Dr. Howard A. Zebker
Stanford University
STAR Lab
232 Durand
Stanford, CA 94305-4055
Co-Investigators:
Charles Elachi, JPL/Caltech
Jakob van Zyl, JPL/Caltech
Radar Interferometric and Penetration Investigations using
SIR-C
Data
OBJECTIVES
To model, experimentally characterize, and verify penetration phenomena in hyperarid
and vegetated regions using the
SIR-C/X-SAR
multiparameter radar system and groundbased
receivers.
To invert measured radar backscatter as a function of frequency and polarization in
terms of geophysical parameters of the surface, subsurface and vegetation canopy
such as surface roughness, subsurface geomorphology, or tree height and density.
To display subsurface and within-canopy features in an image format, thus easing the
interpretability of the results.
PROGRESS
We are currently involved in two study areas associated with
SIR-C.
The first is
the study of penetration phenomena that relate radar backscatter data in vegetated
and hyperarid regions to geophysical factors of the surface cover. The second involves
development of radar interferometry as a technique and is important for the future of
NASA's radar program, which is likely to contain a significant radar interferometry
component.
Interferometric data are requested in raw signal sample format, which we process to
interferograms and subsequent topographic and deformation products. We developed
a
SIR-C
interferometric data processor which we run on workstation computers-- the
software includes the
SAR
processing algorithms, interferogram generation software, baseline
estimation algorithms, and product generation code.
The data for the penetration experiment are also obtained from the raw signal samples.
They are extracted using Fourier spectrum techniques and times series of amplitude
and phase fluctuations are produced. These times series are then cross correlated
to infer the impulse response of sub-canopy radar reflectors as well as the attenuation
of the canopy. These results must be related to parameters of the canopy to derive
the ability of the system to "see" under the trees. This will be of significant
note in future topographic and other interferometric systems aimed at obtaining under-canopy
heights.
Significant Results and Publications
During the past year and one half, the most significant results have been those related
to radar interferometry. We have quantified the performance of repeat pass spaceborne
interferometric topographic maps, and verified the results by comparison with existing maps. We have also developed the three-pass surface deformation technique,
verified it against
GPS
and field survey approaches, and compared results of two-pass
and three-pass analyses of earthquakes.
We have also used
SIR-C
correlation measurements to track active lava flows at Kilauea,
and obtained the most precise estimates of flow volume and mass rates to date.
For this work we developed a theory for the effects of atmospheric variability on
repeat-pass interferometric observations. These are likely the limiting factors
in any practical deformation or repeat-pass topographic system.
FUTURE PLANS
The approach to completing the science studies as identified in the original proposal,
as well as the interferometric experiments that were added to the project science
plan, is a two-fold strategy. Both sets of activities are in the data analysis phase,
that is, the mission and field deployment component of the studies is essentially
complete with the end of the planned flights of the
SIR-C
hardware. The remaining
work concerns data processing, interferometric analysis, penetration analysis, verification with field measurements, and publication and reporting.
Penetration study. During the first and second
SIR-C
flights, special purpose portable
transmitters and receivers were deployed at
SIR-C
supersites. For flight 1, the
site was located at Raco, Michigan, and consisted of temperate forest trees. For
flight 2, the site was located at Kilauea, Hawaii, and consisted of a rain forest region.
During shuttle overflights, ground receivers recorded the absolute level of radar
signals as they were propagated through and attenuated by the vegetation canopy.
In addition, coherent tone generators transmitted narrowband signals back up to the shuttle.
These signals are to be extracted from radar received echo waveforms. In each instance
equipment was simultaneously placed under the canopy and also in an open area to provide a reference signal.
For each overpass, the receiver measurements will be compared between equipment in
the open and in the canopy. The resulting fluctuating power measurements will then
be related to models of scatter by the vegetation. The transmitter measurements
will be extracted from the radar echoes and separated by frequency transform techniques.
