Because of their high pulse rate (1 to 80 kHz depending
on the particular system and operational mode) and
relatively small footprint (system- and operation-
dependant but typically about 20 cm diameter) these
systems are generally successful at penetrating foliage.
In particular, they are able to penetrate to ground level
with sufficient regularity (at least in leaf-off conditions)
to provide “bald-earth’’ DEMs beneath forest canopy
(Reiter, et al., 1999) with respectable, albeit somewhat
de-graded accuracies compared to their bare ground
performance. These systems also demonstrate
advantages in dense urban core areas for acquiring
building and ground elevations because of their
relatively vertical geometry (in contrast to the side-
looking radar geometry) so that loss of data due to
building shadows (occlusions) is less onerous. The two
prime disadvantages of laser from a user's point-of-view
are (1) cost (see Figure 2 below) and (2) delivery. These
issues make the use of laser problematic over large areas.
In this paper we present three examples demonstrating
the relative performance of STAR-3i DEMSs with respect
to three laser systems in three different locations. In this
instance, the laser data are being used as comparative
‘truth’. However, the argument is presented that by
using the systems in a complementary or synergistic
fashion, the advantages of both may be exploited and
their respective disadvantages muted.
In Section 2, we provide a very brief introduction to the
interferometric radar process in deference to the mostly
laser-oriented audience at this workshop. The
assumption is made that symmetry need not be
preserved, as the workshop audience is very
knowledgeable regarding the principles of scanning
lasers. Section 3 provides a brief operational history of
the STAR-3i system and is followed by an overview of
the comparative performance parameters and cost
relationships in Sections 4 and 5. The three application
examples are presented in Section 6 and discussion and
conclusions appear in Section 7.
Throughout this paper we use the term DEM to reference
the scattering surface whether it be bare-earth, canopy or
structures. To differentiate, we refer to the ‘bald-earth
DEM’ as being that surface or DEM from which heights
associated with trees, forests, crops and other objects
such as buildings have been removed.
2. INTERFEROMETRY BACKGROUND
The interferometric process has been widely discussed in
the literature, particularly for the case of repeat pass
interferometry (e.g., Zebker and Villasenor, 1992 and
Goldstein, et al., 1988). Some of the general issues
associated with airborne interferometry have been
discussed, for example, in Gray and Farris-Manning
(1993). The geometry relevant to height extraction, *h',
is illustrated in Figure 1.
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
| | sale > 4 pixel
In L id
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terrain
Figure 1: Schematic of Airborne Interferometric
SAR Geometry
If the two antennas, separated by baseline 'B', receive
the back-scattered signal from the same ground pixel,
there will be a path-difference ‘5’ between the two
wavefronts. The baseline angle ‘0,’ is obtainable from
the aircraft inertial system, the aircraft height is known
from DGPS and the distance from antenna to pixel is the
radar slant range. It is simple trigonometry to compute
the target height ‘h’ in terms of these quantities. The
path-difference is measured indirectly from the phase
difference between the received wavefronts. Because the
phase difference can only be measured between 0 and 27
(modulo 27), there is an absolute phase ambiguity which
is normally resolved with the aid of a coarse ground
elevation estimate and a “phase unwrapping” technique
(e.g. Goldstein, et al., 1988). Thus, the extraction of
elevation is performed on the “unwrapped” phase.
3. SYSTEM HISTORY, SPECIFICATIONS AND
PERFORMANCE
Intermap Technologies has been operating the STAR-3i
system commercially since January, 1997. The system
was developed by ERIM under contract to DARPA
(Defense Advanced Research Projects) and was referred
to as IFSARE at that time. The IFSARE system was
described by Sos, et al. (1994), and is briefly
summarized from an operational point of view in the
following paragraphs.
STAR-3i, an X-band, interferometric SAR, is carried in a
Learjet 36 and is capable, under ideal circumstances, of
imaging 30,000 km? in a single operational day.
Positioning and motion compensation are achieved
through use of a laser inertial reference platform closely
coupled with differentially post-processed GPS. One of
its operational mission modes would be performed at
40,000 ft (12.2 km) ASL and in this mode it would
collect 2.5 m? pixels across a 10 km ground swath. At
lower altitudes, the signal-to-noise ratio is larger and
thus the height noise decreases (Zebker and Villasenor,
1992) thereby improving relative accuracy; however,
swath width is reduced. The DEM created from the
interferometric data is post-processed, and an ortho-
rectified image (ORI) is simultaneously produced. A
International
correlation image,
correlation betweeir
is used for qualit
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acquisition is curre
25,000 ft.
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4. COMPARAT]
OF SEI
In Table 1, we
(selected) of the th
sets described in S
of this table is to i
the laser systems v
standard operation
parameters associa
F
Operational Altitude
Operational Speed
PRF
Incidence Angles (thi
Swath Width (grounc
DEM Sample Spacin
DEM Vertical Accura
DEM Horizontal Acci
Collection Rates
Ortho-Rectified Imag
Sensor Source
Notes:
1 Laser operating p:
2 Laser accuracies :
3 STAR-3i accuraci
4 STAR-3i results a:
5 STAR-3i absolute
6 Typical STAR-3i a
Table 1: Compai
three commercial
