Data Set Information
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| DATA_SET_NAME |
MESSENGER MERCURY RSS/MLA LEVEL 5 DERIVED DATA V1.0
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| DATA_SET_ID |
MESS-H-RSS/MLA-5-SDP-V1.0
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| NSSDC_DATA_SET_ID |
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| DATA_SET_TERSE_DESCRIPTION |
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| DATA_SET_DESCRIPTION |
Data Set Overview : The MESSENGER Radio Science (RS) Reduced Data Archive (RDA) is a collection of derived data products created from raw data collected during the MESSENGER mission to Mercury. These products are generated from radio occultation, MLA, and gravity investigations conducted by members of the MESSENGER Radio Science Team. Gravity data and planet-shape products consist of spherical harmonic models. Personnel at the Goddard Space Flight Center (GSFC) produced spherical harmonic models. Occultation products include measurements of Mercury's radius produced at the Johns Hopkins University Applied Physics Laboratory (JHU/APL). The models are derived from both flyby data and orbit-phase data. MESSENGER's orbit is highly eccentric, with a periapsis altitude between 5 km and 450 km, and an apoapsis altitude initially near 13,000 km. The apoapsis was reduced to 10,000 km in April 2012. The orbit inclination is 84-85 degrees with a periapsis northward of 60N, so the resolution of the gravity models varies with latitude, with the coarsest resolution near the south pole. MESSENGER's eccentric orbit also restricts MLA data to the northern hemisphere, and the shape models rely on occultation measurements of radius for the southern hemisphere. Since MLA measurements are more accurate and much more plentiful than occultations, the shape model is better constrained in the north. The uncertainty of the occultation measurements of radius depends primarily on the quality of the RF signal, which depends on the whether the low-gain or high-gain antenna was used. With the low-gain antennas, the spacecraft attitude, the Earth-spacecraft distance, and the quantity of solar plasma along the RF path are important. In some cases, alternate processing parameters or algorithms can reduce uncertainty. Currently, the measurements with the lowest uncertainty are the measurements from occultations that occur between January and April of each year that MESSENGER was in orbit about Mercury. See PERRYETAL2015 [doi:10.1002/2015GL065101] for additional information on processing of occultation data. The products were generated using radio data collected during flybys and during the orbital phase of the MESSENGER mission. For the first twelve months of the orbital phase, when the orbit period was twelve hours, tracking data were usually obtained for eight hours per Earth day, centered near the apoapsis of every other orbit. After the orbit period was reduced to eight hours in April 2012, tracking was usually six hours per Earth day. MESSENGER used a High-Gain Antenna (HGA) during these periods. There are also data collected during some periapsis passes, usually every other orbit. These data were obtained using the low-gain antennas. During the last six months of the mission, MESSENGER's periapsis was maintained between 5 and 200 km; data from this period contain the highest-resolution signals for analyzing Mercury's gravity. This data set includes three primary data product types: occultation summary files (OCCSUM), Spherical Harmonics ASCII Data Records (SHADR), and Spherical Harmonics Binary Data Records (SHBDR). The MLA data acquisition and processing are described in the MLA archive. Please see documentation in the DOCUMENT directory for additional information on the data files and their format. A description of the mission phases is provided in the MISSION.CAT file included with the archive volume. Parameters : For the gravity and shape products, the data set consists of the spherical harmonic coefficients and their uncertainties. For occultations, the data set contains information on each archived occultation, including the name of the raw data file, the surface location and radius, and quality information such as the effective signal margin and the time uncertainty. See individual Software Interface Specification (SIS) documents, which are located in the DOCUMENT directory, for detailed descriptions of each file contained in this data archive. Processing : Spherical harmonic models and occultation radii are derived from raw radio tracking data, Radio Science Receiver (RDR) data, and MLA data in several steps. The tracking data are processed in large orbit determination programs that integrate the equations of motion (GEODYN at NASA GSFC [PAVLISETAL2007]) and model mathematically the radio science observables (ramped Doppler and range data). The observations are related to the geophysical parameters through the numerical integration and the detailed mathematical modeling of the radio science observables and of all forces acting on the spacecraft trajectory, including Mercury and third body gravity, solar radiation pressure, planetary