| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Dawn Gravity Archive Data Collection of Science Data Products (SDP)
includes data products generated from gravity investigations
conducted by members of the Dawn Gravity Team while the spacecraft
was in orbit around the asteroid Ceres.
Gravity SDPs include spherical harmonic models, maps or images
of those models, and possibly line-of-sight acceleration profiles.
A group at the Jet Propulsion Laboratory JPL under the direction of
Ryan Park produced spherical harmonic models and maps.
At Ceres, the mission was divided into different science orbits.
All the orbits were polar. The RC3 orbit was conducted at an alitutde
of 13500 km, The Survey orbit was performed at a nominal altitude of
4400 km. The High Altitude Mapping Orbit, or HAMO, was performed
at a nominal altitude of 1450 km. The Low Altitude Mapping Orbit,
or LAMO, was performed at a nominal altitude of 375 km. An extended
mission phase XMO1 was conducted at the same altitude as LAMO.
Additional extended mission phases of XMO2, XMO3 and XMO4 were
conducted at altitudes of 1480 km, 7520-9350 km, and 20000 km.
Between these science orbits, the spacecraft as in a transfer phase
using the electric ion engines [RUSSELL&RAYMOND2011].
Modification History
====================
Version 4 of this data set includes includes isostatic, Kaula, and
nominal versions of the CERES70E gravity field coefficients. The
CERES70E gravity model as described in [PARKETAL2020].
Version 3 of the gravity field is consistent with the final SPC shape
model of Ceres [PARKETAL2019]. Additionally, data are weighted
differently in this delivery resulting in small changes to the
uncertainty values of the gravitational field parameters. Version 3
includes all updates to the V2.0 dataset, plus these changes, i.e. fixing
CENTER_LONGITUDE in the data labels, adding an updated coordinate system
document and modification of the data products consistent with this
coordinate system.
Version 2 of the gravity field corrected the location of the Kait crater.
During the development of higher-level data products for Dawn, the
Gravity team discovered that the Gravity team and the imaging team were
using different craters to define the coordinate system. Under
concurrence from the Dawn project, the gravity team corrected the gravity
field products to align with data products used by other teams.
The corrected location of the Kait crater is a rotation of -0.064074 deg
about the z-axis. The corresponding version 2 of the gravity field
re-estimates the gravity field parameters, pole right ascension,
declination, prime meridian, and rotation rate. The version 2 gravity
field fixes the y-axis coordinate to align exactly with the Kait landmark
crater. Details of the coordinate system are described in the
corresponding labels for the DATA/SHADR and DATA/SHBDR data products.
Because the gravity field was re-estimated in this new frame, the
gravitational field parameters and pole frame are slightly different
from the previous version. The change is large enough to affect the
DATA/SHADR and DATA/SHBDR data products, which have been updated in the
version 2 dataset but not large enough to affect the 1-degree resolution
of the DATA/RSDMAP products, which remain unchanged.
Parameters
==========
Spherical harmonic models are tables of coefficients GM, Cmn,
and Smn -- as in equation (1) of [TYLERETAL1992]. These can
be used to represent gravitational potential of Ceres, for
example. ASCII (data type SHA) formatted spherical harmonics
are defined. Each file contains up to four 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); a names table, giving the order in
which coefficients appear; a coefficients table (degree m,
order n, coefficients Cmn and Smn, and their uncertainties).
Radio Science Digital Map files are image representations
of gravity and other parameters. Free air gravity, geoid,
Bouguer anomaly, isostatic anomaly, and topographic values
may be displayed using this data type. Data are formatted
as PDS image objects.
Processing
==========
Spherical harmonic models, maps, and line-of-sight acceleration
profiles are derived from raw radio tracking data in several
steps.
The tracking data are processed in large orbit determination
programs that integrate the equations of motion (DPODP at JPL
[MOYER1971]), 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
planetary and third body gravity, solar radiation pressure,
planetary radiation pressure, atmospheric drag, solid body tides,
and relativity, where applicable.
The gravity field coefficients are obtained by accumulating
normal equations from often hundreds of data arcs, and
solving these systems of linear equations with thousands of unknowns.
The unknowns include arc parameters, particular to one data arc (such
as the spacecraft state, radiation pressure scale factors,
atmospheric drag scale factors, etc.) and common parameters
(such as the gravity coefficients, the planetary gravitational
constant or GM). Radio tracking data are processed in arcs delimited by
propulsive maneuvers, occultations, etc.
The spacecraft periodically performed angular momentum
desaturation maneuvers. These maneuvers allowed the reaction
wheels to spin down to avoid damage, but they had be countered
the use of thrusters. Arcs may be delimited by these maneuvers.
The details of each of these maneuvers specified in the small forces
file of the Dawn Ceres Raw Data Archive.
