors due to the atmosphere are also reduced. 
The elevation measured for any pixel (resolution cell) results 
from a combined signal of scatterers located in the resolution 
cell (sample area). Elevations measure the *volume scatter', i.e. 
there will be some penetration into the canopy and the range 
recorded will not depict the true height of the tree (first surface). 
Therefore, areas covered by vegetation will include more height 
measurement noise than areas covered by specular scatters (i.e. 
buildings). The wavelength of the radar will determine the 
penetration on the signal into the vegetation, X band will not 
penetrate as far as L band. 
In addition, the surface area represented by one pixel may 
consist of a combination of different scatterers. Height 
measurements could be biased due to a interaction of these 
surface features. The backscattered signal (radar response) is 
integrated over a square footprint (resolution cell) somewhat 
larger (about 50%) than the Sm DSM sample distance. (Mercer, 
2002) Therefore, the elevation measured for any DSM sample 
(resolution cell) will result from a combined signal of scattering 
objects located in this sample area. If hedges and shrubs are 
closely located to a road, both, the raised objects and the road 
itself (bald earth) will contribute to the elevation value 
measured for this DSM sample. 
IfSAR has been widely used from spaceborne platforms, the 
ERS Tandem mission and the Shuttle Radar Topography 
Mission (SRTM) are the two prime examples. The main 
airborne IfSAR is the Intermap STAR-3i. This is a single-pass 
across-track IfSAR system operating commercially since 
January 1997. The system is an X-band SAR interferometer 
carried on board a LearJet 36. The two antennae are separated 
by a Im baseline. Accurate positioning and orientation is 
achieved through the use of an on-board laser-based inertial 
navigation system and an on-board differential GPS (Global 
Positioning Systems) system. (Mercer & Schnick, 1999). Other 
airborne SARs are operated by research organisations such as 
NASA and DLR. 
2.3 Products and data providers 
Data from LIDAR or IfSAR is usually provided as digital 
surface models (DSM), digital terrain models (DTM) and 
orthoimages. The generation of the DSM will be done by the 
organisation which has collected the data and will involve 
calculation of the ground co-ordinates from the GPS, INS and 
range measurement, and must include corrections derived from 
the system calibration and from the atmosphere, and of course 
be delivered on a specified datum in a known map projection. 
An image can be formed from the SAR data and may also be 
collected with LiDAR. The DTM and orthoimages can be 
derived from the DSM and image; this will be discussed in 
section 5.