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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
STAR Systems
Parameters STAR-3i TopoSAR STAR-4
Aero
Aircraft Learjet 36A Commander King Air 200
690-1000
Typical flight 750 km/hour 400 km/hour 400 km/hour
velocity
Typical flight
altitude above 3-10 km 3-9km 3-9km
sea level
Ground swith: {3.2 yume 3-8 km* 8-11 km*
width
Center 9.57 GHz 9.55 GHz 9.58 GHz
frequency (X-Band) (X-Band) (X-Band)
Range 67.5 & 135 67.5, 135 &
bandwidth MHz Up t6400 Mtis, | sro Mit
ip 30 — 60° 30 — 60° 30 — 60°
elevation
Polarization HH HH HH
IFSAR 0.9 m 0.6 or 1.8. m 0.98 m
Baseline
ape Typical
a. 1.25m 0.5m 1.25 m
tes Up to 0.5m**
Table 1. Major Technical Specifications of Intermap's
STAR Systems
* Terrain dependent
** Planned - not yet tested
4. AIRBORNE IFSAR MAPPING PROCESS
IFSAR mapping is essentially a process of producing 3-D map
products by processing raw radar data collected by airborne
IFSAR systems. Thematic information for a scene is derived
from the synthetic aperture radar (SAR) images. Height
information is obtained by using the phase difference between
two coherent SAR images simultaneously obtained by two
antennae separated by an across-track baseline in a single-pass
mode (Figure 2). The following outlines the production chain of
a typical airborne IFSAR mission. Figure 3 illustrates a high-
level production flowchart.
“SAR
5 Image
Pixel Amp
Pixel Phase
Differences — -
(interferogram) 5
ote: STAR-3i uses 1 GPS ground station.
lor differential processing; no cther ground
“control points are required.
= Altitude
Figure 2. Concept of Airborne IFSAR Mapping
4.1 Mission Planning and Data Acquisition
Raw radar data, sensor navigation data, and ground GPS data
are collected simultaneously as pre-determined by the mission
planning. Data collection requirements are determined through
a mission planning that takes into account the mission
requirements and terrain conditions. Mission planning
translates mission requirements into operating parameters
required to complete the mission successfully and effectively.
The following are the main components that constitute a typical
mission plan:
e. STAR radar operating parameters
e Flight altitude and speed
e Number, orientation, length and distribution of flight
lines (regular parallel lines and tie lines)
e Multiple look direction requirement
e Number and location of ground-based GPS station(s)
e Number and location of ground control points (not
always necessary) for map product validation and
removal of systematic biases.
4.2 SAR Processing
Collected raw radar data are unloaded from the onboard storage
media. Signals from the two antennae are processed separately
and combined later in the interferometric process. The
navigation processor combines the airborne navigation data
(GPS/INS) with ground based GPS data to generate the precise
information necessary for SAR image formation and
interferometric processing. Single-look complex image pairs
are generated with one image per antenna through an image
formation process.
4.3 Interferometric Processing
For STAR technology image registration is maintained via a
very precisely known baseline. An interferogram is created,
which is a two-dimensional map of phase difference between
the two images. Phase difference contains many integer
multiples of 2x and a fraction part from 0 to 2x. The above-
formed interferogram only represents the fractional part of the
phase difference. To put an IFSAR pixel into 3-D space, the
absolute phase must be determined through a phase unwrapping
process. Once this is complete the phase difference and the
navigation information are used to generate a height for each
sample. The result is a strip of orthorectified image and DEM.
4.4 Post-Processing
These multiple radar strip images and DEMs are merged into a
single image and DEM with a common datum and map
projection in a mosaicking process. Data gaps can be filled
using an appropriate interpolation method or left undefined.
Interactive data editing, primarily for DEMs, is conducted to
detect and correct potential blunders inherent in the dataset, and
for quality control purposes. The finished first surface DEM
can then be further processed and edited to remove objects such
as trees, buildings, towers etc. At the end of the post-
processing, core products that meet pre-defined and consistent
specifications are stored in the Company's online iStore. Core
products include orthorectified radar images, first surface and
bare-earth DEMs.
