Earth Definition

Specify the following parameters related to the Earth:

Earth Definition Parameters
Parameter Description
GravityModel Select a gravity model from the dropdown list.
OverrideGravityProcessNoise Set to true to override process noise information specific to the selected gravity model file (.grv) and use the heritage JGM2 based gravity process noise model. This flag will be enabled when loading scenarios created in ODTK versions 5.1 and earlier when the selected gravity field file did not contain process noise information in the earlier version.
NutationMethod

Provides the capability to set the Nutation method that is used in the fundamental FK5 based coordinate transformations. The following options are provided:

  • Explan1990 - the IAU1980 Theory of Nutation, as published in the 1990 draft "Explanatory Supplement to the Nautical Almanac". It was repeated exactly in the 1992 final version. This model computes explicitly term-wise, evaluating all 106 terms. (A typographical error in Table 3.222.2 has been corrected.)
  • IERS1996 (default) - another representation of the IAU1980 Theory of Nutation, as published in "IERS Technical Note 21, IERS Conventions (1996)" pages 22-25. This model is also evaluated explicitly term-wise.
  • JPLFile - retrieves the IAU1980 nutation transformation from the JPL DE405 planetary ephemeris tape, and uses the JPL-recommended algorithms for interpolating to the time of interest.

In terms of gross orbit accuracy, all three methods are equivalent. However, in terms of validation of compatibility with other products, the selection of a nutation method may be critical. Note the following:

  1. The JPLFile model provides the fastest calculation, but some agencies prefer an explicit evaluation of the algorithm and require it for all segments. There will be differences between these two approaches in some of the least significant digits.
  2. STK uses JPLFile as a default, while ODTK uses IERS1996 as a default. If a user wants to create an ECF ephemeris in STK and import it into ODTK and preserve all significant digits of accuracy, then the user should use the JPLFile option in ODTK.
  3. There are minor differences between the IERS1996 and Explan1990 nutation methods in the Fundamental Arguments, which could impact compatibility of products developed under ODTK.

Note: This selection only affects FK5 related transformations. When SuppressICRF is set to false, the IAU2000A theory is used to relate the inertial frame (ICRF) and the Earth Fixed frame, and this theory is not affected by this setting. However, the transformation between the ICRF and the J2000 (FK5) frame is affected by this setting since this transformation involves computing the transformation between the ICRF and the pseudo-fixed frame and computing the transformation between the pseudo-fixed frame and the J2000 frame.

EclipsingAtmosAlt Used for modeling of solar radiation pressure. Specify a value to be added to the Earth's radius for purpose of determining when solar radiation is eclipsed by the Earth.
GroundReflectionModel File of Earth surface reflectivity coefficients used for modeling Earth albedo and thermal radiation pressure.
EOPData

Set the following attributes:

  • Filename - Click this field, then select and load an EOP file.
  • StartTime - Read-only date/and time field based on data in the EOP file.
  • StopTime - Read-only date/and time field based on data in the EOP file.
  • ValidateTimeSpan - Set to true (default) to issue warning when EOP or flux data is out of date.
  • WarnThreshold - Specify the time span (default = 1 day) to trigger a warning message. If the user runs the filter with the Stop mode set to LastMeasurement, the span from filter start to filter start + threshold is checked to see if the EOP and flux data cover the span. Otherwise, the actual filter stop time is used and the threshold is ignored.
FluxData Read from a file or input static values.
Ionosphere
  • IRI2007DataDirectory - Specify the data directory path which contains the support data for the IRI 2007 ionosphere model.
  • UseAlternateApData - Use Ap values from the space weather source file specified by the AlternateApDataFilename attribute.
  • AlternateApDataFilename - The ionosphere model will use Ap values from this file when UseAlternateApData is true. The default file is SpaceWeather-All-v1.2.txt.

Note:Existing scripts that set IRI2001DataDirectory will need to be changed to IRI2007DataDirectory. For example: this VBScript statement works with ODTK 6.0.4 or earlier:

ODTK.Scenario1.EarthDefinition.Ionosphere.IRI2001DataDirectory = "C:\Documents and Settings\All Users\Application Data\AGI\ODTK 6\DynamicEarthData\IRI2001ModelData\ "

In ODTK 6.1 change IRI2001 to IRI2007:

ODTK.Scenario1.EarthDefinition.Ionosphere.IRI2007DataDirectory = "C:\Documents and Settings\All Users\Application Data\AGI\ODTK 6\DynamicEarthData\IRI2007ModelData\ "

Gravity Models

ODTK makes the following gravity models available:

