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Martin, C. F.

Publications and source records attributed to Martin, C. F..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Five Year Changes in Surface Elevations Of The Greenland Ice Sheet Measured by Aircraft Laser Altimetry

The Arctic Ice Mapping group (Project AIM) at NASA's Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet since 1993, using scanning airborne laser altimeters combined with GPS positioning technology. Flight lines were planned to cover all major ice drainage basins, with the intention to repeat the surveys after a 5-year interval in order to detect changes in the ice-sheet volume. The first resurvey was completed in June/July, 1998, along flight lines in the southern half of Greenland which had been first surveyed in 1993. The northern half of the ice sheet will be resurveyed in 1999. The resulting data sets will provide the first comprehensive examination of regional changes in the the surface elevation of the World's second largest ice sheet. This analysis includes the ice sheet fringe areas, which are expected to be much more climatically sensitive than the interior. Data will be presented which demonstrate the combination of the ATM (Airborne Topographic Mapper) sensors and GPS (Global Positioning System) yields measurements which are repeatable at the 10 cm level over baselines in excess of 1000 km.

Krabill, W. B.↗
NASA NTRS1999

Airborne Laser/GPS Mapping of Beaches

Results are presented from topographic surveys of the Assateague National Seashore Park using recently developed airborne laser and Global Positioning System (GPS) technology. During November, 1995, and again in May, 1996, the NASA Arctic Ice Mapping (AIM) group from the NASA Goddard Space Flight Center's Wallops Flight Facility conducted surveys as a part of technology enhancement activities or warm-up missions prior to conducting elevation measurements of the Greenland Ice Sheet as part of NASA's Global Climate Change program. The resulting data are compared to surface surveys using standard techniques. The goal of these projects is to make these measurements to an accuracy of 10 cm. The measurements were made from NASA's 4-engine P-3 Orion aircraft using the Airborne Topographic Mapper (ATM), a scanning laser system. The necessary high accuracy vertical as well as horizontal positioning are provided by Global Positioning System (GPS) receivers located both on board the aircraft and at a fixed site at Wallops Island.

Krabill, W. B.↗
NASA NTRS1997

Airborne Laser/GPS Mapping of Assateague National Seashore Beach

Results are presented from topographic surveys of the Assateague Island National Seashore using recently developed Airborne Topographic Mapper (ATM) and kinematic Global Positioning System (GPS) technology. In November, 1995, and again in May, 1996, the NASA Arctic Ice Mapping (AIM) group from the Goddard Space Flight Center's Wallops Flight Facility conducted the topographic surveys as a part of technology enhancement activities prior to conducting missions to measure the elevation of extensive sections of the Greenland Ice Sheet as part of NASA's Global Climate Change program. Differences between overlapping portions of both surveys are compared for quality control. An independent assessment of the accuracy of the ATM survey is provided by comparison to surface surveys which were conducted using standard techniques. The goal of these projects is to mdke these measurements to an accuracy of +/- 10 cm. Differences between the fall 1995 and 1996 surveys provides an assessment of net changes in the beach morphology over an annual cycle.

Kradill, W. B.↗
NASA NTRS1992

Decentralized-feedback pole placement of linear systems

A projectile product spaces model is used to analyze decentralized systems. The degree of the pole placement map is computed. The conditions under which the degree is odd are also given. Twin lift systems are studied. It is proved that the poles of a twin lift system can be assigned to any values by local static and local dynamic feedback laws if and only if the system is jointly controllable.

Wang, X.↗
NASA NTRS1990

Satellite altimeter calibration techniques

This paper examines calibration techniques which can most effectively satisfy the requirements of future satellites carrying high-accuracy radar altimeters, such as the ESA ERS-1 and the NASA/CNES Topex/Poseidon satellites scheduled for launch during the next five years. The calibration accuracies and the advantages and disadvantages of the four currently proposed calibration techniques for over-water calibration are discussed: (1) a tide gauge on a tower at-sea and a nearby laser, (2) a laser and a tide gauge on an island with an offshore satellite pass and a geoid tie between the satellite ground track and the laser, (3) a tide gauge on a tower at-sea with satellite positioning from multiple lasers and a GPS, and (4) a laser and a tide gauge on a tower at-sea. Error budgets for these techniques, developed on the basis of state-of-the-art tracking systems, were found to have one sigma height uncertainties in the 2.8 to 4.9 cm range.