Useful references which describe the procedures applied in
general to processing Ceres orbiter tracking data include
[PARKETAL2016].
[THORNTON&BORDER2003] is a general reference for Orbit Determination.
Data
====
Data are available online through the Planetary Data System
(http://pds.nasa.gov). A volume of reduced data was prepared for
the Dawn mission at Vesta and a similar volume for Ceres.
ASCII spherical harmonic models are stored in the DATA/SHA directory
with file names of the form GTsss_nnnnvv_SHA.TAB where:
'G' denotes the generating institution
'J' for the Jet Propulsion Laboratory
'T' indicates the type of data represented
'G' for gravity field
'sss' is a 3-character modifier specified by the data producer.
This modifier is used to indicate the source spacecraft or
project, such as V for Vesta or C for Ceres.
'_' the underscore character is used to delimit modifiers in
the file name for clarity.
'nnnnvv' is a 4- to 13-character modifier specified by the data
producer. Among other things, this modifier may be used to
indicate the target body, whether the SHADR contains primary
data values as specified by 'T' or uncertainties/errors,
and/or the version number. For Dawn, this specifies the
degree and version of the field.
'_' the underscore character is used to delimit information in
the file name for clarity.
'SHA' denotes that this is an ASCII file of Spherical Harmonic
coefficients
'.TAB' indicates the data is stored in tabular form.
Each SHADR file is accompanied by a detached PDS label; that label is
a file in its own right, having the name GTsss_nnnnvv_SHA.LBL.
Binary spherical harmonic models are stored in the DATA/SHB directory
with file names of the form GTsss_nnnnvv_SHB.DAT where:
'G' denotes the generating institution
'J' for the Jet Propulsion Laboratory
'T' indicates the type of data represented
'G' for gravity field
'sss' is a 3-character modifier specified by the data producer.
This modifier is used to indicate the source spacecraft or
project, such as V for Vesta or C for Ceres.
'_' the underscore character is used to delimit modifiers in
the file name for clarity.
'nnnnvv' is a 4- to 6-character modifier specified by the data
producer. Among other things, this modifier may be used to
indicate the target body, whether the SHBDR contains primary
data values as specified by 'T' or uncertainties/errors,
and/or the version number. For Dawn, this specifies the
degree and version of the field.
'_' the underscore character is used to delimit information in
the file name for clarity.
'SHB' denotes that this is an Binary file of Spherical Harmonic
coefficients and covariance
'.DAT' indicates the data is stored in a binary data file.
Each SHBDR file is accompanied by a detached PDS label; that label is
a file in its own right, having the name GTsss_nnnnvv_SHB.LBL.
Radio Science Digital Map products are stored in the DATA/RSDMAP
directory with file names of the form GTsss_ffff_nnnn_cccc.IMG where:
'G' denotes the generating institution
'J' for the Jet Propulsion Laboratory
'T' indicates the type of mission data represented
'G' for gravity field
'sss' is a 3-character modifier specified by the data producer.
This modifier is used to indicate the source spacecraft or
project, such as V for Vesta or C for Ceres.
'_' the underscore character is used to delimit information
in the file name for clarity.
'ffff' is a 4- to 6-character modifier specified by the data
producer to indicate the degree and order of the
solution for the gravity field, topography or magnetic
field.
'_' the underscore character is used to delimit information
in the file name for clarity.
'nnnn' is a 4- to 8-character modifier indicating the type
of data represented
'ANOM' for free air gravity anomalies
'ANOMERR' for free air gravity anomaly
errors (1)
'GEOID' for geoid
'GEOIDERR' for geoid errors (1)
'BOUG' for Bouguer anomaly
'ISOS' for isostatic anomaly
'TOPO' for topography
'MAGF' for magnetic field
(1) Geoid and gravity anomaly errors are computed
from a mapping of the error covariance matrix
of the gravity field solution.
'_' the underscore character is used to delimit information
in the file name for clarity.
'cccc' is a 4-character modifier specified by the data producer
to indicate the degree and order to which the potential
solution (gravity, topography or magnetic field) has
been evaluated. In the case of the error maps for the
gravity anomalies or geoid error, this field indicates to
which maximum degree and order the error covariance was
used to propagate the spatial errors
'.IMG' indicates the data is stored as an image.
Each RSDMAP file is accompanied by a detached PDS label; that label is a
file in its own right with name GTsss_ffff_nnnn_cccc.LBL.
Coordinate System
=================
Dawn Gravity SDP files use a Ceres centered body-fixed coordinate system
similar to the IAU coordinate system. The values differ slightly
because the the orientation of Ceres is estimated in the orbit
determination process.
See the coordinate system document in the DOCUMENTS directory:
DOCUMENT/CERES_COORD_SYS_180628.LBL
and the labels of specific gravity products for details.
Software
========
None.
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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