Gravity Models
Model Central
Body
Description
WGS84 Earth World Geodetic System 1984, an Earth-fixed reference frame, including an earth model, with primary parameters defining the shape of an Earth ellipsoid, its angular velocity and the earth mass that is included in the ellipsoid reference, and secondary parameters defining a detailed gravity model of the Earth. WGS 84 was created by the Defense Mapping Agency (DMA). This model uses a gravitational parameter (GM) of 3.986004418E+14 and a reference distance of 6,378,137.0 m.
EGM96 Earth Earth Gravity Model 1996, a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. Developed jointly by NIMA, NASA Goddard and Ohio State University. This model uses a gravitational parameter (GM) of 3.986004415E+14 and a reference distance of 6,378,136.3 m.
EGM2008 Earth Earth Gravity Model 2008, a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. Developed jointly by NIMA, NASA Goddard, and Ohio State University. This model uses a gravitational parameter (GM) of 3.986004415E+14 and a reference distance of 6,378,136.3 m.
GEMT1 Earth Goddard Earth Model T1. This model uses a gravitational parameter (GM) of 3.98600436000e+14 and a reference distance of 6,378,137.0 m.
GGM01C Earth GRACE Gravity Model 01C, first version of gravity field based on GRACE satellite data and constrained with terrestrial gravity information. Source: http://www.csr.utexas.edu/grace/gravity/ggm01/
GGM02C Earth GRACE Gravity Model 02C, second version of gravity field based on GRACE satellite data and constrained with terrestrial gravity information. Source: http://www.csr.utexas.edu/grace/gravity/ggm02/
GGM03C Earth GRACE Gravity Model 03C, third version of gravity field based on GRACE satellite data and constrained with terrestrial gravity information. Source: http://www.csr.utexas.edu/grace/gravity/
JGM2 Earth Joint Gravity Model version 2, a model that describes the Earth gravity field up to degree and order 70, developed by NASA/GSFC Space Geodesy Branch, the University of Texas Center for Space Research and CNES. This model uses a gravitational parameter (GM) of 3.98600441500e+14 and a reference distance of 6,378,136.300000 m.
JGM3 Earth Joint Gravity Model version 3, a model that describes the Earth gravity field up to degree and order 70, developed by the University of Texas and NASA/GSFC. This model uses a gravitational parameter (GM) of 3.98600441500e+14 and a reference distance of 6,378,136.300000 m.
WGS72_ZonalsToJ4 Earth

Contains the gravitational parameter and zonal coefficients through J4 used by STK's SGP4 propagator. It uses a gravitational parameter of 3.986007996E+14 and a reference distance of 6378135.0 m.

Caution: This model is not recommended for use with ODTK.

WGS84 EGM96 Earth This model uses the coefficients from EGM96 with the shape model of WGS84. It uses a gravitational parameter (GM) of 3.986004418E+14 and a reference distance of 6,378,137.0 m
WGS84 old Earth Old version of WGS84. This model uses a gravitational parameter (GM) of 3.986005e+14 and a reference distance of 6,378,137.0 m.

Click here for information on the gravity (*.grv) file format.

EOP Data

Use the EOPData.Filename attribute to select an EOP file (.txt) provided by CelesTrak, which can be downloaded from http://celestrak.com/SpaceData (default), or an EOP file (.dat) located in the DynamicEarthData directory.

.dat Files

These files contain Earth Orientation Parameters that include the pole wander values and the UT1-UTC time corrections. These parameters are used when transforming between Earth Fixed and Earth Mean J2000 coordinate frames. All the EOP files have identical formats. The data for these files comes from the USNO series 7 / IERS Bulletin A. The format for the data is as seen in the IERS rapid service section of Bulletin A:

EOP Data Format
Element Unit
MJD N/A
x " (arc second)
error " (arc second)
y " (arc second)
error " (arc second)
UT1-UTC s (second)
error s (second)

The total number of data records is the first entry in the file. The EOP.dat file contains recently observed data (about 5 years worth) and about one year of predict data. This file is updated with every release of STK, and so the actual dates spanned by this file have changed with every release. Older versions of the EOP.dat file have also been included in this directory for users that desire to maintain backward compatability to previous versions. Version identifiers are included in the filename. Users who choose to use the more recent file (EOP.dat) may notice small differences over the previous version of STK. Position differences are generally less than a few meters. Timely updates of the EOP.dat file can be found on AGI's ftp site:

ftp://ftp.agi.com/pub/STKData/CentralBodies/Earth/

The file EOP.dat.all is provided for users that have a need for more historical data; this file contains the more recent data (just as in the EOP.dat file) but in addition contains observed data from as far back as about 2 Jan 1973. Updates of this file are also available on our website.

ODTK's Data Update Utility allows you to update the EOP data files.

Notes

True of Date to Earth Centered Pseudo-Fixed Transformation (Pseudo ECF)

The transformation from TOD to Earth Centered Pseudo-Fixed (Pseudo ECF) involves a single rotation about the z-axis by the apparent Greenwich hour angle. The angle is computed as the sum of the mean Greenwich hour angle and the equation of equinox. The former is also computed as the sum of the mean Greenwich hour angle at zero hour UT1 and the offset angle. The mean Greenwich hour angle is computed using a cubic polynomial in Julian Date (JD) UT1 time past the J2000 epoch. The coefficients of the polynomial are converted from US Naval Observatory circular No. 163, the Document CG-SCF-225C Code Ident 23892 and from the Explanatory supplement to the Astronomical Almanac. The offset angle is based on the Earth rotation rate, which is updated linearly as a function of zero hour JD past the J2000 epoch. The computation of the zero hour UT1 also requires tabulated values of UT1-UTC from the Earth Orientation Parameter (EOP) table.

Pole Wander Transformation

Transformation from the Pseudo ECF reference frame to the Earth Centered Fixed reference frame is based on two small angles taking into account continental drift. The angles are obtained from the Earth Orientation Parameters (EOP) table, which is constructed based on data from the US Naval Observatory. This transformation is the motion of the rotational pole.