International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
cross track displaced antennae. These antennae can be mounted
on a single platform — single-pass mode, or with a single
antenna passing over the area twice — dual-pass mode. While
space systems typically use a dual-pass configuration (the
notable exception is SRTM), most modern airborne
implementations are single-pass across-track. Single-pass mode
is desirable as it eliminates the primary problem with dual-pass
mode — the scene and atmosphere change during the period of
acquiring both datasets causes temporal decorrelation.
2.2 Airborne and Spaceborne Implementation
Compared with their spaceborne counterparts, airborne IFSAR
implementations have many operational advantages such as
flexible system deployment, higher spatial resolution, and a
lesser degree of influence from the atmosphere. These
advantages provide for the creation of a product of greater
accuracy. The most aggressive spaceborne IFSAR application
was the SRTM project of which Intermap played a key role in
production. The SRTM data is the most ambitious product to
date providing elevation data between 56° south and 60° north
latitude with a 16-m vertical accuracy.
2.3 Capabilities and Advantages
2.3.1 Weather Independence: IFSAR is an active microwave
sensor and can operate in conditions and environments where
other mapping technologies cannot. These environments
include: operation at night, through cloud cover, through light
rain or snow and dust. This makes IFSAR very useful and cost-
effective to map through the smoke of a forest fire, rain clouds
during a flood, or at night.
2.3.2 First Surface and Terrain Models: By nature of the
radar sensor, the original elevation models generated are of the
first surface and not the underlying bare-earth. While the first
surface DEMs have many applications, bare-earth DEMs are
traditionally expected and required for many topographic
mapping purposes. A bare-earth DEM can be satisfactorily
derived from the first surface DEMs for many terrain types
using appropriate processing technologies, such as Intermap’s
TerrainFit& (Wang et al, 2001). The combined use of both the
first surface and bare-earth DEMs has expanded applications of
the DEMs.
2.3.3 Orthorectified Radar Imagery: The STAR systems also
generate high-resolution orthorectified radar imagery together
with DEMs. This imagery is beneficial for users to create
value-added mapping products in a cost-effective way and is
also useful for many remote sensing applications.
2.3.4 Quick Turn-around Time: STAR technology can
efficiently map large areas in a short time frame due to its
weather independence, fast data acquisition and high level of
production automation. This is attractive for many emergency-
mapping, regional and nation-wide mapping applications.
2.3.5 Cost competency: The cost to generate high resolution
and highly accurate mapping data for large areas becomes
insurmountable for other mapping technologies when mapping
products with similar quality are expected. STAR technology is
very cost competent.
2.4 Limitations and Mitigations
As with any remote sensing technology, airborne IFSAR
mapping also has limitations that need to be understood and
addressed. IFSAR service and data providers are adopting
various ways to mitigate the effects caused by those limitations.
2.4.1 ‘Area-like’ Sensing: IFSAR is an averaging sensor that
essentially averages through the integration process for each
ground resolution cell. An IFSAR elevation observation is
made up from the integration of responses over extent of the
cell. As a result, the spatial density of the height samples is
dependent on the radar resolution applied within the
interferometric process. The STAR technology will represent a
5 x 5-m? averaging area for the height estimate. Higher
resolutions are available if the aircraft is flown lower. However,
this is not a standard product.
2.4.2 Side-looking Imaging Geometry: IFSAR is a side-
looking sensor and does not view the nadir. For certain terrain
conditions, foreshortening, layover, and shadow phenomena
may appear on the radar imagery and influence the product
quality. These phenomena affect correlation between the two
interferometric channels, causing a loss in accuracy and
possibly a loss in data for the affected region. Interpolation can
be used to fill small areas of missing data. However, multiple
flight lines with different radar look direction may also be
required to minimize those artifacts when the situation is
severe, typically over areas with significant terrain.
2.4.3 Weather Restriction: While IFSAR can be operated in
nearly all kinds of weather conditions, there still are some
situations that should be avoided to ensure data accuracy. One
issue is air turbulence at acquisition altitudes. IFSAR missions
require the platform to be as reasonably stable as for accurate
trajectory reconstruction from the navigation data. Turbulence
makes this task difficult and thus degrades the quality of the
resultant mapping products. If the weather is too turbulent to
successfully collect radar data, flights are postponed until
conditions improved. Heavy moisture accumulation in clouds
(severe thunderstorm clouds) must also be avoided as they can
absorb a significant portion of the radar energy.
3. INTERMAP'S STAR SYSTEMS
A strong heritage of photogrammetric and radar mapping
experience positions Intermap Technologies very well as a
manufacturer of geospatial data product. Its commercial
capacity and professional reputation to meet demand for high-
quality and low-cost geospatial data are attracting worldwide
attention, especially since the launch of its NEXTMap® effort
and off-the-shelf data availability business model.
Intermap currently owns and operates three aircraft outfitted
with its advanced STAR technology. These are called STAR-
3i®, installed in a Learjet; TopoSAR®, installed in an Aero
Commander; and STAR-4, installed in a King Air 200 (Figure
1) Table 1 lists major technical specifications of the three
systems. These systems are configured as an across-track X-
band SAR interferometer with single-pass 3-D radar mapping
capabilities.
STAR-3i TopoSAR STAR-4
Figure 1. Intermap’s STAR Systems
Interna
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Aircraft
Typical
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Typical
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Ground
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Center
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Best im:
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