radiation pressure, solid body tides, and relativity. The gravity field coefficients are obtained by accumulating normal equations from hundreds of data arcs and solving these systems of linear equations for thousands of unknowns. The unknowns include arc parameters, particular to one data arc (such as the spacecraft state and radiation pressure scale factors) and common parameters (such as the gravity coefficients and the Mercury gravitational constant or GM). Radio tracking data are processed in arcs, usually 24 hours long. The occultation radius measurements use RF data obtained at the DSN when Mercury occults the RF transmissions from MESSENGER. The RF power history for each occultation is extracted from Radio Science Receiver measurements. Fits to a diffraction curve then provide the occultation time and its uncertainty. Combined with an ephemeris, this time provides Mercury's radius at the point where the RF path grazes the surface with the assumption that Mercury's surface is smooth. The radius derived and described in DOCUMENT/OCCSUM.TXT in this archive volume is not corrected for local topographic effects, which may have moved the occulting edge from the smooth-sphere location. The OCCSUM.TXT software interface specification document contains more information on the processing. See PERRYETAL2015 for further details. The shape model coefficients are based on weighted, constrained, least- squares fit of spherical harmonics to the MLA and occultation data. The MLA data comprise more than 25 million individual measurements of radius. In order to fit spherical harmonic coefficients of high degree and order without undue computational effort, the MLA observations are conservatively assigned an uncertainty of 1 km, averaged into 0.5 by 0.5 degree bins, and weighted according to the number of observations in each bin. While the topography in each of the 335,571 bins measured by MLA is known on 20 to 80-m-diameter footprints with uncertainties of a few meters radially, the average height in a region is not as well known owing to undersampling of topography. Occultation data are weighted based on an uncertainty of 0.33 km, which reflects the uncertainty in the accuracy of the Digital Terrain Model (DTM) used to adjust the raw occultation radius for the effects of local topography as well as the areal averaging employed. The distribution of data on the surface of Mercury is extremely sparse in the southern hemisphere, with only 1% of the observations south of the equator, and only 313 observations south of 20 degrees south latitude. The spherical harmonic expansion to degree and order 155 results in a least-squares fit of >20,000 coefficients to a total of 110,752 observations. While formally overconstrained, the solution is numerically ill-conditioned, and the solution is truncated at degree and order 150 to avoid aliasing. A planetary power law is applied in a similar manner as that employed for gravity [e.g., MAZARICOETAL2014] to damp the least-squares equations, resulting in a better-conditioned solution, at the expense of a negligibly worse fit to the observations. A damping term of (2n+1)n^2 is added to terms of degree n on the normal matrix diagonal, to constrain the solution variance at degrees n>0. The formal uncertainties, given as 1 standard deviation (s.d.), are obtained from the diagonal terms of the solution covariance matrix, and reflect the lack of knowledge of the true coefficient power as well as the power law assumptions. Thus, the formal uncertainties must increasingly be regarded as underestimates at higher degrees. The (0,0) term corresponding to the mean radius is unconstrained, as are the lowest degree-and-order shape coefficients, however the sparseness of southern occultations reduces confidence in the coefficients of degrees higher than 2. Data : Users should consult listings in the INDEX.TAB file to ascertain full coverage of each data type. The DATA directory contains the SHADR, SHBDR, and OCCSUM products. OCCSUM files are named MESS_RS_OCC_V.CSV. The ASCII text spherical harmonic files are named GGMES_V_SHA.TAB and GTMES_V_SHA.TAB for the gravity and shape models respectively and the binary version of the gravity model is named GGMES_V_SHB.DAT. or indicates the degree and order of the model used. is a version number, range 01..99. The file naming conventions for these products are described in detail in their associated SIS documents. Every file in the directory has a PDS label. Except for files in the root and in the CATALOG directories and the *INFO.TXT files in the other directories, the label is a separate file in the same directory with the same name but a LBL extension. Coordinate System : The MESSENGER RDR products use the Mercury planetocentric fixed body coordinate system. The MESSENGER mission has adopted the Planetocentric model for Mercury. At 2439.4 km, the geodetic reference radius is slightly smaller than the 2439.7 km IAU value [ARCHINALETAL2011]. Software : Software is not included with this archival data set. Media/Format : This is an electronically stored and transferred data set and may be downloaded from http://pds.nasa.gov.