A cross-correlation of the open and canopy transmitter signals then yields an effective
impulse response of the radar to targets hidden in the canopy independent of shuttle
motion. This again will be related to scattering models of the canopy for both the
temperate and rain forest terrain types.
Interferometer investigations.
SIR-C
collected two types of interferometric data.
These were data with a six-month temporal baseline collected during both flight
1 and flight 2, and data with a one-, two-, or three-day temporal baseline collected
wholly during flight 2. We propose to investigate this data set using two types of analysis:
i) generation of topographic maps and ii) measurement of surface deformation.
Since
SIR-C
is a multifrequency instrument, it offers the unique opportunity to compare
results at different frequencies. Phenomenological differences in the scattering
behavior at 24 cm and 6 cm wavelength will comprise a major part of this investigation.
We will process these data from the raw signal sample format, as delivered by the
SIR-C
ground data processing facility. We will generate topographic maps of several
of the supersite areas at both frequencies, and compare them to conventionally derived
digital elevation models to assess their accuracy. These will then be made available
to the science team to aid in other investigations.
We also will examine the potential of
SIR-C
to determine surface deformation. The
principal data set here will be the six-month data as the centimeter-level motion
we are sensitive to will be much more apparent over six months than three days.
A significant part of the latter study is to understand the effects of the atmosphere
on the received signals. This will involve processing some of the 1, 2, and 3 day
data to look for apparent motion caused by atmospheric irregularities. These interfering signals will be quantified and assessed for their impact in future space-based
radar systems.
The benefits from these investigations will accrue from both sets of activities.
The penetration studies will aid in design of radar systems capable of imaging beneath
canopies, important for ecological studies as well as for target detection systems.
They also will provide new insights into the physics behind scattering from dense canopies.
It has now been demonstrated that interferometric radar techniques benefit a wide
variety of Earth science investigations, including production of digital elevation
models of the Earth's surface, centimeter-scale surface deformation measurements
of coseismic displacement fields, and centimeter-per-day velocity maps of ice sheets and glaciers.
The
SIR-C
experiments will give us the first quality set of multifrequency data
that can be used to examine phenomenological changes dependent on radar wavelength.
It also will permit the first full examination of the artifacts induced by atmospheric
interference. This approach will enable the fastest and most thorough means to realize
the full potential of radar interferometry for geoscience applications.
PUBLICATIONS
Zebker, H. A., P. Rosen, S. Hensley, and P. Mouginis-Mark, Analysis of active lava
flows on Kilauea Volcano, Hawaii, using
SIR-C
radar correlation measurements, submitted
to Nature, August 1995.
Zebker, H. A., R. Goldstein, P. Rosen, and S. Hensley, Effect of atmospheric variability
on interferometric deformation and topography measurements, in preparation, August
1995.
Farr, T. G., D. Evans, H. A. Zebker, D. Harding, J. Bufton, T. Dixon, S. Vetrella,
and D. Gesch, Mission in the works promises precise global topographic data, EOS Transactions
, Vol. 76, No. 22, pp.225-228, May 30, 1995.
Zebker, H. A., T. G.Farr, R. P. Salazar, and T. H. Dixon, Mapping the world's topography
using radar interferometry: the TOPSAT mission, IEEE Proceedings,
Vol. 82, No. 12, pp 1774-1786, December 1994.
Zebker, H. A., P. A. Rosen, R. M. Goldstein, A. Gabriel, and C. Werner, On the derivation
of coseismic displacement fields using differential radar interferometry: the Landers
earthquake, Journal of Geophysical Research
- Solid Earth, Vol. 99, No. B10, pp 19617-19634, October 10, 1994.
Zebker, H. A., C. L. Werner, P. Rosen, and S. Hensley, Accuracy of topographic maps
derived from
ERS-1
radar interferometry, IEEE Transactions on Geoscience and Remote Sensing
, Vol. 32, No. 4, pp 823-836, July 1994.