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At least 523 records · Page 29

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NASA NTRS1974

A practical obstacle detection system for the Mars Rover

Discussion of an obstacle detection system which uses a laser range finder as the principal sensing device. The laser scans the scene ahead and stores the range data for the terrain in a matrix form. This matrix can be thought of as a 'range image'. This range varies from point to point, but sharp changes are caused by the presence of obstacles. Some existing algorithms and one developed by the authors were used to obtain the outlines of obstacles from the 'range image'. All these algorithms end with a thresholding operation. A theoretical analysis shows how a proper value of the threshold may be chosen, given the type of irregularity of the terrain to be detected. The analysis also shows how large the statistics of measurement noise can be allowed without leading to false alarms.

Reed, M.↗

NASA NTRS2002

Lunar geophysics, geodesy, and dynamics

Experience with the dynamics and data analyses for earth and moon reveals both similarities and differences. Analysis of Lunar Laser Ranging (LLR) data provides information on the lunar orbit, rotation, solid-body tides, and retroreflector locations.

lunar laser ranging lunar science↗

NASA NTRS2015

Proposed SLR Optical Bench Required to Track Debris Using 1550 nm Lasers

A previous study has indicated that by using approx.1550 nm wavelengths a laser ranging system can track debris objects in an "eye safe" manner, while increasing the expected return rate by a factor of approx. 2/unit area of the telescope. In this presentation we develop the optical bench required to use approx.1550nm lasers, and integration with a 532nm system. We will use the optical bench configuration for NGSLR as the baseline, and indicate a possible injection point for the 1550 nm laser. The presentation will include what elements may need to be changed for transmitting the required power on the approx.1550nm wavelength, supporting the alignment of the laser to the telescope, and possible concerns for the telescope optics.

Satellite Laser Ranging↗

NASA NTRS2022

Simulation of a Future SLR Satellite to Improve Low-Degree Gravity Estimates

The Gravity Recovery and Climate Experiment (GRACE; 2002-2017) and GRACE Follow-On (2018-present) have observed Earth's monthly mass change with unprecedented spatial resolution. These missions have long relied on satellite laser ranging (SLR) measurements to replace the C 2,0 coefficient, which GRACE recovers poorly. Recent work has also shown the need for SLR-determined C 3,0 when GRACE operates with a single accelerometer. However, it was not until the 2012 launch of the Laser Relativity Satellite that the SLR data gained the sensitivity to recover C 3,0 accurately. These low-degree gravity coefficients represent large-scale mass transport and small changes in their values have implications for ice sheet, ocean mass, and water storage estimates. To fully exploit SLR's utility for time-variable gravity (TVG), future satellite orbits should be selected to maximize their sensitivity to the gravity field. In this work, we present results from a simulation study of a hypothetical SLR satellite in which we generate 1 year of data to satellites placed across varying inclinations. We also simulate seven current SLR satellites to show realistic improvements from the new satellite. When compared to the known truth input, a low-inclination satellite ( <~ 45°) most improves the low-degree gravity terms, especially the even zonals which show a significant decorrelation. From this, we investigate recovery of the annual variability in the simulated signal and find recovery of the sine component improves by up to 41%. This has important implications when considering future SLR satellites in the context of TVG.

Satellite laser ranging↗

NASA NTRS1978

Determining crustal strain rates with spaceborne geodynamics ranging system data. 1: Baseline analysis

A satellite-borne laser ranging system is proposed that is capable of making highly precise geodetic measurements over baselines ranging from a few tens of kilometers to several hundred kilometers. The precision with which crustal strain rates are derived from measurements made with this system is analyzed by using simple site configurations, intersite distances of about 25-70 kilometers, and measurement programs ranging from a few years to fifteen years. It is concluded that precisions of several parts in 10 to the 9th power per year are achievable. Compared to the expected shear strain rates of about 7 x 10 to the minus 7th power yr/1, this produces very favorable signal-to-noise ratios.

