Engineering topics

Lemoine, Frank G.

Publications and source records attributed to Lemoine, Frank G..

62 records · Page 4

NASA NTRS1998

Gravity Field Mapping of Mars with MGS

Tracking of the MGS spacecraft in orbit at Mars by the Deep Space Network since last September has provided doppler and range measurements that are being used to improve the model of the Mars gravity field. During most of October 1997, April 1998, and June thru August 1998 high quality tracking data were obtained while the periapse was in the northern hemisphere at altitudes in the 170 to 190 km range. The eccentric orbit had a period of about 11.5 hrs and an inclination of about 96.2 degrees so that low altitude tracking was obtained over most of the northern hemisphere, including the north polar icecap. Data from the earlier Mariner 9 and Viking missions have been added to the MGS data and a series of experimental gravity models developed from the combined datasets. These models have generally been of degree and order 70 and are a significant improvement over earlier models that did not include the MGS data. Gravity anomalies over the north polar cap region of Mars are generally less than 50 to 100 mgals and show no obvious correlation with the topography. Successive MGS orbits derived using these new models are showing agreement at the 100 meter level, and this has been confirmed with the laser altimeter (MOLA) on MGS These comparisons are expected to improve significantly as more tracking data get included in the solution and the MGS orbit becomes more circular giving a more balanced geographical distribution of data at low altitude. This will happen early in 1999 as the orbit approaches the mapping configuration of a circular orbit at about 400 Km.

Smith, David E.↗

NASA NTRS1998

Modelling and Estimation of Dynamic Ocean Topography Within Global Geopotential Solutions

In this poster, we will present a report on the ongoing investigation "Improved Gravitational and Dynamic Height Models Through the use of Oceanographic Data." we have used a two year mean (1993-1994) of the Dynamic ocean Topography (DOT) field implied by the Semtner and Chervin POCM_4B model, and developed normal equations in surface spherical harmonics to degree and order 30. These normal equations, were combined with normal equations derived from TOPEX and ERS-1 altimeter data over the same time period. Combination solutions (based on satellite tracking data, altimeter data, surface gravity data and OCM data) were were developed Test solutions were obtained estimating the DOT field to 2Ox2O and 3ox3O. These solutions were tested with independent DOT values computed over 38 WOCE hydrographic sections, which contained a total of 3072 stations and represented 216000 km of travelled lines. The weighted standard deviation of the differences between the DOT obtained from the hydrographic data and and the field estimated from the joint combination solutions was computed for each of the test models, the weighted standard deviation for the baseline combination solution excluding the POCM4_B data was 9.7 cm for a DOT solution to 30x30, where introducing the POCM4_B data into the combination model reduced the standard deviation to 9.2 cm, indicating the the introduction of oceanographic information benefits the solution for the dynamic ocean topography. We will discuss the weighting schemes applied and the method of solution. Another aspect of our investigation involves testing alternate parametric representations of the dynamic height field. We looked at the alternative representations in terms of the Proudman functions (PF), and compared these to the use of spherical harmonics (SH) to represent the dynamic ocean topography, using once again the 1993 and 1994 output of the POCM4_B OCM as the reference model. A significant advantage of PF's over SH's is that the former require no "fill-in" values over areas where the DOT is undefined. We show that the PF and SH results using equal number of parameters agree quite well in overall content. In addition, PFs appear better suited for representation of high frequency signals close to the signal boundary. The development of the PF solutions for the DOT will be described and the statistics of the comparisons will be presented.

Lemoine, Frank G.↗

NASA NTRS1997

Topography of the Moon from the Clementine Lidar

Range measurements from the lidar instrument carried aboard the Clementine spacecraft have been used to produce an accurate global topographic model of the Moon. This paper discusses the function of the lidar; the acquisition, processing, and filtering of observations to produce a global topographic model; and the determination of parameters that define the fundamental shape of the Moon. Our topographic model: a 72nd degree and order spherical harmonic expansion of lunar radii, is designated Goddard Lunar Topography Model 2 (GLTM 2). This topographic field has an absolute vertical accuracy of approximately 100 m and a spatial resolution of 2.5 deg. The field shows that the Moon can be described as a sphere with maximum positive and negative deviations of approx. 8 km, both occurring on the farside, in the areas of the Korolev and South Pole-Aitken (S.P.-Aitken) basins. The amplitude spectrum of the topography shows more power at longer wavelengths as compared to previous models, owing to more complete sampling of the surface, particularly the farside. A comparison of elevations derived from the Clementine lidar to control point elevations from the Apollo laser altimeters indicates that measured relative topographic heights generally agree to within approx. 200 in over the maria. While the major axis of the lunar gravity field is aligned in the Earth-Moon direction, the major axis of topography is displaced from this line by approximately 10 deg to the cast and intersects the farside 24 deg north of the equator. The magnitude of impact basin topography is greater than the lunar flattening (approx. 2 km) and equatorial ellipticity (approx. 800 m), which imposes a significant challenge to interpreting the lunar figure. The floors of mare basins are shown to lie close to an equipotential surface, while the floors of unflooded large basins, except for S.P.-Aitken, lie above this equipotential. The radii of basin floors are thus consistent with a hydrostatic mechanism for the absence of significant farside maria except for S.P.-Aitken, whose depth and lack of mare require significant internal compositional and/or thermal heterogeneity. A macroscale surface roughness map shows that roughness at length scales of 10(exp 1) - 10(exp 2) km correlates with elevation and surface age.

