Engineering topics
Elachi, C.
Publications and source records attributed to Elachi, C..
Keynote Address DESCANSO International Symposium
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Space Technology and the Discovery of the Lost City of Ubar
The location of the legendary city of Ubar, a desert caravansary which supported the ancient and lucrative frankincense trade, has likely been found at the edge of the Arabian Peninsula's Empty Quarter in modern day Oman. The archaeological site was located by an unusual combination of historical research, and application of space technology, in support of traditional archaeology.
Cassini Radar Investigation of Titan
Saturn's largest moon, Titan, boasts organic chemistry that may hold clues to how life formed on the primitive Earth. The Cassini Mission, to be launched in October 1997, will explore Saturn and its moons, especially Titan. Mapping the surface of Titan will be accomplished with a radar instrument that acts as a 14 GHz synthetic aperture radar, microwave radiometer, altimeter, and scatterometer.
(abstract) Radiophysical Properties of Venusian Impact Craters
An analysis of 222 large (greater than 20-km-diameter) impact craters on Venus using both cycle 1 and cycle 2 Magellan data is being conducted to determine the radiophysical properties (i.e., backscatter cross section, emissivity, reflectivity, rms slope) of the craters and to search for correlations with target region properties and subsequent geological history.
Advances in spaceborne radar remote sensing
Radar sensors are being more commonly used for spaceborne sensing of Earth and Planetary surfaces and atmosphere. This is a result of advances in: better understanding wave/natural surface interaction mechanisms, measurement techniques, hardware development and data handling/visualization. A number of spaceborne radar systems are presently operational or will be launched in the next few years. In addition airborne instruments utilizing multispectral, polarmetric and interferometric capabilities are opening new vistas which will lead to major advances in radar sensing by the end of the decade. This paper is a review of recent developments and expected future advances.
Low-emissivity impact craters on Venus
An analysis of 144 impact craters on Venus has shown that 11 of these have floors with average emissivities lower than 0.8. The remaining craters have emissivities between 0.8 and 0.9, independent of the specific backscatter cross section of the crater floors. These 144 impact craters were chosen from a possible 164 craters with diameters greater than 30 km as identified by researchers for 89 percent of the surface of Venus. We have only looked at craters below 6053.5 km altitude because a mineralogical change causes high reflectivity/low emissivity above the altitude. We have also excluded all craters with diameters smaller than 30 km because the emissivity footprint at periapsis is 16 x 24 km and becomes larger at the poles.
Global digital topography mapping with a synthetic aperture scanning radar altimeter
Global digital topography data of the land surface is of importance in a variety of geoscientific and application disciplines. Such a database, with a spatial resolution of 150 to 500 m and height accuracy of 5 m or better can be acquired from an orbiting platform using a synthetic aperture scanning radar altimeter. Near-global coverage can be achieved within 14 days from an orbiting platform in a polar or near-polar orbit.
Construction and analysis of simulated Venera and Magellan images of Venus
The 1-3 km resolution Venera 15 and 16 images of Venus and the expected 120-300 m resolution Magellan mission image data are presently simulated through a digital processing of Seasat radar images covering a desert dune complex in the Gran Desierto of Sonora, accreted terranes in the central interior of Alaska, and the Appalachian Valley and Ridge Province. The simulations suggest that the nature and extent of terrain modification on Venus by such exogenic processes as atmosphere-surface weathering, erosion, and deposition, will remain uncertain, since the length scale of features indicative of such processes may be too small to be discerned from Venera data; Magellan data may provide this critical fine-scale morphological data, however, and thereby allow the testing of the two competing resurfacing scenarios.
Overview of the Shuttle Imaging Radar (SIR-C)
The Shuttle Imaging Radar-C (SIR-C) experiment will provide increased capability over Seasat and the two previous Shuttle Imaging Radars by acquiring digital images simultaneously at two microwave frequencies (L- and C-band) with multiple signal polarizations (HH, VV, HV, VH). The SIR-C is a dual bandwidth system so that investigators can choose a high or low resolution mode (20 and 10 MHz, respectively). An X-band SAR with VV polarization will be flown with SIR-C, resulting in a three-frequency capability. The SIR-C prototype data will be acquired using an airborne imaging radar to test designs that will be implemented on SIR-C, to develop the overall calibration strategy for SIR-C, and to develop data analysis tools for SIR-C data.
