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Zebker, Howard A.

Publications and source records attributed to Zebker, Howard A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2020

Joint retrieval of soil moisture and permafrost active layer thickness using L-band InSAR and P-band PolSAR

Seasonal subsidence measured by repeat-pass interferometric synthetic aperture radar (InSAR) can be used to infer the active layer thickness (ALT) in permafrost regions. The differential volume of soil water undergoing the phase change over the thaw season is one of the factors impacting the seasonal subsidence and is a function of both soil moisture profile and thaw depth. Without the information about soil moisture, this InSAR approach can have large biases in the ALT estimates when soil moisture profile is below saturation. Soil moisture and ALT can also be estimated from polarimetric synthetic aperture radar (PolSAR) backscatter observations but the sensing depth of the PolSAR approach is limited when deep ALT is present. In this paper, we integrated these two approaches and applied a joint retrieval method to estimate the soil moisture profiles and ALT from the L-band InSAR and P-band PolSAR data acquired over the Arctic-boreal region during the 2017 Arctic-Boreal Vulnerability Experiment (ABoVE) airborne campaign.

Schaefer, Kevin M.↗
NASA NTRS2011

Airborne Radar Interferometric Repeat-Pass Processing

Earth science research often requires crustal deformation measurements at a variety of time scales, from seconds to decades. Although satellites have been used for repeat-track interferometric (RTI) synthetic-aperture-radar (SAR) mapping for close to 20 years, RTI is much more difficult to implement from an airborne platform owing to the irregular trajectory of the aircraft compared with microwave imaging radar wavelengths. Two basic requirements for robust airborne repeat-pass radar interferometry include the ability to fly the platform to a desired trajectory within a narrow tube and the ability to have the radar beam pointed in a desired direction to a fraction of a beam width. Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) is equipped with a precision auto pilot developed by NASA Dryden that allows the platform, a Gulfstream III, to nominally fly within a 5 m diameter tube and with an electronically scanned antenna to position the radar beam to a fraction of a beam width based on INU (inertial navigation unit) attitude angle measurements.

Hensley, Scott↗
NASA NTRS2006

Cassini RADAR Observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe

The Cassini mission includes 34 investigations of Saturn's icy satellites by the 2.2-cm-wavelength (13.8-GHz) RADAR instrument, operating both as a scatterometric radar and a passive radiometer. These measurements are sensitive to near-surface electrical properties and structure at scales about six times smaller than the only groundbased radar wavelength available to study the satellites (13 cm) and 22 times longer than the millimeter wavelengths at the limit of Cassini's Composite Infrared Spectrometer (CIRS). Here we present Cassini's first radar results for seven of the satellites.

Saturn↗
NASA NTRS2004

Validation and Improvement of SRTM Performance over Rugged Terrain

We have previously reported work related to basic technique development in phase unwrapping and generation of digital elevation models (DEM). In the final year of this work we have applied our technique work to the improvement of DEM's produced by SRTM. In particular, we have developed a rigorous mathematical algorithm and means to fill in missing data over rough terrain from other data sets. We illustrate this method by using a higher resolution, but globally less accurate, DEM produced by the TOPSAR airborne instrument over the Galapagos Islands to augment the SRTM data set in this area, We combine this data set with SRTM to use each set to fill in holes left over by the other imaging system. The infilling is done by first interpolating each data set using a prediction error filter that reproduces the same statistical characterization as exhibited by the entire data set within the interpolated region. After this procedure is implemented on each data set, the two are combined on a point by point basis with weights that reflect the accuracy of each data point in its original image. In areas that are better covered by SRTM, TOPSAR data are weighted down but still retain TOPSAR statistics. The reverse is true for regions better covered by TOPSAR. The resulting DEM passes statistical tests and appears quite feasible to the eye, but as this DEM is the best available for the region we cannot fully veri@ its accuracy. Spot checks with GPS points show that locally the technique results in a more comprehensive and accurate map than either data set alone.

Zebker, Howard A.↗
NASA NTRS1996

Radar Interferometric and Penetration Investigations using SIR-C Data

Progress, significant results, publications and future plans are discussed in relation to the following objectives: (1) 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; (2) 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; and (3) To display subsurface and within-canopy features in an image format, thus easing the interpretability of the results.