Kolenkiewicz, R.↗
NASA NTRS1990

A global nonlinear tracking problem

The tracking problem for a globally minimum phase nonlinear system is considered. It is assumed that the signal to be tracked is slowly varying and a priori bounds on its magnitude are known. It is shown that if the system has bounded derivatives and exponentially stable zero dynamics then the system admits an output feedback controller which solves the tracking problem.

Dayawansa, W. P.↗
NASA NTRS1985

Determination of highly accurate orbits for altimeter satellites over limited geographic areas

Accurate orbits are necessary for altimeter satellites for operational computation of sea surface heights and for the determination of any bias in the altimeter measurement. These satellites require high accuracy throughout the orbit, in general a difficult task due to gravity model, solar radiation pressure, and atmospheric drag effects. Extensive analysis has reduced such errors for Seasat only to the 50 cm level. For calibration purposes the use of such an orbit would require a very large number of tracks to reduce the error to the sub-10 cm level even if all passes had only random errors. Since altimeter calibration orbits require high accuracy only over a limited geographical area, the use of a ranging station, such as a laser tracker in the calibration area can provide the required accuracy. The Bermuda laser provided such orbits for Seasat, with satellite-station height accuracy estimated to be at the sub-3 cm level. Because the station height is accurately known, such orbits can also be used to test global orbit accuracies.

Kolenkiewicz, R.↗
NASA NTRS1984

Seasat observations of lithospheric flexure seaward of trenches

Lithospheric flexure seaward of deep ocean trenches is evident in Seasat altimeter observations of the marine geoid. In fact, mechanical models of lithospheric flexure can be tested directly on the Seasat altimeter data. A simple elastic model has been used for the oceanic lithosphere and, after least squares adjustments, estimates have been recovered of model parameters including outer rise (OR) amplitude, OR wavelength, and effective lithospheric thickness. Effective lithospheric thicknesses have been recovered for six regions: the Mariana, the Kuril, the Philippine, the Aleutian, the Izu-Bonin, and the Middle America OR's. These results support the proposition that effective thickness Te increases with age of lithosphere in approximate accord with the relation Te approximately C x square root of age where C approximately 4 km/square root of (m.y.). In fact, altimetric results agree more closely with this relation than do published results based on bathymetric data. The close agreement with the thickness-age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the Kuril, Marianas, and Izu-Bonin trenches. This thickness-age relation implies that flexural strength of the oceanic lithosphere is temperature controlled.

Mcadoo, D. C.↗
NASA NTRS1983

Riccati group invariants of linear hamiltonian systems

The action of the Riccati group on the Riccati differential equation is associated with the action of a subgroup of the symplectic group on a set of hamiltonian matrices. Within this framework various sets of canonical forms are developed for the matrix coefficients of the Riccati differential equation. The canonical forms presented are valid for arbitrary Kronecker indices, and it is shown that the Kronecker indices are invariants for this group action. These canonical forms are useful for studying problems arising in the areas of optimal decentralized control and the spectral theory of optimal control problems.

Garzia, M. R.↗
NASA NTRS1983

On families of systems and deformations

Until recently, there has not been any systematic effort towards a theory of systems with parameter variations. It is presently suggested that the concept of families of systems is basic to such an effort, and entails the techniques of Lie theory, differential geometry, and algebraic geometry. Attention is given to the geometric characterization of certain families of systems that appear in control and identification problems. The ways in which families of systems degenerate as parameter variations become large are isolated, using the topological rather than algebraic geometric case.