Cohen, S. C.↗

NASA NTRS1990

A laser-sheet flow visualization technique for the large wind tunnels of the National Full-Scale Aerodynamics Complex

A flow visualization technique for the large wind tunnels of the National Full Scale Aerodynamics Complex (NFAC) is described. The technique uses a laser sheet generated by the NFAC Long Range Laser Velocimeter (LRLV) to illuminate a smoke-like tracer in the flow. The LRLV optical system is modified slightly, and a scanned mirror is added to generate the sheet. These modifications are described, in addition to the results of an initial performance test conducted in the 80- by 120-Foot Wind Tunnel. During this test, flow visualization was performed in the wake region behind a truck as part of a vehicle drag reduction study. The problems encountered during the test are discussed, in addition to the recommended improvements needed to enhance the performance of the technique for future applications.

Reinath, M. S.↗

NASA NTRS1997

Using Laser Altimetry to Detect Topographic Change in Long Valley Caldera, California

Long Valley caldera California, is a site of extensive volcanism, persistent seismicity, and uplift of a resurgent dome, currently at a rate of about 3 cm/year. Airborne laser altimetry was used to determine the surface topography of the region in 1993. A repeat mission occurred in 1995. Three different laser altimeters were flown, dubbed ATLAS, SLICER and RASCAL. Data processing consists of the combination of the aircraft trajectory and attitude data with the laser range, the determination of an atmospheric delay, laser pulse timing errors, laser system biases, and data geolocation to obtain the position of the laser spot on the ground. Results showed that using the ATLAS and SLICER instruments, the elevation of an overflown lake is determined to precisions of 3.3 cm and 2.9 cm from altitudes of 500 m and 3 km above the ground, and about 10 cm using the RASCAL instrument from 500 m above ground. Comparison with tide gauge data showed the laser measurements are able to resolve centimeter-level changes in the lake elevation over time. Repeat pass analysis of tracks over flat surfaces indicate no systematic biases affect the measurement procedure of the ATLAS and SLICER instruments. Comparison of GPS and laser-derived elevations of easily-identifiable features in the caldera confirm the horizontal accuracy of the measurement is within the diameter of the laser footprint, and vertical accuracy is within the error inherent in the measurement. Crossover analysis shows that the standard error of the means at track intersection points within the caldera, and dome (i.e., where zero and close to the maximum amount of uplift is expected) are about I cm, indicating elevation change at the 3 cm/year level should be detectable. We demonstrate one of the powerful advantages of scanning laser altimetry over other remote sensing techniques; the straightforward creation of precise digital elevation maps of overflown terrain. Initial comparison of the 1993-1995 data indicates uplift occurred, but filtering is required to remove vegetation effects. Although research continues to utilize the full potential of laser altimetry data, the results constitute a successful demonstration that the technique may be used to perform geodetic monitoring of surface topographic change.

Hofton, M. A.↗

NASA NTRS1997

Using Laser Altimetry to Detect Topographic Change at Long Valley Caldera, California

Long Valley caldera, California, is a site of extensive volcanism, persistent seismicity, and uplift of a resurgent dome, currently at a rate of approximately 3 cm/year. Airborne laser altimetry was used to determine the surface topography of the region in 1993. A repeat mission occurred in 1995. Three different laser altimeters were flown, dubbed ATLAS, SLICER and RASCAL. Data processing consists of the combination of the aircraft trajectory and attitude data with the laser range, the determination of an atmospheric delay, laser pulse timing errors, laser system biases, and data geolocation to obtain the position of the laser spot on the ground. Results showed that using the ATLAS and SLICER instruments, the elevation of an overflown lake is determined to precisions of 3.3 cm and 2.9 cm from altitudes of 500 m and 3 km above the ground, and approximately 10 cm using the RASCAL instrument from 500 m above ground. Comparison with tide gauge data showed the laser measurements are able to resolve centimeter-level changes in the lake elevation over time. Repeat pass analysis of tracks over flat surfaces indicate no systematic biases affect the measurement procedure of the ATLAS and SLICER instruments. Comparison of GPS and laser-derived elevations of easily-identifiable features in the caldera confirm the horizontal accuracy of the measurement is within the diameter of the laser footprint, and vertical accuracy is within the error inherent in the measurement. Crossover analysis shows that the standard error of the means at track intersection points within the caldera and dome (i.e., where zero and close to the maximum amount of uplift is expected) are about 1 cm, indicating elevation change at the 3 cm/year level should be detectable. We demonstrate one of the powerful advantages of scanning laser altimetry over other remote sensing techniques; the straightforward creation of precise digital elevation maps of overflown terrain. Initial comparison of the 1993-1995 data indicates uplift occurred, but filtering is required to remove vegetation effects. Although research continues to utilize the full potential of laser altimetry data, the results constitute a successful demonstration that the technique may be used to perform geodetic monitoring of surface topographic change.

