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IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India,2002
GENERATION AND VALIDATION OF DEM USING
SAR INTERFEROMETRIC TECHNIQUES AND DIFF ERENTIAL GPS
P. Jayaprasad*, B. Narender, S.K. Pathan and Ajai
Forestry, Land use and Photogrammetry Group,
Space Applications Centre, ISRO, Ahmedabad. 380 015, India
Jp_pallipad @ yahoo.com
KEYWORDS: SAR Interferometry, Coherence, DEM, Differential GPS
ABSTRACT:
Synthetic Aperture Radar (SAR) interferometry or InSAR is a useful technique for topographic mapping from Space. It utilizes phase
content of the radar signal derived from the complex radar data as additional information for extracting three-dimensional information of
the surface. The objective of the present study was to generate DEM using InSAR techniques in a moderately undulating terrain and
validate it using differential GPS measurements. DEM was generated using InSAR software developed at SAC. Differential GPS survey
was carried out to establish GCP's for validating DEM.
1. INTRODUCTION
Synthetic Aperture Radar (SAR) interferometry or InSAR is a
useful technique for topographic mapping from space. It utilizes
phase content of the radar signal as an additional information
source derived from the complex radar data for extracting three-
dimensional information of the surface. SAR images record both
intensity and the phase values of the backscattered signal. The
intensity gives the nature of the surface and the phase values give
the terrain elevation or topography of the surface. Complex
images recorded by different antennas or at different times are
combined to form interferograms. If the two antennas are within
the scattered beam of a ground resolution cell, then due to
coherent nature of the radar signals, the signals reflected from the
same scatterer on the ground will interfere each other. The phase
components of a pair of images interfere to produce a fringe
pattern. This resulting phase difference image is known as
interferogram. Since phase is a function of distance, the fringes
represent the elevation pattern in an encoded form due to
wrapping of phase from — zm to +r. Using one known scene
elevation, phase unwrapping can be carried out and these phase
values can be related to the terrain elevation using the SAR
geometry.
SAR interferometry offers advantages with respect to other
techniques in having all weather capability and high accuracy
especially for differential height measurements. Differential
interferometry allows one to map elevation changes at centimeter
level by means of three SAR images of the same area separated in
time with adequate baseline.
This study attempts to further explore the use of this technique for
DEM estimation in a moderately undulating terrain. The
validation of DEM was done using differential GPS
measurements.
2. InSAR THEORY
A radar interferometer is formed by relating the signals from two
spatially separated radar antennas,The separation of the two
antennas is called the baseline. The two antennas may be mounted
on a single platform or a synthetic interferometer may be realized
by utilizing a single antenna on a satellite in a nearly exact
repeating orbit. However, for repeat orbit case, temporal
decorrelation constitutes an important source of error in
topographic mapping.
The geometry of interferometer is as shown in fig.1. Al and A2
are two satellite positions separated by baseline B. a is the base
line angle i.e. the angle of the baseline with respect to the
horizontal and 0 the look angle. R and R+3 are the slant ranges, z
the surface topography and H the satellite height. Radar signals
transmitted and received from two antennae will, when properly
resampled and cross multiplied, form an interferogram. For repeat
pass imaging geometries, on each pass the radar acts as both a
transmitter and receiver, therefore the total path difference for
each radar observation to a given point on the surface is twice
what would be expected if a single spacecraft or aircraft with two
physical antennas were used. The phase difference at each
location is proportional to the path difference 2 8 with a
component of proportionality 2 z/ A. From the geometry the
following relations can be derived.
ô = 2/42 Q where 97 2z/4*26 (1)
(R8) 2 R^ « B? - 2RB Sin (a - 0) (2)
Sin (a — 0) = ((R - 8) - R? -B?y 2RB (3)
z= h-RCos 0 (4)
Using this equation topographic heights can be estimated using
values of 0 and B. Knowing the height differences in X and Y
directions we can estimate slope of the terrain. The phase errors
and attitude errors are the two sources of error in height
estimation.