DATA_SET_DESCRIPTION |
Data Set Overview : The gravitational signature of the Moon was determined from velocity perturbations of the Lunar Prospector (LP) spacecraft as measured from the Doppler shift of the S-band radio tracking signal. LP was tracked by NASA's Deep Space Network (DSN) at Goldstone, California, Canberra, Australia, and Madrid, Spain. The tracking data were used to determine the LP orbit about the Moon, as well as the lunar gravity field [KONOPLIVETAL1998]. The LP data were combined with S-band tracking observations from Lunar Orbiters 1, 2, 3, 4, and 5 and from the Apollo 15 and 16 subsatellites [KONOPLIVETAL1993B] and from Clementine [LEMOINEETAL1997]. In advance of the LRO mission, the Lunar Prospector and other historical tracking data were re-analyzed and the GLGM-3 model was developed. The model is documented in a JGR paper [MAZARICOETAL2010]. In addition, the GLGM-3 model was extended to degree 200 using a Line-of-Sight (LOS) analysis technique [HANETAL2011] to produce the LPE200 model. Data : There are 2 data types for the gravity products found on this volume: tabular and array data. The files containing the spherical harmonic coefficients of the Moon's gravity field (GLGM3150, LPE200) are in tabular (ASCII) and array (BINARY) format, with each row in the table containing the degree index m, the order index n, the coefficients Cmn and Smn, and the uncertainties in Cmn and Smn. The binary array file is a little-endian, row ordered upper triangular matrix. Coordinate System : The coordinate system for the gravity data, and the coefficients in the GLGM3150 and LPE200 gravity fields is selenocentric, center of mass, longitude positive east. The location of the pole and the prime meridian are defined by the principal axes as given by the integrated lunar librations of DE421 [WILLIAMSETAL2008]. Processing : The GLGM3 gravity solution consists of 4,324,171 observations, of which 3,570,901 were contributed by LP. The data were divided into 1225 independent arcs based on considerations of data coverage and timing of maneuvers. The table below summarizes the number of observations and arcs from each spacecraft: Satellite Number Total Periapsis Apoapsis of Arcs Observ. (km) (km) Lunar Orbiter-1 70 48,575 50 1830 Lunar Orbiter-2 90 77,726 50 1870 Lunar Orbiter-3 73 62,264 50 1820/320 Lunar Orbiter-4 32 48,688 2700/75 6000/4000 Lunar Orbiter-5 70 42,916 100/170 1750/2000 Apollo-15 subsatellite 93 52,500 75 160 Apollo-16 subsatellite 46 42,579 30 190 Clementine 40 378,022 370 2960 LP (nominal) 184 2,198,751 90 110 LP (extended) 127 1,372,150 25 45 Total 1225 3,570,901 For each arc certain parameters were determined: for example, the spacecraft state (position and velocity), solar radiation pressure coefficients, Doppler biases for each station over the arc to account for frequency biases, and increments in velocity to account for spacecraft manuevers. The DE421 set of planetary and lunar ephemerides was used in the analyses. Average data fits and the final effective data weight for each set of the data in the solution are as follows: Satellite Avg. RMS of fit Eff. Data Wt. (cm/s) (cm/s) Lunar Orbiter 1 0.24 3.16 Lunar Orbiter 2 0.11 3.16 Lunar Orbiter 3 0.07 3.16 Lunar Orbiter 4 0.05 0.55 Lunar Orbiter 5 0.21 0.49 Apollo 15 subsatellite 0.12 0.95 Apollo 16 subsatelite 0.15 0.32 Clementine 0.31 1.34 Lunar Prospector (nom) 0.02 1.41 Lunar Prospector (ext) 0.25 3.78 Media/Format : This data set is stored online at the Planetary Data System (http://pds.nasa.gov/) and may be downloaded using a web browser or FTP software. A copy may be requested on physical media if downloading is not possible. The Planetary Data System maintains backup copies of this data set on various media.
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CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview : The data noise on the historic Doppler data ranged from 0.3 to several mm/s, depending on the arc, and most of the data were at a count interval of 60 seconds. The Clementine Doppler data from the DSN stations had a data noise of 0.25 mm/s with a count interval of 10 seconds. Clementine was also tracked by a 30 meter antenna of the Naval Research Lab in southern Maryland. [KONOPLIVETAL2001] provides a detailed description of the Lunar Prospector tracking data. The overall data accuracy was about 0.3 mm/s but there was some variability due to antenna type (34 m vs. 26 m). In addition during the extended mission, [KONOPLIVETAL2001] reports that multipath signals make the LP Doppler data closer to the poles more noisy. We note that [KONOPLIVETAL2001] applied a deweighting scheme (cf pp.5 of [KONOPLIVETAL2001]) due to convergence of the ground tracks. No such deweighting by latitude was performed with GLGM-3. Errors in the Free-air gravity anomalies computed from the full error covariance to 150x150 range from a minimum of 2.4 mGals on the nearside equator to a maximum of 78.1 mGals on the mid latitude regions of the lunar farside. The lunar nearside is characterized by errors of 2-30 mGals. The lunar farside is characterized by errors of 30 to 78 mGals. Similarly errors in the geoid, computed from the full error covariance to 150x150 range from a minumum of 0.1 meters on the nearside to a maximum of 24.8 meters on the mid-latitude regions of the farside. On the nearside, the geoid errors are under 8 meters. Geoid errors on the lunar farside range from 8 to 25 meters. These errors are computed as commission error to degree 150, and do not include omission error (the contribution of error from degrees higher than 150). Review : The volume containing the LP gravity dataset was formally reviewed by the PDS in October 2011. Data Coverage/Quality : Lunar Prospector is the first circular polar orbiter for the Moon in a low altitude orbit of 100 km. The only previous low altitude data (about 100 km) was from the Lunar Orbiters and the Apollo subsatellites at low inclination (which provide coverage at < 30 degrees of latitude). Lunar Orbiters 4 and 5 provided some near equatorial data at low altitude from their polar eccentric orbit, but no low altitude data for the high latitudes. Clementine, with its eccentric near-five hour orbit, had a periapse altitude of 415 km and so provided long wavelength information on the gravity field. Because of the Moon's synchronous rotation, spacecraft cannot be directly tracked from Earth over a large part of the lunar farside, so there is no tracking data from 120 to 240 degrees longitude in the +/- 70 degree latitude band. Limitations : See Data Coverage/Quality above.
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