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| DATA_SET_RELEASE_DATE |
2016-05-06T00:00:00.000Z
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| START_TIME |
2008-01-07T12:00:00.000Z
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| STOP_TIME |
2015-04-30T11:13:55.000Z
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| MISSION_NAME |
MESSENGER
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| MISSION_START_DATE |
2004-08-03T12:00:00.000Z
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| MISSION_STOP_DATE |
2015-04-30T12:00:00.000Z
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| TARGET_NAME |
MERCURY
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| TARGET_TYPE |
PLANET
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| INSTRUMENT_HOST_ID |
MESS
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| INSTRUMENT_NAME |
RADIO SCIENCE SUBSYSTEM
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| INSTRUMENT_ID |
RSS
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| INSTRUMENT_TYPE |
RADIO SCIENCE
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| NODE_NAME |
Geosciences
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| ARCHIVE_STATUS |
ARCHIVED
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| CONFIDENCE_LEVEL_NOTE |
Overview : Data in this archive have been reduced as part of mission data analysis activities of the MESSENGER mission. Products of questionable validity have been flagged or omitted. Review : This archival data set was reviewed by the MESSENGER RS Team prior to submission to the Planetary Data System (PDS). Data set design, documentation, and sample products have passed a PDS peer review. Prior to creation of the final version of the archival data set, key elements of the archive were distributed for preliminary review. These included electronic versions of example PDS labels, CATALOG files, and Software Interface Specifications. These materials were distributed to PDS personnel, the experiment investigator, and others, as appropriate. The archived data set incorporates changes based on the reviews. Data Coverage and Quality : This volume contains gravity and shape models generated from MESSENGER data collected through April 2015 for the gravity model and shape model. The tracking data include flyby data and orbit-phase data. Due to MESSENGER's eccentric orbit with a periapsis that is northward of 60N, the spatial resolution of the gravity field is higher in the northern hemisphere than in the southern hemisphere. There are 170 occultation measurements of radius that overlap with the MLA data. The occultation measurements are an average of 5 meters higher than the MLA data with a standard deviation of 130 meters, in agreement with the average time-based uncertainty. The time-based uncertainty is the uncertainty in the time of occultation multiplied by the perpendicular speed and the data set only includes occultations where this uncertainty is 500 meters or less. The data are also filtered for excessive difference from the image-based DTM and for low noise. Future, improved processing may add more occultations, reduce uncertainty, or both. Since most of the egress events had large time uncertainties, the archived occultations consist almost exclusively of ingresses. For some of the carrier power calculations (Pc/No) in the occultation summary product with Pc/No greater than 25 dB-Hz, the reported power is low by up to 5 dB-Hz because a portion of the carrier power was inadvertently included in the noise, reducing the power-to-noise ratio. Limitations : The limitations in this data set follow from the quality of the execution, which is described above under Data Coverage and Quality.
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| CITATION_DESCRIPTION |
Perry, M.E., Lemoine, F.G., Smith, D.E., Zuber, M.T., Neumann, G. A., Solomon, S.C., MESSENGER Radio Science Reduced Data Record (MESS-H-RSS-5-SDP-V1.0), NASA Planetary Data System, 2016.
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| ABSTRACT_TEXT |
This data set contains archival results from radio science investigations conducted during the MESSENGER mission. Radio measurements were made using the MESSENGER spacecraft and Earth-based stations of the NASA Deep Space Network (DSN). The data set includes three products: 1) high-resolution spherical harmonic models of Mercury's gravity field. These results were derived from raw radio tracking data; 2) measurements of Mercury's radius at several locations around the planet. These radii are derived from radio occultations; and 3) spherical harmonic models of Mercury's surface shape. Both Mercury Laser Altimeter (MLA) data and occultation data were used to generate the shape models. The investigations were conducted by personnel at JHU/APL, GSFC, and JPL.
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| PRODUCER_FULL_NAME |
MARK E. PERRY
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| SEARCH/ACCESS DATA |
Geosciences Web Services
Mercury Orbital Data Explorer
Geosciences Online Archives
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