Interferometric Processing of SIR-C Data for DEM Generation
in Mt. Kunlun, West China
Wang Chao , Zhang Hong , Pan Guangdong , Yang Qingyou
Institute of remote Sensing Applications,
Chinese Academy of Sciences, Beijing 100101
Tel:+86 10 6488 9546 Fax:+86 10 6488 9786
E-mail: cwang@public.bta.net.cn
Abstract
The Spaceborne Imaging radar (SIR-C) onboard the space shuttle Endeavor acquired
interferometric synthetic aperture radar (InSAR) data over Mt. Kunlun, West China, during
its second flight in Oct. 1994. An interactive INSAR data processor is developed for DEM
Generation from SIR-C interferometric data. The main procedure includes coregistration of
SLC data, phase unwrapping by weighted least-squares algorithms, InSAR parameter
estimation with ground control points, Image slant range to ground range correction with
resulting height information. The rms error of resulting DEM in Karakax test site of Mt.
Kunlun is around 16m.
Introduction
During last decade interferometric
synthetic aperture radar (InSAR) shows as
powerful technique for three dimensional
information retrieval of earth surface.
INSAR takes advantages of not only
capabilities of conventional SAR systems
but also phase information of radar signals,
which provides geometric information of
surface. Since late 1980’s InSAR becomes
one of advanced topics of remote sensing.
In Oct. 1994 during the second SIR-C
mission, repeat-pass interferometry was
experimented from its 8
th
mission days.
Interferometric data was acquired over
several test sites, including Karakax Valley
of Mt. Kunlun, West China. In this study,
the SIR-C interferometric data was
processed for DEM generation over
Karakax Valley test site.
Test Site and Data Description
The properties of this data set are given in
table I.
Table I Parameters of SIR-C
interferometric data in kunlun , xinjiang
|
Data take |
143.40 | 159.40
|
| Acquisition date
| 941009
| 941010
|
| Platform altitude (km)
| 213.6539764
| 213.4776459
|
| Latitude |
N36.0724678 | N36.0473557 |
| Longitude |
E79.1982880 | E79.2181244 |
| Band | L | L |
| Wavelength (m)
| 0.2422721 | 0.2422721 |
| Incidence angle (degree)
| 50.681 | 50.780 |
| Polarization | V V | V V |
| Azimuth pixel spacing (m) | 4.4907508 | 4.4907508 |
| Range pixel spacing (m)
| 3.3312409 | 3.3312409 |
| Slant range(near) (m)
| 318.5835876 | 318.5835876 |
Description of technique
The processor is divided in the
following blocks: image registration,
interferogram generation, flat-earth phase
removal, phase unwrapping, interferometric
parameters estimation, and DEM generation
(see Fig 1).
(1) Conregistration. The image
conregistration is performed in two stages:
coarse and fine registration. Coarse
registration is based in a cross correlation in
a window at the center of the scene. Fine
registration must compensate the pixel
misregistration and its variation along the
image with an accuracy of sub-pixel. The
interferomeric coherence is defined by
g
1and g
2 denote pixel values in each of the
two images (see Fig.2) respectively and E{}
denotes expectation value.
(2) Interferogram generation. The
interferogram is generated by multiplying
one image by the complex conjugate of the
other one. The order of the multiplication is
determined in order to assure the correct
sign of the phase.
(3) “Flat earth” phase removal. The "flat
earth" phase is computed by measuring the
dominant fringe frequency in azimuth and
range directions of the interferogram, and
then it can be removed, so that the
remaining fringes are due only to the relief.
(4) Phase Unwrapping. Phase unwrapping is
the reconstruction of a function on a grid
given the value modulo 2
p of the function
on the grid. In order to reduce the phase
noise and to facilitate the phase
unwrapping , the interferogram is filtered
and enhanced . The next step is phase
unwrapping. In the last few years an
increasing interest has been devoted to
phase unwrapping. In our study, weighted
least-squres algorithm using discrete cosine
transform and branch-cutting algorithm are
tested.
(5) Interferometric parameters
estimation.Since ERS-1/2,SIR-C has less
precise tracking of the orbit and basseline
parameters, the InSAR parameters
estimation from ground control points is
performed using least squares algorithm as
equation (2):
where, B is baseline,
Fi is absolute
interferometric phase at point (h
i,r
i).
(6) DEM generation. The conversion
process between the unwrapped phase in the
slant range and a geocoded DEM is carried
out in two steps. First, the relation between
phase and height can be set up by two ways,
ground control points or interferometric
parameters. The resulting height map is
given in slant range coordinates. The
second step is geocoding, the
transformation onto a cartographic
reference system of the slant range height
map. Geocoding was done using the orbit
information to project the slant range image
over the ground.