Zebker, Howard A.↗
NASA NTRS1995

Techniques For Topographical Mapping Via Interferometric SAR

Two techniques for processing data acquired by airborne interferometric synthetic-aperture-radar (SAR) system yield terrain-height maps. Predicated on availability of accurate navigational system that yields data on motion and orientation of airplane that carries SAR system and, also effectively gives orientation of baseline between two radar antennas of system. Three-dimensional coordinates of target point on terrain determined from (1) position and orientation of airplane as given by navigational system and (2) slant range gamma and phase shift phase as measured by SAR system.

Madsen, Soren N.↗
NASA NTRS1994

Exploiting Decorrelations In Synthetic-Aperture Radar

Temporal decorrelation between synthetic-aperture-radar data acquired on subsequent passes along same or nearly same trajectory serves as measure of change in target scene. Based partly on mathematical models of statistics of correlations between first- and second-pass radar echoes. Also based partly on Fourier-transform relations between radar-system impulse response and decorrelation functions particularly those expressing decorrelation effects of rotation and horizontal shift of trajectories between two passes.

Zebker, Howard A.↗
NASA NTRS1994

On the derivation of coseismic displacement fields using differential radar interferometry: The Landers earthquake

We present a map of the coseimic displacement field resulting from the Landers, California, June 28, 1992, earthquake derived using data acquired from an orbiting high-resolution radar system. We achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques. Data from the ERS 1 synthetic aperture radar instrument acquired in April, July, and August 1992 are used to generate a high-resolution, wide area map of the displacements. The data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30-m resolution element in a 113 km by 90 km image. Our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake. Fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing. Comparison of these data with Global Positioning System and electronic distance measurement survey data yield a correlation of 0.96; thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format. The technique we use is (1) more automatic, (2) more precise, and (3) better validated than previous similar applications of differential radar interferometry. Since we require only remotely sensed satellite data with no additioanl requirements for ancillary information. the technique is well suited for global seismic monitoring and analysis.

Zebker, Howard A.↗
NASA NTRS1994

Software For Analysis Of SAR Data

MacSigmaO is interactive program for Macintosh computer enabling user to display and make computations from data collected by JPL AIR-SAR, ERS-1, JERS-1, and Magellan radar systems. Provides four basic functions: synthesis of images (if necessary), statistical analysis of selected areas, analysis of corner reflectors as calibration measure (if appropriate and possible), and informative mouse tracking. Written in C for use on Macintosh series of computers.

Norikane, Lynne↗
NASA NTRS1994

Development Of Topographic Mapping With SAR

Report summarizes development of airborne interferometric synthetic-aperture-radar system (SAR) for topographic mapping. Basic principle of topographic mapping by use of interferometric SAR and performance of earlier version of system described in "Topographical Mapping With Synthetic-Aperture Radar" (NPO-16665).

Zebker, Howard A.↗
NASA NTRS1993

Techniques and applications of SAR interferometry for ERS-1

The ERS-1 Synthetic Aperture Radar (SAR) demonstrated that it is possible to measure high resolution topography over large areas using interferometry with repeat orbit data sets. Given that the ERS-1 orbit is known with high precision, an automatic system for generation of interferograms is presented. Least squares estimation using ground control points provides an accurate method for determining the precise interferometric baseline. The system parameters which affect the accuracy of the Digital Elevation Models (DEM's) are errors in the interferometric baseline, decorrelation caused by baseline separation, thermal noise, and surface change. An adaptive filter based on the local interferometric coherence was developed to improve phase unwrapping of the interferogram. Final accuracies of the DEM's generated for single scene pairs in Alaska (U.S.) and Bonn (Germany) are on the order of 5 to 10 m.

Werner, Charles L.↗
NASA NTRS1993

Measurement and Simulation of Signal Fluctuations Caused by Propagation through Trees

We present measured magnitude and phase fluctuations of UHF, L band, and C band signals that were transmitted from the ground through a forest canopy to an airborne radar. We find that the measured fluctuations are similar to those calculated by a simple Monte Carlo simulation. Both observed and calculated RMS fluctuations are typically several decibels in magnitude and tens of degrees in phase at all three frequencies.

Durden, Stephen L.↗
NASA NTRS1993

Comparison Of Two Methods Of Calibrating Polarimetric SAR

Report presents comparative analysis of two approaches to calibration of synthetic-aperture radar (SAR). Performed in context of NASA/JPL SAR system mounted in DC-8 airplane, also applicable to any polarimetric SAR system of similar design. Demonstrates functional equivalence of two approaches to calibration. In one, known as Klein's approach, SAR data calibrated supplied in scattering-matrix format. Other named after van Zyl, data calibrated supplied in Stokes-matrix (compressed) format.

Freeman, Anthony↗