Krishnaprasad, P. S.↗
NASA NTRS1983

SEASAT observations of lithospheric flexure seaward of trenches

Lithospheric flexure seaward of deep ocean trenches in SEASAT altimeter observations of the marine geoid. In fact, mechanical models of lithospheric flexure can be tested directly on the SEASAT altimeter data. A simple elastic model was used for the oceanic lithosphere and, after least squares adjustments, estimates of model parameters were recovered including Outer Rise (OR) amplitude, OR wavelength, and effective lithospheric thickness. Effective lithospheric thickness was recovered for five regions: the Mariana, the Kuril, the Philippine, the Aleutian and the Middle America OR. These results support the suggestion of Bodine et al. (1981) that effective thickness, T, increased with age of lithosphere in approximate accord with the relation T approximately equals x age to the 1/2 power where C approximately equals 4 km x my to the -1/2 power. Altimetric results agree more closely with this relation than do published results based on bathymetric data. The close agreement with the thickness-age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the Kuril, Marianas and Izo-Bonin trenches. This thickness-age relation implies that flexural strength of the oceanic lithosphere is temperature controlled.

Mcadoo, D. C.↗
NASA NTRS1983

Linear quadratic optimal control for symmetric systems

Special symmetries are present in many control problems. This paper addresses the problem of determining linear-quadratic optimal control problems whose solutions preserve the symmetry of the initial linear control system.

Lewis, J. H.↗
NASA NTRS1983

Orbit structure of Hamiltonian systems arising from Lie transformation group actions

This paper associates the Riccati group and its group action on linear-quadratic optimal control problems to the action of a Lie transformation group on a set of Hamiltonian matrices. In this Lie theoretic setting results are presented concerning the associated orbit structure and the structure of the group itself. These results are of importance in understanding the solution structure of matrix Riccati differential equations, and thus also of importance in linear-quadratic optimal control.

Garzia, M. R.↗
NASA NTRS1982

Baseline monitoring using aircraft laser ranging

The use of aircraft laser ranging for the determination of baselines between ground based retroreflectors was investigated via simulations and with tests at Wallops Flight Center using the Airborne Oceanographic Lidar (AOL) on the Wallops C-54 aircraft ranging to a reflector array deployed around one of the Wallops runways. The aircraft altitude and reflector spacing were chosen on the basis of scaled down modeling of spacecraft tracking from 1000 km of reflectors separated by some 52 km, or of high altitude (10 km) aircraft tracking of reflectors separated by some 500 m. Aircraft altitudes flown for different passes across the runway reflector array varied from 800 m to 1350 m, with 32 reflectors deployed over an approximtely 300 m x 500 m ground pattern. The AOL transmitted 400 pulses/sec with a scan rate of 5/sec in a near circular pattern, so that the majority of the pulses were reflected by the runway surface or its environs rather than by retroreflectors. The return pulse characteristics clearly showed the high reflectivity of portions of the runway, with several returns indistinguishable in amplitude from reflector returns. For each pass across the reflector field, typically six to ten reflector hits were identified, consistent with that predicted by simulations and the observed transmitted elliptical pulse size.

Krabill, W. B.↗
NASA NTRS1982

Global differential geometry: An introduction for control engineers

The basic concepts and terminology of modern global differential geometry are discussed as an introduction to the Lie theory of differential equations and to the role of Grassmannians in control systems analysis. To reach these topics, the fundamental notions of manifolds, tangent spaces, vector fields, and Lie algebras are discussed and exemplified. An appendix reviews such concepts needed for vector calculus as open and closed sets, compactness, continuity, and derivative. Although the content is mathematical, this is not a mathematical treatise but rather a text for engineers to understand geometric and nonlinear control.

Doolin, B. F.↗
NASA NTRS1982

Relativistic perturbations on the motion and tracking of the LAGEOS satellite

The modifications to Geodyn to substitute the Einstein gravitational theory for Newtonian gravitation are described. This results in modifications to both the satellite equations of motion and to the modeling of satellite tracking measurements. Since the Newtonian theory is a very close approximation to the Einstein theory (or general theory of relativity), observable deviations from Newtonian theory are small, and the applicable equations of motion or observation equations can be formulated so that they differ only by small terms which are denoted as relativistic perturbations. In the implementation of the Einstein theory in an orbital data reduction program, station coordinates and satellite coordinates which resemble the normal Cartesian coordinates are considered.

Martin, C. F.↗