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Wall, Stephen D.

Publications and source records attributed to Wall, Stephen D..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

The Cassini Radar Investigation

The Cassini/Huygens Mission is a nineteen-year multinational project to design, construct and execute an investigation of the Saturn system, with emphasis on its largest moon, Titan. Titan's atmosphere is nearly opaque at optical wavelengths, so a Ku-band radar imaging system was required to map its surface. In this paper we describe the radar instrument, discuss some of the challenges to its design, and review its operating modes. We briefly summarize the surprises that the radar instrument has revealed while investigating Titan.

Cassini↗
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

Model-Based Engineering Design for Trade Space Exploration throughout the Design Cycle

This paper presents ongoing work to standardize model-based system engineering as a complement to point design development in the conceptual design phase of deep space missions. It summarizes two first steps towards practical application of this capability within the framework of concurrent engineering design teams and their customers. The first step is standard generation of system sensitivities models as the output of concurrent engineering design sessions, representing the local trade space around a point design. A review of the chosen model development process, and the results of three case study examples, demonstrate that a simple update to the concurrent engineering design process can easily capture sensitivities to key requirements. It can serve as a valuable tool to analyze design drivers and uncover breakpoints in the design. The second step is development of rough-order- of-magnitude, broad-range-of-validity design models for rapid exploration of the trade space, before selection of a point design. At least one case study demonstrated the feasibility to generate such models in a concurrent engineering session. The experiment indicated that such a capability could yield valid system-level conclusions for a trade space composed of understood elements. Ongoing efforts are assessing the practicality of developing end-to-end system-level design models for use before even convening the first concurrent engineering session, starting with modeling an end-to-end Mars architecture.

models↗
NASA NTRS1999

Reinventing The Design Process: Teams and Models

The future of space mission designing will be dramatically different from the past. Formerly, performance-driven paradigms emphasized data return with cost and schedule being secondary issues. Now and in the future, costs are capped and schedules fixed-these two variables must be treated as independent in the design process. Accordingly, JPL has redesigned its design process. At the conceptual level, design times have been reduced by properly defining the required design depth, improving the linkages between tools, and managing team dynamics. In implementation-phase design, system requirements will be held in crosscutting models, linked to subsystem design tools through a central database that captures the design and supplies needed configuration management and control. Mission goals will then be captured in timelining software that drives the models, testing their capability to execute the goals. Metrics are used to measure and control both processes and to ensure that design parameters converge through the design process within schedule constraints. This methodology manages margins controlled by acceptable risk levels. Thus, teams can evolve risk tolerance (and cost) as they would any engineering parameter. This new approach allows more design freedom for a longer time, which tends to encourage revolutionary and unexpected improvements in design.

Wall, Stephen D.↗
NASA NTRS1995

User guide to the Magellan synthetic aperture radar images

The Magellan radar-mapping mission collected a large amount of science and engineering data. Now available to the general scientific community, this data set can be overwhelming to someone who is unfamiliar with the mission. This user guide outlines the mission operations and data set so that someone working with the data can understand the mapping and data-processing techniques used in the mission. Radar-mapping parameters as well as data acquisition issues are discussed. In addition, this user guide provides information on how the data set is organized and where specific elements of the set can be located.

Wall, Stephen D.↗
NASA NTRS1994

(abstract) Science-Project Interaction in the Low-Cost Mission

Large, complex, and highly optimized missions have performed most of the preliminary reconnaisance of the solar system. As a result we have now mapped significant fractions of its total surface (or surface-equivalent) area. Now, however, scientific exploration of the solar system is undergoing a major change in scale, and existing missions find it necessary to limit costs while fulfilling existing goals. In the future, NASA's Discovery program will continue the reconnaisance, exploration, and diagnostic phases of planetary research using lower cost missions, which will include lower cost mission operations systems (MOS). Historically, one of the more expensive functions of MOS has been its interaction with the science community. Traditional MOS elements that this interaction have embraced include mission planning, science (and engineering) event conflict resolution, sequence optimization and integration, data production (e.g., assembly, enhancement, quality assurance, documentation, archive), and other science support services. In the past, the payoff from these efforts has been that use of mission resources has been highly optimized, constraining resources have been generally completely consumed, and data products have been accurate and well documented. But because these functions are expensive we are now challenged to reduce their cost while preserving the benefits. In this paper, we will consider ways of revising the traditional MOS approach that might save project resources while retaining a high degree of service to the Projects' customers. Pre-launch, science interaction can be made simplier by limiting numbers of instruments and by providing greater redundancy in mission plans. Post launch, possibilities include prioritizing data collection into a few categories, easing requirements on real-time of quick-look data delivery, and closer integration of scientists into the mission operation.

mission operation science community interaction co↗
NASA NTRS1994

Wind Streaks on Venus: Clues to Atmospheric Circulation

Magellan images reveal surface features on Venus attributed to wind processes. Sand dunes, wind-sculpted hills, and more than 5830 wind streaks have been identified. The streaks serve as local "wind vanes," representing wind direction at the time of streak formation and allowing the first global mapping of near-surface wind patterns on Venus. Wind streaks are oriented both toward the equator and toward the west. When streaks associated with local transient events, such as impact cratering, are deleted, the westward component is mostly lost but the equatorward component remains. This pattern is consistent with a Hadley circulation of the lower atmosphere.