On the design issues for a spaceborne rain mapping radar
The design concept for the spaceborne rain mapping radar for the Tropical Rain Measuring Mission (TRMM) of Thiele (1987) is examined. The TRMM requirements for sampling strategy, spatial and height resolutions, observable altitude, and rain rate measurement range are described. The adaptive scanning and nadir Doppler measurement modes for the radar are discussed.
Global digital topography mapping using a scanning radar altimeter
The conceptual design of a Scanning Radar Altimeter system capable of collecting less than 300-m spatial and less than 3-m height resolution digital topography data for the entire globe, from an orbital platform, is presented. A 37-GHz frequency SRA system is used to achieve the requisite resolution while reducing antenna length in the along-track dimension. Near-global coverage in a short time period is obtained by scanning the antenna beam cross-track, in a swath of about 100 km. Attention is given to the algorithm that will be used to retrieve pixel height from the return waveform.
SIR-B - The second Shuttle Imaging Radar experiment
On October 5, 1984, the second Shuttle Imaging Radar (SIR-B) was launched into orbit aboard the Space Shuttle Challenger. SIR-B is part of an evolutionary radar program designed to progressively develop a multifrequency, multipolarization synthetic aperture radar with a variable earth-imaging geometry. The SIR-B instrument is an upgraded version of SIR-A, with the additional capability of tilting the antenna mechanically to acquire imagery at variable incidence angles ranging from 15 to 60 deg. The variable look angle capability provided a means of acquiring multiple incidence angle imagery over specific targets on successive days of the mission. These data are being used to classify surface features by their backscatter signatures as a function of incidence angle and for topographic mapping. In addition to the antenna tilt capability, a digital data-handling system was added to increase the dynamic range, the resolution was improved by a factor of two over SIR-A, and a calibration subsystem was added to improve the radiometric accuracy of the data. The mission had a number of problems, including loss of the primary digital data path between the Shuttle and the ground. In spite of these problems, approximately 20 percent of the planned digital data were collected over the 8-day Shuttle mission corresponding to an areal coverage of about 6.4 million sq km.
Microwave penetration and attenuation in desert soil - A field experiment with the Shuttle Imaging Radar
Receivers buried in the Nevada desert were used with the Shuttle Imaging Radar to measure microwave attenuation as a function of soil moisture in situ. Results agree closely with laboratory measurements of attenuation and suggest that penetration of tens of centimeters in desert soils is common for L-band (1.2-GHz) radar.
Overview of the Shuttle Imaging Radar-B preliminary scientific results
Data collected with the Shuttle Imaging Radar-B (SIR-B) on the October 5, 1985 Shuttle mission are discussed. The design and capabilities of the sensor which operates in a fixed illumination geometry and has incidence angles between 15 and 60 deg with 1 deg increments are described. Problems encountered with the SIR-B during the mission are examined. the The radar stereo imaging capability of the sensor was verified and three-dimensional images of the earth surface were obtained. The oceanography experiments provided significant data on ocean wave and internal wave patterns, oil spills, and ice zones. The geological images revealed that the sensor can evaluate penetration effect in dry soil from buried receivers and the existence of subsurface dry channels in the Egyptian desert was validated. The use of multiincidence angle imaging to classify terrain units and derive vegetation maps and the development of terrain maps are confirmed.
Modelling of vegetation volumes
The purpose is to describe work that is being done to find theoretical models to describe radar backscatter from vegetation layers. The geometry of the problem is shown. The information that one would like to find through the application of the results of these models would include: the thickness of the layer; the absorption in the layer (i.e., density, moisture content, and biomass); the geometry of the scatterers (i.e., shape and orientation); how much of the received power is due to volume scattering only; and a way to enhance the ratio of scattering that has some interaction with the ground surface. The proposed ways to find this information are discussed.