Smith, David E.↗

NASA NTRS1996

The Lunar Crust: Global Structure and Signature of Major Basins

New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (approx. 20-120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact-related redistribution. The 6l-km average crustal thickness, constrained by a depth-to-Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center-of-figure from the center-of-mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole-Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three- dimensional, minimum-slope and curvature algorithm. Both mare and non-mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (greater than 300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.

Neumann, Gregory A.↗

NASA NTRS1992

The processing of the Viking Orbiter range data and its contribution to Mars gravity solutions

The processing of Doppler data has been the primary method for deriving models of the Mars gravity field. Since the Mariner 9 and Viking spacecraft were placed in orbit about Mars, many models from degree and order 6 to degree and order 50 have been developed. However, during the Viking mission, some 26,000 range measurements to the two Viking Orbiters were also obtained. These data have not previously been used in the derivation of Mars gravity models. A portion of these range data have been processed simultaneously with the Doppler data. Normal equations were generated for both sets of data and were used to create two solutions complete to degree and order 30: a nominal solution including both the range and the Doppler data (MGM-R100), and another solution including only the Doppler data (MGM-R101). Tests with the covariances of these solutions, as well as with orbit overlap tests indicate that the interplanetary range data can be used to improve the modeling of the Mars gravity field.

Lemoine, Frank G.↗

NASA NTRS1992

Constraints on Energy Dissipation in the Earth's Body Tide From Satellite Tracking and Altimetry

The phase lag by which the earth's body tide follows the tidal potential is estimated for the principal lunar semidiurnal tide M(sub 2). The estimate results from combining recent tidal solutions from satellite tracking data and from Topex/Poseidon satellite altimeter data. Each data type is sensitive to the body-tide lag: gravitationally for the tracking data, geometrically for the altimetry. Allowance is made for the lunar atmospheric tide. For the tidal potential Love number kappa(sub 2) we obtain a lag epsilon of 0.20 deg +/- 0.05 deg, implying an effective body-tide Q of 280 and body-tide energy dissipation of 110 +/- 25 gigawatts.

Ray, Richard D.↗

NASA NTRS1990

Sensitivity studies of Mars orbiters for Mars gravity recovery

The purpose of the study is to assess the Mars orbiters with regard to their sensitivity to the Mars gravity field. The sensitivity is evaluated and quantified in terms of the magnitude and frequency of the orbiter position and velocity perturbations. The strengths and weaknesses of each orbiter, including Mariner 9, Viking 1 and 2, Mars Observer, Phobos, and Deimos, when used as a sensor of the Mars gravity field are assessed. It is shown that Mariner 9 and Viking data supply strong information on the low degree and order coefficients, with Mariner 9 contributing to the even-ordered terms out to about degrees 10 to 12. Phobos and Deimos are shown to be able to contribute to the lowest degrees of the gravity field, while the Mars Observer, in addition to the lower-degree and -order harmonics, will be responsive to coefficients as high as degree and order 50. Emphasis is placed on the spacecraft orbit plane orientation allowing the radial velocity perturbations to be measured by the Doppler data, thus enhancing the sensitivity to the gravitational signal.

Rosborough, George W.↗

NASA NTRS1990

Modeling issues in precision orbit determination for Mars orbiter

This paper examines the accuracy of recent Mars gravity models and the importance of perturbations due to the Mars radiation pressure and the Martian moons, Phobos and Deimos, on the trajectories of Mars orbiters. A linear orbit perturbation theory is used to characterize the patterns of gravity field near resonances for the Viking and Mariner 9 spacecraft. These resonances are shown to have considerable power and their potential for contributing to Mars gravity solutions is emphasized. It is shown that some of the same resonance orders which appear in the Viking orbits, dominate the radial orbit error spectrum for Mars Observer. Results of orbit determination simulations at the Goddard Space Flight Center show that the perturbations caused by the Martian moons and the Mars radiation pressure are larger than 0.1 mm/s, the expected precision of the Mars Observer Doppler tracking data. Tests with the Viking Doppler data indicate that best analysis of these data mandates the inclusion of the Phobos gravitational perturbation in the modeling of Viking spacecraft trajectories.

Lemoine, Frank G.↗