Hofton, M. A.↗

NASA NTRS2010

Swept Frequency Laser Metrology System

A swept frequency laser ranging system having sub-micron accuracy that employs multiple common-path heterodyne interferometers, one coupled to a calibrated delay-line for use as an absolute reference for the ranging system. An exemplary embodiment uses two laser heterodyne interferometers to create two laser beams at two different frequencies to measure distance and motions of target(s). Heterodyne fringes generated from reflections off a reference fiducial X(sub R) and measurement (or target) fiducial X(sub M) are reflected back and are then detected by photodiodes. The measured phase changes Delta phi(sub R) and Delta phi (sub m) resulting from the laser frequency swept gives target position. The reference delay-line is the only absolute reference needed in the metrology system and this provides an ultra-stable reference and simple/economical system.

Zhao, Feng↗

NASA NTRS2019

Observational Constraint on the Radius and Oblateness of the Lunar Core‐Mantle Boundary

Lunar laser ranging (LLR) data and Apollo seismic data analyses, revealed independent evidence for the presence of a fluid lunar core. However, the size of the lunar fluid core remained uncertain by _55 km (encompassing two contrasting 2011 Apollo seismic data analyses). Here we show that a new description of the lunar interior's dynamical model provides a determination of the radius and geometry of the lunar core-mantle boundary (CMB) from the LLR observations. We compare the present day lunar core oblateness obtained from LLR analysis with the expected hydrostatic model values, over a range of previously expected CMB radii. The findings suggest a core oblateness (f(c) = (2.2 ± 0.6) x 10(exp 14)) that satisfies the assumption of hydrostatic equilibrium over a tight range of lunar CMB radii (R(CMB) = 381 ± 12 km). Our estimates of a presently-relaxed lunar CMB translates to a core mass fraction in the range of 1.59 - 1.77% with a present-day Free Core Nutation (FCN) within (367±100) years.

Moon↗

NASA NTRS2012

Lunar Science

Lunar Laser Ranging will provide critical data to determine the initial composition of the Moon and the Earth-Moon system. Specifically, LLR uniquely measures the properties of the lunar core via librations. LUNAR is advancing LLR: APOLLO recovery of Lunokhod 1, thus dramatically improving measurements of librations; Technology development of next generation retroreflectors; New drilling techniques. Dual-use Low Frequency Radio arrays will constrain: Lunar ionosphere density via riometry; Nano-size interplanetary dust particles responsible for surface weathering.

lunar laser ranging↗

NASA NTRS2006

Combinations of Earth Orientation Measurements: SPACE2005, COMB2005, and POLE2005

Independent Earth orientation measurements taken by the space-geodetic techniques of lunar and satellite laser ranging, by very long baseline interferometry, and by the Global Positioning System have been combined using a Kalman filter. The resulting combined Earth orientation series, SPACE2005, consists of values and uncertainties for Universal Time, polar motion, and their rates that span from September 28, 1976, to January 7, 2006, at daily intervals and is available in versions whose epochs are given at either midnight or noon. The space-geodetic measurements used to generate SPACE2005 have then been combined with optical astrometric measurements to form two additional combined Earth orientation series: (1) COMB2005, consisting of values and uncertainties for Universal Time, polar motion, and their rates that span from January 20, 1962, to January 7, 2006, at daily intervals and which is also available in versions whose epochs are given at either midnight or noon; and (2) POLE2005, consisting of values and uncertainties for polar motion and its rate that span from January 20, 1900, to December 21, 2005, at 30.4375-day intervals.

