?* NLREROS.TXT
/*
/* This file contains information from the original, submitted data set
/* catalog file for the NLR spherical harmonic shape and gravity models
/* and associated files.
/*
NLR Shape and Gravity Models of 433 Eros
Produced by D. E. Smith
BEAR DATA_SET_ID: "NEAR-A-NLR-6-EROS-MAPS-MODELS-V1.0"
The files present spherical harmonic shape and gravity models of
433 Eros, with accompanying maps and images.
The full, original data set description follows. This description
has been edited to contain the present names of the files and for
any changes made since the original delivery.
Note that the gravity covariance matrix mentioned below has
subsequently been produced and submitted, and is included in this
dataset as the file N393COVAR.TAB.
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.
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.
Review
======
This archival data set was reviewed by the NEAR/NLR Science
Team prior to final submission to the Planetary Data
System (PDS).
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, example data files, CATALOG files, and other
relevant documentation. These materials were distributed to
PDS personnel, the experiment investigator, and others, as
appropriate.
Data Coverage and Quality
=========================
These files contain gravity and shape models and gridded maps
generated from NEAR data collected through February 2001.
The NEAR data included tracking results from the EROS Orbit
Phase.
NLRSHAPE.TAB is a spherical harmonic gravity field model
complete to degree and order 16.
SHAPEMAP_I2.FIT and SHAPEMAP_R4.FIT are two versions of the
same gridded shape map of Eros at a resolution of 0.125 by
0.125 degrees, based on altimetry data acquired by the NLR
instrument. The data include edited NLR observations from 200-km
orbits and lower, using the NLR393 orbit solutions produced at
NASA-GSFC.
COVERMAP.FIT is a data file that accompanies the shape map of
Eros SHAPEMAP_*.FIT at a resolution of 0.125 by 0.125 degrees. It
contains the number of observations per pixel that have been
used to produce the NLR shape data product. The data include
edited NLR observations from 200-km orbits and lower.
NLRSHAPE.TAB is a 1 degree resolution tabular data file that
contains global shape data of Eros, including the latitude and
longitude of the NLR laser bounce point, the radius, and the
number of observations (counts).
Limitations
===========
There is NO gravity covariance matrix on these volumes.
This product is useful in understanding the errors in the gravity
solution. The gravity field covariance represents the complete
error description for the gravity field solution, including the
coefficient variances (sigmas^2), and the correlations between
all the coefficients.
This product was not available at the time this archive was
completed. The file and documentation will be available at a
later date on the World Wide Web/Internet at the following URL:
http://pds-geophys.wustl.edu/pds/near_nlr.