Showing papers by "Frank G. Lemoine published in 1997"

The Development of the NASA GSFC and NIMA Joint Geopotential Model

Abstract: The NASA Goddard Space Flight Center, the National Imagery and Mapping Agency (NIMA; formerly the Defense Mapping Agency or DMA) and The Ohio State University have collaborated to produce EGM96, an improved degree 360 spherical harmonic model representing the Earth’s gravitational potential. This model was developed using: (1) satellite tracking data from more than 20 satellites, including new data from GPS and TDRSS, as well as altimeter data from TOPEX, GEOSAT and ERS-1. (2) 30’ x 30’ terrestrial gravity data from NIMA’s comprehensive archives, including new measurements from areas such as the former Soviet Union, South America, Africa, Greenland, and elsewhere. (3) 30’ x 30’ gravity anomalies derived from the GEOSAT Geodetic Mission altimeter data, as well as altimeter derived anomalies derived from ERS-1 by KMS (Kort and Matrikelstyrelsen, Denmark) in regions outside the GEOSAT coverage. The high degree solutions were developed using two different model estimation techniques: quadrature, and block diagonal. The final model is a composite solution consisting a combination solution to degree 70, a block diagonal solution to degree 359, and the quadrature model at degree 360. This new model will be used to define an undulation model that will be the basis for an update of the WGS-84 geoid. In addition, the model will contribute to oceanographic studies by improving the modeling of the ocean geoid and to geodetic positioning using the Global Positioning System (GPS).

Topography of the Moon from the Clementine lidar

Abstract: 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.

A 70th degree lunar gravity model (GLGM‐2) from Clementine and other tracking data

Abstract: A spherical harmonic model of the lunar gravity field complete to degree and order 70 has been developed from S band Doppler tracking data from the Clementine mission, as well as historical tracking data from Lunar Orbiters 1-5 and the Apollo 15 and 16 subsatellites. The model combines 361,000 Doppler observations from Clementine with 347,000 historical observations. The historical data consist of mostly 60-s Doppler with a noise of 0.25 to several mm/s. The Clementine data consist of mostly 10-s Doppler data, with a data noise of 0.25 mm/s for the observations from the Deep Space Network, and 2.5 mm/s for the data from a naval tracking station at Pomonkey, Maryland. Observations provided Clementine, provide the strongest satellite constraint on the Moon's low-degree field. In contrast the historical data, collected by spacecraft that had lower periapsis altitudes, provide distributed regions of high-resolution coverage within +/- 29 deg of the nearside lunar equator. To obtain the solution for a high-degree field in the absence of a uniform distribution of observations, we applied an a priori power law constraint of the form 15 x 10(exp -5)/sq l which had the effect of limiting the gravitational power and noise at short wavelengths. Coefficients through degree and order 18 are not significantly affected by the constraint, and so the model permits geophysical analysis of effects of the major basins at degrees 10-12. The GLGM-2 model confirms major features of the lunar gravity field shown in previous gravitational field models but also reveals significantly more detail, particularly at intermediate wavelengths (10(exp 3) km). Free-air gravity anomaly maps derived from the new model show the nearside and farside highlands to be gravitationally smooth, reflecting a state of isostatic compensation. Mascon basins (including Imbrium, Serenitatis, Crisium, Smythii, and Humorum) are denoted by gravity highs first recognized from Lunar Orbiter tracking. All of the major mascons are bounded by annuli of negative anomalies representing significant subsurface mass deficiencies. Mare Orientale appears as a minor mascon surrounded by a horseshoe-shaped gravity low centered on the Inner and Outer Rook rings that is evidence of significant subsurface structural heterogeneity. Although direct tracking is not available over a significant part of the lunar farside, GLGM-2 resolves negative anomalies that correlate with many farside basins, including South Pole-Aitken, Hertzsprung, Korolev, Moscoviense, Tsiolkovsky, and Freundlich-Sharonov.

Topography of the Moon from the Clementine

Abstract: 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 °. The field shows that the Moon can be described as a sphere with maximum positive and negative deviations of -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 -200 m 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 ° to the east and intersects the farside 24 ° north of the equator. The magnitude of impact basin topography is greater than the lunar flattening (~2 km) and equatorial ellipticity (-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, Iie 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 l0 T-102 km correlates with elevation and surface age.

from Clementine and other tracking data

Abstract: . A spherical harmonic model of the lunar gravity field complete todegree and order 70 has been developed from S band Doppler tracking data fromthe Clementine mission, as well as historical tracking data from Lunar Orbiters1-5 and the Apollo 15 and 16 subsatellites. The model combines 361,000 Dopplerobservations from Clementinewith 347,000 historical observations. The historicaldata consist of mostly 60-s Doppler with a noise of 0.25 to several mm/s. TheClementine data consist of mostly 10-s Doppler data, with a data noise of 0.25mm/s for the observations from the Deep Space Network, and 2.5 mm/s for thedata from a naval tracking station at Pomonkey, Maryland. Observations providedClementine, provide the strongest satellite constraint on the Moon's low-degreefield. In contrast the historical data, collected by spacecraft that had lower periapsisaltitudes, provide distributed regions of high-resolution coverage within +29 ° ofthe nearside lunar equator. To obtain the solution for a high-degree field in the_bsence of a uniform distribution of observations, we applied an a priori power lawconstraint of the form 15 × lO-S/l 2 which had the effect of limiting the gravitationalpower and noise at short wavelengths. Coefficients through degree and order 18 arenot significantly affected by the constraint, and so the model permits geophysicalanalysis of effects of the major basins at degrees 10-12. The GLGM-2 modelconfirms major features of the lunar gravity field shown in previous gravitationalfield models but also reveals significantly more detail, particularly at intermediatewavelengths (103 kin). Free-air gravity anomaly maps derived from the new modelshow the nearside and farside highlands to be gravitationally smooth, reflecting astate of isostatic compensation. Mascon basins (including Imbrium, Serenitatis,Crisium, Smythii, and Humorum) are denoted by gravity highs first recognizedfrom Lunar Orbiter tracking. All of the major mascons are bounded by annuliof negative anomalies representing significant subsurface mass deficiencies. MareOrientale appears as a minor mascon surrounded by a horseshoe-shaped gravitylow centered on the Inner and Outer Rook rings that is evidence of significantsubsurface structural heterogeneity. Although direct tracking is not available over ,a significant part of the lunar farside, GLGM-2 resolves negative anomalies thatcorrelate with many farside basins, including South Pole-Aitken, Hertzsprung,Korolev, Moscoviense, Tsiolkovsky, and Freundlich-Sharonov.IntroductionUntil the launch of Clementine, on January 24, 1994,the sources of tracking data for gravity models derivedby U.S. investigators have been the Lunar Orbiters andthe Apollo spacecraft. The Lunar Orbiters were in-