Walter E. Brown

Models of radar imaging of the ocean surface waves

A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation:tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed.

Radar mapping, archeology, and ancient maya land use.

A severe incongruity has long existed between the well-known complexity of ancient Maya civilization and the relatively feeble economic base that could be reconstructed for it. Recent fieldwork has ihdicated that much more intensive cultivation patterns were used than was previously thought. Data from the use of synthetic aperture radar in aerial surveys of the southern Maya lowlands suggest that large areas were drained by ancient canals that may have been used for intensive cultivation. Ground checks in several limited areas have confirmed the existence of canals, and excavations and ground surveys have provided valuable comparative information. Taken together, the new data suggest that Late Class period Maya civilization was firmly grounded in large-scale and intensive cultivation of swampy zones.

Variations in surface roughness within Death Valley, California: Geologic evaluation of 25-cm-wavelength radar images

Images — processed from 25-cm wavelength, side-looking airborne radar — of the salt flats and gravel fans on the floor of Death Valley, California, show distinctive variations in radar backscatter (that is, variations in image gray tones) that can be correlated with systematic changes in the surface roughness of different geologic units. Well-developed desert pavements on the oldest boulder gravel units of late Pleistocene age are clearly delineated as weak backscatterers on the images. A gradation in the size of gravel constituents near the base of the giant gravel fans is associated with an abrupt change in the backscatter energy. The change takes place at gravel radii between 0.08λ and 0.14λ (2.0 and 3.5 cm). A breakpoint observed in the Rayleigh scattering region of the total radar cross section is virtually independent of the antenna depression angle as long as the resolution area does not lie in the first pulse width of the echo (the 90° depression angle). With the longer wavelength radar systems, antenna depression angles of 45° to 90° appear to be well suited for investigations of surface roughness because of the suppression of radar shadows and the increased radar return from the near range.

Commentary: Rebuttal to Pope and Dahlin

AbstractExamination of the synthetic aperture radar imagery and of the hydraulic characteristics of Maya lowland swamps has led Kevin O. Pope and Bruce H. Dahlin (“AncientMaya Wetland Agriculture: New Insights from Ecological and Remote Sensing Research,” Journal of Field Archaeology 16 [1989] 87–106) to conclude that there is no connection between lattice patterns in the imagery and raised fields in the swamps. They further conclude that there are no significant numbers of raised fields in periodic swamps. We examine the arguments and points raised and find them tenuous at best, and disingenuous at worst. We conclude that the SAR imagery does indeed indicate raised field and canal systems and that they exist in the large periodic swamps of the Maya lowlands.

Preliminary results from the NASA/JPL multifrequency, multipolarization synthetic aperture radar

A brief description is given of the three-frequency polarimetric synthetic-aperture radar built and tested at the Jet Propulsion Laboratory. The radar has the capability to simultaneously acquire fully polarimetric data at P-, L-, and C-bands from a DC-8 airborne platform. The radar has an instantaneous data rate of approximately 2.1 Gb/s and has selectable record rates between 80-240 Mb/s. The system has a wide dynamic range featuring 8-b analog-to-digital converters and full digital floating-point processing. The processing is accomplished offline on a minicomputer system assisted by an array processor. Sample images are presented.<<ETX>>

Measurement Problems of Space Vehicle Experiments

The need for conducting remote measurements by use of experimental sensors on space vehicles, in an environment where there is often a large uncertainty in the parameter to be measured, establishes a new area of endeavor for the researcher. The conditions peculiar to this problem are in many instances vastly different from those normally encountered in laboratory research, and are related to the over-all spacecraft design requirements. These new conditions attach a new significance to calibration philosophy, sampling rates, conversion accuracies, and reliability. Furthermore, the ground data handling problem of extracting information from the large quantities of data is extremely severe and usually requires careful planning by the experimenters several months prior to the actual measurement. The purpose of this paper is to survey the current and future aspects of acquiring accurate and intelligible information from the spacecraft sensors by the utilization of techniques which tend to be more compatible with the spacecraft design requirements.