Greeley, Ronald↗
NASA NTRS1993

An operations concept methodology to achieve low-cost mission operations

Historically, the Mission Operations System (MOS) for a space mission has been designed last because it is needed last. This has usually meant that the ground system must adjust to the flight vehicle design, sometimes at a significant cost. As newer missions have increasingly longer flight operations lifetimes, the MOS becomes proportionally more difficult and more resource-consuming. We can no longer afford to design the MOS last. The MOS concept may well drive the spacecraft, instrument, and mission designs, as well as the ground system. A method to help avoid these difficulties, responding to the changing nature of mission operations is presented. Proper development and use of an Operations Concept document results in a combined flight and ground system design yielding enhanced operability and producing increased flexibility for less cost.

Ledbetter, Kenneth W.↗
NASA NTRS1992

Aeolian features on Venus - Preliminary Magellan results

About 44 percent of the surface of Venus has been searched in a reconnaissance mode for wind-related features using primarily cycle 1 Magellan radar data. The aeolian features, including possible dune fields, yardangs, and various types of wind streaks, are described, and the geological settings and properties of the surfaces in which they occur are assessed. Attention is also given to possible modes of formation of the most common aeolian features (wind streaks), with reference made to terrestrial examples. The relationships between aeolian features and atmospheric circulation patterns on Venus are considered.

Greeley, Ronald↗
NASA NTRS1992

Images and topographic relief at the north pole of Venus

The analysis of unique coverage of the north pole of Venus using a subset of 13 images from a total of about 775 synthetic aperture radar orbits during cycle 1 of the Magellan mission is reported. Images at 13 distinctly different azimuths are reported that range over 360 deg in longitude and that show the topographic relief at vastly different imaging geometries at intervals of about 20 deg. A pair of intersecting or crossing orbits is used to assess the topographic relief with stereo radargrammetric techniques, and to refine this assessment using a multiimage clinometric approach. It is shown that the pole is located in accentuated topography with elevation differences in the range of no more than 1 km. It is concluded that pinpointing the location of the pole from the overlapping images and standard mission data is accurate to within about 100 pixels or 8 km (RMS).

Leberl, Franz W.↗
NASA NTRS1991

Design of mission operations systems for scientific remote sensing

The present work describes the mission operations system (MOS) design process for remote-sensing missions. A MOS is defined as the system required to perform, monitor, and control an operation, encompassing personnel, hardware, software and/or documentation. Attention is given to telecommunications and remote-sensing instrumentation, MOS definition program phases and reviews, and MOS organization, management, and staffing. Also treated are the uplink and downlink processes, anomalies and contingency plans, the illustrative case of the MOS for the Magellan radar sensing mission, and a projection of future MOSs incorporating AI.

Wall, Stephen D.↗
NASA NTRS1990

Large-scale patterns of eolian sediment transport on Venus - Predictions for Magellan

The atmosphere of Venus is likely to exert a major influence on the surface. Arecibo and Venera 15/16 observations of the population of impact craters with bright halos give direct evidence of surface modification, on the scale of centimeters, to smooth the surface on time scales of 50 - 250 my. Both chemical and mechanical modifications probably occur. Diurnal winds in the planetary boundary layer can transport particles. Using boundary layer theory, including the effects of topography, surface stresses and resulting transport and deposition of sand-size particles are calculated. Regional slopes are sites of largest surface stresses. Sand will be generally transported downhill, although there is a preferential net transport from east to west, in the same direction as the atmospheric superrotation. It is predicted here that streaks may be seen in the Magellan radar images which will indicate directions of net eolian transport on the surface of Venus.

Saunders, R. S.↗
NASA NTRS1988

Radiometric calibration analysis of SIR-B imagery

The second flight of the NASA Spaceborne Imaging Radar (SIR-B) collected nearly 8 hours of digital synthetic-aperture radar (SAR) data of the earth's surface. This paper analyzes the performance of the SAR to determine the accuracy of the relative radiometric calibration of these data. Procedures are presented for deriving accuracy estimates as well as limitations under which these estimates are valid. The impact on calibration of an RF breakdown in the primary antenna feed system cable is evaluated. An analysis of the platform stability based on the SAR echo data is also presented in conjunction with its resultant effect on the calibration accuracy. Finally, numerical error bounds are derived with guidelines on their utilization.

Wall, Stephen D.↗