Engineering topics

Zebker, Howard A.

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

60 records · Page 4

NASA NTRS1987

Imaging Radar Polarimeter

Radar measures full polarization tensor of each element in scene in one sweep. New system comprises dual-polarized antenna, single transmitter, and four-channel receiver and digital recorder installed in aircraft, plus digital processor on ground. Produces radar-backscatter images corresponding to 10- by 10-km regions on ground. Signals recorded from orthogonal linearly polarized antennas combined in computer after flight to synthesize any desired combination of transmitted and received polarizations. Data recorded on single flight processed to provide multiple images.

Zebker, Howard A.↗

NASA NTRS1987

Imaging radar polarization signatures - Theory and observation

Radar polarimetry theory is reviewed, and comparison between theory and experimental results obtained with an imaging radar polarimeter employing two orthogonally polarized antennas is made. Knowledge of the scattering matrix permits calculation of the scattering cross section of a scatterer for any transmit and receive polarization combination, and a new way of displaying the resulting scattering cross section as a function of polarization is introduced. Examples of polarization signatures are presented for several theoretical models of surface scattering, and these signatures are compared with experimentally measured polarization signatures. The coefficient of variation, derived from the polarization signature, may provide information regarding the amount of variation in scattering properties for a given area.

Van Zyl, Jakob J.↗

NASA NTRS1987

Subcentimeter-size particle distribution functions in planetary rings from Voyager radio and photopolarimeter occultation data

Analysis of measurements of the scattered and direct components of Voyager 1 radio occultation signals at 3.5 and 13 cm wavelengths yield estimates of the distribution functions of supracentimeter-size particles and thickness of relatively broad regions in Saturn's rings. If mearurements of signal amplitude at a shorter wavelength are combined with the previously analyzed data, the shape of the distribution functions characterizing the smaller particles can be constrained. If size distributions of arbitrary form were considered, many solutions are found that are consistent with the three available observations of signal amplitude. The best-fit power law was calculated to the three observations at three wavelengths for several of the embedded Saturn ringlets. Mie scattering theory predicts that the measured phase of the radio occultation signal is highly sensitive to particles ranging from 0.1 to 1.0 wavelengths in size, thus additional constraints on the subcentimeter-size distribution functions for both the Saturn and Uranus rings can in principle be derived from radio phase measurements.

Zebker, Howard A.↗

NASA NTRS1987

Imaging radar polarimetry from wave synthesis

A new approach is reported to the measurement of the complete polarization signature of each resolution element in an image implemented with an airborne synthetic aperture radar system. Signals recorded on one data pass from orthogonal linearly polarized antennas are utilized. The signals are combined in a data processor to synthesize any desired combination of transmit and receive polarizations. The technique permits measurement of the complex, multichannel reflectivity of a scene on a single aircraft pass and to late reprocess the data to provide multiple image maps, with each representing the backscattered energy from the scene measured with a different combination of observational transmit and recieve polarizations. The resulting polarization signature measurements indicate optimum polarizations for observations of certain classes of objects and give insight into the identification of dominant scattering mechanisms for each kind of object. The mathematical model for polarization synthesis is summarized, and some theoretical polarization measurements are illustrated for several types of targets. The overall radar system implementation is described in detail. Some analyses of data acquired on three aircraft flights are presented. The technique has been applied to mapping and differentiation of lava flows and to differentiation of forested and clear-cut areas.

Zebker, Howard A.↗

NASA NTRS1987

Radar polarization signatures of vegetated areas

A simple model is presented for the prediction of the full polarization signature of vegetation resembling tall grass. This polarization signature can be used to detect the presence of vegetation even in those cases in which the vegetation layers are comparatively thin. Also presented is a model which predicts the polarization dependence of different tree types. Attention is given to the cases of pine and deciduous forest model predictions; both types of forest can be expected to contain terms representing the scatter from the ground, as well as forward, double reflections from the ground and limbs/trunk.

Van Zyl, Jakob J.↗

NASA NTRS1986

Imaging radar polarimetry from wave synthesis

It was shown that it is possible to measure the complete scattering matrix of an object using data acquired on a single aircraft pass, and can combine the signals later in the data processor to generate radar images corresponding to any desired combination of transmit and receive polarization. Various scattering models predict different dependence on polarization state of received power from an object. The imaging polarimeter permits determination of this dependence, which is called the polarization signature, of each point in a radar image. Comparison of the theoretical predictions and observational data yield identification of possible scattering mechanisms for each area of interest. It was found that backscatter from the ocean is highly polarized and well-modeled by Bragg scattering, while scattering from trees in a city park possesses a considerable unpolarized component. Urban regions exhibit the characteristics expected from dihedral corner reflectors and their polarization signatures are quite different from the one-bounce Bragg model.

Zebker, Howard A.↗