laser ranging↗

NASA NTRSDate not supplied

The GEODYN Precision Orbit Determination Software

GEODYN-II is NASA’s state-of-the-art geodetic parameter estimation and precision orbit determination (POD) system, developed at Goddard Space Flight Center and maintained within the Geodesy and Geophysics Laboratory. GEODYN supports POD and geodetic parameter estimation for both Earth orbiting and planetary/planetary body missions. It implements a comprehensive set of high-fidelity force models for integrating satellite orbits. GEODYN supports approximately 100 measurement types, including support for Global Navigation Satellite System (GNSS), Satellite Laser Ranging (SLR), Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS), Very Long Base Interferometry (VLBI), landmark (camera data), landmark crossovers, radar and laser direct altimeter ranging, and dynamic altimetry crossovers.

Joseph B Nicholas↗

NASA NTRS1979

Ion-molecule processes in lasers

Three classes of molecular electronic transition lasers produced by hybrid pumping of high pressure rate gas mixtures are discussed. These are (1) rare gas dimer lasers (such as excited Ar2, Kr2, and Xe2) lasing in the VUV, (2) rare gas halide lasers or excimer lasers (such as KrF, ArF, and XeCl excimers) lasing in the UV, and (3) the charge transfer molecular ion laser (such as N2/+/) lasing in the visible range. Laser excitation methods and kinetic sequences are examined for these lasers.

Laudenslager, J. B.↗

NASA NTRS1988

Sensor fusion of range and reflectance data for outdoor scene analysis

In recognizing objects in an outdoor scene, range and reflectance (or color) data provide complementary information. Results of experiments in recognizing outdoor scenes containing roads, trees, and cars are presented. The recognition program uses range and reflectance data obtained by a scanning laser range finder, as well as color data from a color TV camera. After segmentation of each image into primitive regions, models of objects are matched using various properties.

Kweon, In SO↗

NASA NTRS1993

The first satellite laser echoes recorded on the streak camera

The application of the streak camera with the circular sweep for the satellite laser ranging is described. The Modular Streak Camera system employing the circular sweep option was integrated into the conventional Satellite Laser System. The experimental satellite tracking and ranging has been performed. The first satellite laser echo streak camera records are presented.

Hamal, Karel↗

NASA NTRS1985

On the geodetic applications of simultaneous range differences to Lageos

While the majority of the currently deployed satellite laser systems are of centimeter level precision, the accuracy of the geodetic products corresponds to the accuracy in which geodesists are interested. On several occasions, the best models are either not good enough or result in tedious computations which are required to achieve accuracies comparable to those of the observations. A possible alternative is related to the development of a new method of data analysis. This method involves the minimization of the effect of the systematic errors and, in particular, the minimization of errors arising from an incorrectly modeled orbit. The present paper provides a summary of the results of a study of this option, taking into account laser ranging data from Lageos.

Pavlis, E. C.↗

NASA NTRS1978

Precision orbit computations for Starlette

The Starlette satellite, launched in February 1975 was designed to minimize the effects of nongravitational forces and to obtain the highest possible accuracy for laser range measurements. Analyses of the first four months of laser tracking data from nine stations have confirmed the stability of the orbit and the precision to which the satellite's position can be established. Initial orbit computations using the GSFC GEM - 7 gravity model produced rms fits of about 8 to 10 meters for arc lengths of 5 days. After tailoring a gravity model specifically to Starlette, the rms fits for the 5 day arcs were reduced significantly to the 1 to 2 meter level. An rms fit of 4.3 meters was obtained for a 90 day arc. Five day arcs overlapped by 2.5 days showed rms satellite position differences generally less than 2 meters. Prediction errors at the end of two months were less than 30 milliseconds.

Marsh, J. G.↗