141 
RECENT ADVANCES IN AIRBORNE INSAR FOR 3D APPLICATIONS 
Bryan Mercer, Qiaoping Zhang 
Intermap Technologies Corp., #1200, 555 - 4 th Avenue SW, Calgary, AB, Canada T2P 3E7 
{bmercer, qzhang}@intermap.com 
Commission I, WG 1/2 
KEY WORDS: DEM, DSM, DTM, InSAR, Interferometric SAR, Mapping 
ABSTRACT: 
The objective of this paper is to provide an update on two programs which have been evolving recently that will have significant 
impact on several geomatics application areas requiring 3D information over extended areas. The first program relates to the 
creation of nation-wide and continental-scale DEM databases using airborne Interferometric SAR (InSAR) at a level of detail 
intermediate between that of airborne lidar and space-bome systems. This program is operational and well advanced in its goal. The 
second program is developmental, and concerns a first-of-its-kind airborne, single-pass L-Band, fully polarimetric InSAR 
(PolInSAR) system. The primary goal of this research is to determine how well ground elevation can be extracted beneath forest 
canopy of different types in the absence of temporal decorrelation effects. Some preliminary results are presented here. 
1. INTRODUCTION 
The use of Digital Elevation Models (DEMs) is widely spread 
and growing, not only in the traditional mapping world but 
increasingly in support of new applications that are driven by 
consumer interests. In this new environment, not only do 
required levels of detail and implicit accuracy vary according to 
application, but price and current availability are major 
considerations for the user, many of whom come from outside 
the geomatics industry. An additional consideration is that some 
applications, in order to be effective, transcend local political 
boundaries and require uniform data-sets across regional, 
national and even continental scales. Meanwhile the advances 
of enabling technologies such as GPS, communications 
bandwidth, storage capacity and processing power have been 
instrumental in the growth of both numbers and capability of 
systems for DEM creation including both passive and active 
systems. Among the active systems, both lidar and 
Interferometric SAR (InSAR) have become major sources of 
three-dimensional information. 
In particular, airborne InSAR, as demonstrated in the following 
sections, is contributing to the wide-spread availability of 
DEMs over continent-sized areas and across national 
boundaries with properties of accuracy, resolution and price 
that are intermediate between those of lidar and SRTM. The 
objective of the first part of this paper is to provide an update 
on the NEXTMap® programs for creating DEMs of Western 
Europe and the USA using well-developed operational airborne 
X-Band InSAR technology. The second part of the paper, by 
contrast, describes the early phases of a developmental program, 
the objective of which is to extract bare-earth DEMs from 
beneath forest canopy using single-pass airborne L-Band 
Polarimetric InSAR (PolInSAR) techniques. 
In the following sections we will first provide a brief 
background with respect to the InSAR technology, and 
summarize the specifications and validation of the various 
DEM and image products created by the STAR-series of 
airborne InSAR platforms. This will be followed by a 
discussion of the NEXTMap® concept with an update of the 
current implementation status and a description of the current 
capacity of the acquisition and processing elements required to 
achieve the NEXTMap® goals and schedule. The 
developmental program will then be addressed, first discussing 
some aspects of L-Band PolInSAR. A few preliminary results 
from the recent tests single-pass tests will then be presented. 
2. INSAR BACKGROUND 
2.1 InSAR Summary 
The interferometric process has been widely discussed in the 
literature, (e.g. Zebkor and Villsenor, 1992; Bamler and Hartl, 
1998; Rodriguez and Martin, 1992). The geometry relevant to 
height extraction, h, is illustrated in Figure 1. If the two 
antennas, separated by baseline B, receive the back-scattered 
signal from the same ground pixel, there will be a path- 
difference 8 between the two received wave-fronts. The 
baseline angle Ob 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. 
sin(6f- Ob) = 8/B 
(1) 
8/A = (p/(2*n) + n 
(2) 
h =H-r s cos (Oj) 
(3) 
The path-difference 8 is measured indirectly from the phase 
difference (p between the received wave fronts (Equation 2). 
Because the phase difference cp can only be measured between 
0 and 2n (modulo 27i), there is an absolute phase ambiguity (« 
wavelengths) which is normally resolved with the aid of 
relatively coarse ground control. A “phase unwrapping” 
technique completes the solution. Thus the extraction of 
elevation is performed on the “unwrapped” phase. Often the