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NEAR NLR LEVEL 2 DATA PRODUCTS V1.0 (PDS)

NSSDCA ID: PSPG-00782

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= The NEAR Laser Rangefinder (NLR) Level 2 Data Products include the along-track profiles of NLR data in SI units, together with spacecraft position, orientation, and timing data. The radius of 433 Eros with respect to its center of mass is the primary profile parameter. Surface slope and topographic relief are not as easily defined on an irregular object as on the Earth. We provide the geopotential arising from the mass of the asteroid and its rotation, based on the NLR shape model NLR125AR.IMG, a rotation rate of 3.3166E-04 rad s^(-1), and an assumed homogeneous density of 2670 kg m^(-3). Geophysical studies may use the potential as a proxy for topographic height, scaled by a regional average gravitational acceleration [CHENGETAL2001]. Ranges from level 1 NLR data are the source of the level 2 data product. Precision orbits were generated at NASA-GSFC, and supplemented where appropriate by JPL navigation team orbits, ancillary timing and attitude information supplied by the NEAR SDC and instrument data derived from in-flight analysis. These data were merged to create the calibrated profile data. Each level 2 data product is a text format table corresponding to a daily level 1 EDR file, and is described by a detached label. The file naming convention is LyydddNv.TAB, where L=laser, and yyddd contains the two-digit year and the three-digit day of observation. The letter v stands for processing version. Version 'D' is based on the most recent JPL navigation team orbits, version 'G' and 'J' on GSFC's precision orbit series 393, where version 'J' is supplemented by JPL orbits at times of spacecraft maneuvers. There is only one version of the data for each specific orbit on the volume. The orbital mission phase employed only normal format packets. In these packets, the spacecraft mission elapsed time (MET) is recorded at the time of the first major frame in the packet. A normal rate packet contains up to 56 observations, so that the last observation may occur up to 55 seconds after the nominal packet time in the case of 1 Hz data. The SPACECRAFT_CLOCK_STOP_COUNT and STOP_TIME values reflect the time recorded in the last packet for a given UTC day. Thus the last observation may occur later than the nominal STOP_TIME. Parameters ========== Profiles are tables of longitude, latitude, and radius at the laser bounce point, together with time stamps, the spacecraft position, and instrument threshold setting. Processing ========== Ranges are converted from counts to meters of one-way range from NLR to 433 Eros as described in the calibration document NLRCALIB.TXT. Precision orbit data for the NEAR-Shoemaker spacecraft is provided by the NASA-GSFC analysis team and the JPL navigation team, and is archived in the form of SPICE format spacecraft ephemeris (SPK) files. The MET time associated with each shot, with minor frame given in decimal seconds, is converted to Ephemeris Time in seconds past J2000 (ET) using the project's time conversion (SCLK) file. The spacecraft position at the ET shot time is interpolated from the SPK file. The spacecraft orientation SPICE file (CK) is resampled at 1-second intervals of whole seconds. Its quaternions are convolved with a 9-point, centered, finite-impulse-response filter to reduce high-rate gyro noise before conversion to matrix form. The filtered spacecraft orientation matrix transforms vectors from the J2000 inertial reference frame to the instrument deck (NEAR_SC_BUS_PRIME) frame. Observations are directed toward the positive X'-axis in this frame. A fixed boresight offset with respect to this frame is assumed. The Euler angles of the boresight are provided as a SPICE instrument kernel (IK) file, from which a transformation matrix from NEAR to NLR frame is derived. The J2000-to-NLR frame matrix is calculated from the smoothed CK by matrix composition. The one-way time-of-flight from the laser range is added to the laser fire time to obtain the bounce point time, and the calibrated one-way range vector is projected from the spacecraft state at this time along the boresight to obtain the surface bounce point position in inertial coordinates with respect to the asteroid center of mass. The inertial position is then rotated to the asteroid body-fixed coordinate system using satellite/planet position and orientation files (PCK) supplied by the project. The asteroid-located radius vector is given in both radial coordinates and as a Cartesian position vector. The angle subtended by the bounce point to NEAR spacecraft position vector and the radius vector is calculated. This angle is loosely termed the EMISSION_ANGLE, assuming a spherical body. Normally an emission angle greater than 90 degrees would indicate a missed shot, but due to the irregular shape of 433 Eros, emission angles >120 degrees were obtained. The angle subtended by the NEAR to Eros center vector and the NEAR to bounce point vector (OFF_NADIR angle) is also provided. Lastly, the potential due to gravity and rotation at the shot location is calculated [CHENGETAL2001] assuming a uniform density of 2670 kg m^-3 and the NLR125AR.IMG shape model. The potential is evaluated via a surface integral over a 2 by 2 degree grid, refined to 1/4 by 1/4 degree in the vicinity of the shot. Data ==== Data are stored on CD-WO volumes. A CD volume was created for delivery of data to PDS. ASCII profiles are stored in the /LEVEL2/TOPO directory with file names of the form LyydddNv.TAB, where yy and ddd are numeric year and day-of-year, v is a version identifier, and TAB denotes that this is an ASCII table. Ancillary Data ============== None. Coordinate System ================= NLR data files use an EROS centered body-fixed coordinate system with positive east longitude. The transformation from inertial (J2000) coordinates to body-fixed is represented by the project-supplied SPICE format attitude and orientation files used to generate level 2 data products. Software ======== None. Media/Format ============ The archival data set was written on CD-WO by the NEAR/NLR Project. The CD-WO volumes conform to ISO 9660 Level 2 standards. Data Set Overview ================= The NEAR Laser Rangefinder (NLR) Level 3 Data Products includes spherical harmonic shape and gravity models, and gridded maps or images of those models. Parameters ========== Spherical harmonic models are tables of coefficients GM, Cmn, and Smn. ASCII data type (SHA) formats are defined. Each file contains 2 tables: a header table containing general parameters for the model (gravitational constant, its uncertainty, degree and order of the field, normalization state, reference longitude, and reference latitude) and a coefficients table (degree m, order n, coefficients Cmn and Smn, and their uncertainties). Gridded Digital Map files are image representations of shape and other parameters. Gravity and topographic values may be displayed using this data type. Data are formatted as PDS image objects. Processing ========== The Level 2 NLR profiles in the 200 by 200 km orbital phases and lower are the source of the NLR shape models. Where two ground tracks intersect, the residual is calculated by linear interpolation between adjacent shots. The crossover residuals are used as a measure of data quality and can be used to remove systematic errors in spacecraft ephemeris and pointing. Crossover corrections are implemented in two stages via the software described in [NEUMANNETAL2001], as modified for NLR. The first stage adjusts for long-wavelength errors due to mis-modeled ephemeris and pointing, at a period of four cycles per asteroid revolution. The second stage adjusts higher-frequency errors at a period of ~5 minutes, primarily due to residual pointing mismodeling. Adjustments are iterated until the crossover residual root-mean-square error is less than 10 m. The adjusted profiles are merged by binning observations within a given grid cell, uniformly spaced in longitude and latitude. The median value and the number of observations in each bin comprise the gridded data product. The gravity field coefficients were obtained by solving systems of equations with thousands of unknowns. Radio tracking data in long arcs delimited by propulsive maneuvers, occultations, etc. were used in solutions that were obtained iteratively and by least squares. Data ==== Data are stored on CD-WO volumes. A CD volume was created for delivery of data to PDS. ASCII spherical harmonic models are stored in the /LEVEL3/SHA directory with file names of the form NLRknnnv.SHA, where nnn is a 3-character modifier specified by the data producer, v is a version identifier, and k is replaced by G for gravity field or T for topography/shape field, and SHA denotes that this is an ASCII file of spherical harmonic coefficients. Gridded map products are stored in the /LEVEL3/IMG directory. Each image is a binary array of 16-bit integers with one image line per file record. The image file name is in the form NLRnnnvk.IMG, where nnn is the bin size, v is a version identifier, and k is replaced by C for counts or R for radius, and IMG denotes that this is a PDS image object. For example, a one-eighth degree resolution topography image would be named NLR125vR.IMG. Its format would be described by a detached PDS label in the file NLR125vR.LBL. An ASCII tabular file containing global shape data of Eros, including the latitude and longitude of the NLR laser bounce point, the radius, and the number of observations (counts) can be found in the /LEVEL3/ASCII directory. Ancillary Data ============== None. Coordinate System ================= NLR data files use an EROS centered body-fixed coordinate system with positive east longitude. The transformation from inertial (J2000) coordinates to body-fixed is represented by the project-supplied SPICE format attitude and orientation files used to generate level 2 data products. Software ======== None. Media/Format ============ The archival data set was written on CD-WO by the NEAR/NLR Project. The CD-WO volumes conform to ISO 9660 Level 2 standards. These data are available on-line from the Planetary Data System (PDS) at: http://pds-geosciences.wustl.edu/GEO/near-a-nlr-5-cdr-eros-orbit-v1/nleros_2001/

Alternate Names

  • NEAR NLR LEVEL 3 DATA PRODUCTS V1.0 (PDS)
  • NEAR-A-NLR-5-CDR-EROS/ORBIT-V1.0
  • NEAR-A-NLR-6-EROS-MAPS-MODELS-V1.0

Discipline

  • Planetary Science: Geology and Geophysics

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Maria T. ZuberData ProviderMassachusetts Institute of Technologyzuber@tharsis.gsfc.nasa.gov
Dr. David E. SmithGeneral ContactNASA Goddard Space Flight Centerdavid.e.smith@nasa.gov
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