Gravity Model Description

The Geosciences Node archives spherical harmonic models and digital maps of the gravity fields of Mars, Mercury, Venus, and Earth's Moon.  Input data are from radio tracking of the spacecraft.

Types of Gravity Data

Spherical harmonic models are tables of coefficients that can be used to represent gravitational potential of a planet.  Both ASCII (data type SHA) and binary (data type SHB) formats exist, with the latter being preferred for large models that include covariance terms.  A spherical harmonic data product contains up to four tables:  a header table containing general parameters for the model; a names table, giving the order in which coefficients appear; a coefficients table; and a covariance table.

Digital maps 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 images.

File Naming Conventions  

ASCII spherical harmonic model file names have the general form

IDsss_nnnnvv_SHA.TAB

where

I denotes the generating institution, one of the following:
   'J' for Jet Propulsion Laboratory
   'G' for Goddard Space Flight Center
   'C' for Centre National d'Etudes Spatiales
   'M' for Massachusetts Institute of Technology  

D indicates the type of data represented, one of:
   'G' for gravity field
   'T' for topography
   'M' for magnetic field  

sss is used to indicate the source spacecraft or project, such as MRO for the Mars Reconnaissance Orbiter  

nnnn  can be used to indicate the degree and order of the gravity field solution or to indicate the target body  

vv is a version letter or number  

SHA denotes that the file contains Spherical Harmonic coefficients as ASCII text.  

For example, JGMRO_110B2_SHA.TAB contains a spherical harmonic gravity model to degree and order 110, version B2, generated from MRO data by a science team at JPL.  

Binary spherical harmonic models have similar file names except that SHB is used to indicate the binary format; for example, JGMRO_110B_SHB.DAT.  

This naming scheme came into common use with the products generated from the MRO mission. Earlier spherical harmonic model files use variations on this scheme, but all include SHA or SHB to indicate ASCII or binary data. All spherical harmonic products are described by a detached PDS label with the same name, extension LBL.

Radio Science Digital Map product file names have the form IDsss_ffff_nnnn_cccc.IMG.  The IDsss portion uses the same convention as the ASCII and binary formats. The 'ffff' modifier is used to indicate the degree and order of the gravity field solution. The 'nnnn' modifier indicates the type of data represented; for example, 'ANOM' for free air gravity anomalies or 'BOUG' for Bouguer anomaly.  The 'cccc' modifier specifies the degree and order to which the potential solution has been evaluated.

Detailed documentation for each gravity model is found in the PDS labels and in the catalog and document directories of the archive volumes.

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