baseline angle '0,' is obtainable from the aircraft inertial system, 
the aircraft height is known from differential GPS and the 
distance from antenna to pixel is the radar slant range. Then it 
is simple trigonometry to compute the target height *h" in terms 
of these quantities as shown in equations 1-3. 
A, | 
radar BE | 
; _enlargement of rl ^. 3 vu (0, - 8,) | 
radar antennas | INE t | 
Te | js 4 
10, | N 
{ i a." : | 
beam | P eT beam | 
| 9, | 
n r. 
a 4 | 
| 
Tg 4 pixel 
jh ; 
terrain 
Figure 1. Schematic of Airborne IFSAR Geometry. 
sin(0; - 60,) = 5/B (1 
0/4 = ¢/(2*m) + n (2 
hzH-r,cos(0,) (3 
) 
) 
) 
The path-difference ‘5’ is measured indirectly from the phase 
difference between the received wavefronts (eqn. 2). Because 
the phase difference can only be measured between 0 and 2x 
(modulo 2x), there is an absolute phase ambiguity (*n' 
wavelengths) which is normally resolved with the aid of 
relatively coarse ground control. A “phase unwrapping" 
technique (e.g. Goldstein et al, 1988) completes the solution. 
Thus the extraction of elevation is performed on the 
"unwrapped" phase. Often the IFSAR is operated in a so-called 
ping-pong mode which effectively doubles the value of the 
geometric baseline B. These equations become the basis for 
sensitivity and error analysis (e.g. Rodriguez and Martin (1996). 
A direct consequence is the recognition that for the airborne 
IFSAR system STAR-3i, the dominant error source is *phase 
noise” so that the signal-to-noise ratio, which is a function of 
flying height among other factors, becomes a means of (partly) 
controlling height error specifications. 
When there is a fixed, rigid baseline separating the two antennas, 
the signals are collected simultaneously (single-pass 
interferometry). The same principles apply if the data are 
received by the same antenna in subsequent passes (repeat-pass 
interferometry). Significant issues then become (1) for 
satellites: temporal de-correlation due to change in target 
between passes (for example Radarsat has a 24 day repeat 
cycle) and (2) for airborne systems: positional uncertainty. 
This schematic idealization is replaced of course by many 
factors in the practical implementation of IFSAR. For example 
a complex image containing phase and magnitude information 
is created from the signal received at each antenna. Subsequent 
operations on the complex images allow three ortho-rectified 
products to be derived: DEM, Magnitude and Correlation. The 
DEM, as noted earlier, is usually referred to as a DSM in 
recognition that the received signal relates to the scattering 
surface which may be the terrain or could be an object upon the 
terrain, natural or otherwise. The magnitude is often referred to 
simply as an ORI (Ortho-Rectified Image). In relatively open 
urban or forest situations, it is possible to create a DTM (Digital 
842 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
Terrain Model) from the DSM (Wang, et. al. (2002)) and this is 
offered as a Core Product along with the DSM and ORI (see 
Table 2 below). 
2.1 Two Airborne IFSAR Implementations 
Two examples of IFSAR implementation are shown in Figure 2. 
These two systems (STAR-3/ and TopoSAR) are both deployed 
operationally by Intermap. A third system, STAR-4, has 
recently been developed by Intermap and is currently being 
flight tested. 
  
  
Figure 2. STAR-3i (left) and TopoSAR (right) 
STAR-3i was originally designed and built by ERIM, but has 
subsequently had major upgrades in hardware and software 
(Tennant, et. al., 2003). TopoSAR was originally developed by 
AeroSensing under the name AeS-1 (Hoffman, et. al., 2001) and 
has also experienced upgrades — mostly in the processing area. 
Some of the salient characteristics of the two systems are shown 
in Table 1. The STAR-3i system has a higher data acquisition 
capacity while the TopoSAR can achieve finer resolution. Of 
greater interest however is its multi-polarization P-Band 
capability. 
  
  
  
Typical Parameters STAR-3i TopoSAR 
Platform Lear Jet AeroCommander 
Altitude (km) 6.5-9.5 3.5-6.5 
Speed (km/hr) 700 450 
Frequency Band X X P 
Centre Wavelength (cm) 3 3 74 
Image Resolution (m) 1:25 0.5-2 2 
Polarization HH HH HH, VV,HV/VH 
Swath Width (km) 5, 10 2,4,7 4 
IFSAR Mode Single Pass Single | Repeat-Pass 
DEM Spacing (m) 5 12.5.5 2.5 
  
  
  
  
  
  
Table 1: Typical operating parameters of STAR-3i and 
TopoSAR airborne IFSAR systems. 
A consistent set of Core Product DEM and ORI specifications 
irrespective of platform has been created and is summarised in 
Table 2. Varying flying altitudes and operating modes, enables 
different accuracy specifications to be achieved which may be 
reflected in cost and other factors. 
  
  
  
Product DSM DTM 
Type RMSE  Spacing|| RMSE Spacing 
| 0.5 5 0.5 5 
Il 1 5 1 5 
Ill 3 10 - - 
  
  
  
  
  
  
Table 2: Intermap Core Product specifications for IFSAR 
DEMs. All units are meters. RMSE refers to vertical accuracy 
Intern 
an 
(D 
WA 
Be 
an 
fe: 
Sp 
Int 
su 
sp 
Th 
Ph 
ac 
pe 
En 
In 
Th 
de 
ab 
En 
  
Fig 
the 
el. 
abc 
pre