The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
data are simultaneously collected by two antennas of the radar 
system. The SAR images are obtained by using the digital 
compression process. The range difference of two antennas to 
the scene is estimated by the interferogram of two coherent 
SAR images. The onboard-integrated Global Positioning 
System (GPS) and Inertial Measurement Unit (IMU) provide 
the navigation measurements. The system platform position and 
orientation are obtained by the GPS/INS integration post 
processing. Combining the accurate GPS/INS navigation 
solution and the interferogram measurements the orthorectified 
SAR image and DEM can be obtained by applying the geo- 
referencing algorithm. Figure 1 illustrates the geometry relevant 
to the height extraction based on the interferometric 
observations of SAR images. 
Figure 1: Geometry of IFSAR height extraction 
Considering the attitude of antenna platform the height 
difference can be determined by the following equations: 
h T = h P - p- [sin(<z> - a)cos(/?)] 
Ap 1_A 
B 4n 
COS (p = 
(1) 
(2) 
where 
hp ... the altitude of the antenna center Aq , 
a ... the roll and pitch of antenna platform, 
p ... the slant range between antenna and the target, 
(p ... the angle between the baseline and the line of sight / , 
Ap ... the range difference, 
<f>... the phase difference between two SAR antennas, 
B ... the baseline length of the vector b between two antennas, 
A ... the wavelength of SAR pulse signal. 
In the equation (1) wavelength A and the baseline length B are 
predetermined. The position of the antenna center and the 
orientation of antenna platform are normally obtained by the 
precise GPS/INS navigation solution. The slant range between 
antenna and the target p is calculated by the time delay of SAR 
image pixel and the orientation angle (p of the line of sight can 
be determined by the phase difference <p of interferometric 
measurements. Thus combing the highly accurate GPS/INS 
navigation solution with the SAR Interferometric measurement 
makes 3-D mapping using IFSAR technology possible. The 
detailed description of the interferometric process of SAR 
image data can be found in Rodriguez and Martin (1992). 
2.2 Intermap’s Airborne IFSAR System: STAR System 
Intermap has developed and currently operates five advanced 
airborne IFSAR systems. All systems are single-pass, side 
looking interferometric SAR system. The first of these systems 
Star-3 i® was originally developed by the Environmental 
Research Institute of Michigan (ERIM) and has been operated 
commercially by Intermap since 1996. In 2001 Intermap has re 
designed and modified the system to improve the operational 
efficiency and product quality. Based on the newly modified 
and upgraded system Intermap has developed a new 
interferometric SAR architecture, called STAR technology, see 
Keith et al (2003) for details. In last few years Intermap has 
developed other IFSAR systems: Star-4, Star-5 and Star-6, 
based on the STAR technology in order to increase the 
production capacity to meet the requirement of Intermap 
worldwide MEXTMap program. The details of NEXTMap 
program can be found in Bryan, 2007. The last system, 
TopoSAR was upgraded from the Aes-1 IFSAR system and is 
currently used mainly as a research and development platform 
for new system, such as repeat-pass P-band sensor and single 
pass polarized L-band system. 
Table 1 summarizes the specifications of the four STAR 
systems used in the NEXTMap program, more details of STAR 
systems can be found in Tennant et al, 2003, Li et al, 2004, 
Bryan, 2007. The major advantages of the newly developed 
Intermap’s STAR systems are summarized as follows: 
• High resolution SAR images with signal bandwidth of 
135 or 270 MHz, 
• High accuracy of DEM with RMS of 0.5 ~ 1.0 meter 
for the SAR data collected at altitude of 10000 meter, 
• Operational efficiency: single-pass SAR system with 
capability of collecting data from either left or right 
looking direction, 
• Considerably effective acquisition rate by optimal 
acquisition procedure: 10000 to 20000 km 2 /flight. 
These advantages provide effective technology to generate 
DEM products with higher accuracy and greater spatial detail. 
System/ 
Parameter 
Star-3i 
Star-4 
Star-5 
Star-6 
Operational 
year 
1996/2001 
2004 
2007 
2007 
Platform 
Leanet 
36A 
King 
Air 200 
King 
Air 200 
Leariet 
36A 
Typical 
speed 
(km/hr.) 
720-750 
400- 
430 
400- 
430 
720- 
750 
Typical 
flight 
altitude (km) 
6-12 
4-8.5 
4-7.5 
6-12 
Ground 
swath width 
(km)* 
8- 15 
6-11 
8-15 
6-11 
Center 
frequency 
(GHz) 
9.605 
9.605 
9.605 
9.605 
Range 
bandwidth 
(MHz) 
135 
135/270 
135/270 
135 
Polarization 
HH 
HH 
HH 
HH