Earth Definition

Specify the following parameters related to Earth:

Earth Definition Parameters
Parameter	Description
GravityModel	Select a gravity model from the drop-down menu list.
OverrideGravityProcessNoise	Set this 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. For ODTK versions 5.1 and earlier, ODTK will enable this flag when loading scenarios and when the selected gravity field file does not contain process noise information.
NutationMethod	This provides the capability to set the Nutation method that ODTK uses in the fundamental FK5-based coordinate transformations. Choose from the following options: Explan1990 - This is 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 termwise, evaluating all 106 terms. (A typographical error in Table 3.222.2 has been corrected.) IERS1996 (default) - This is another representation of the IAU1980 Theory of Nutation, published in "IERS Technical Note 21, IERS Conventions (1996)", pages 22-25. This model is also evaluated explicitly termwise. JPLFile - This 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: 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. STK uses JPLFile as a default, while ODTK uses IERS1996 as a default. If you want to create an ECF ephemeris in STK and import it into ODTK and preserve all significant digits of accuracy, then you should use the JPLFile option in ODTK. There are minor differences between the IERS1996 and Explan1990 nutation methods in the Fundamental Arguments that could impact compatibility of products developed under ODTK. 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	ODTK uses this 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	This is a 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 - This is a read-only date and time field based on data in the EOP file. StopTime - This is a read-only date and time field based on data in the EOP file. ValidateTimeSpan - Set this to true (default) to issue a warning when EOP or flux data is out of date. WarnThreshold - Specify the time span (default = 1 day) to trigger a warning message. If you run the filter with the Stop mode set to LastMeasurement, ODTK checks the span from filter start to filter start + threshold to see if the EOP and flux data cover the span. Otherwise, ODTK uses the actual filter stop time and ignores the threshold.
FluxData	ODTK reads this from a file or from input static values.
Ionosphere	IRIxxDataDirectory - Specify the data directory path that contains the support data for the IRI ionosphere model. By default, the IRI model uses the Ap information found in the specified space weather file, but it can also use the data found in the file ap.dat, located in the IRIxxDataDirectory. ApSource - The IRI model requires timely geomagnetic index data (Ap) as input. Choose between using the Ap data located in the IRI data directory (IRIDataFile) and a selected space weather file (SpaceWeatherFile) in support of computation of ionosphere effects on measurements. SpaceWeatherFilename - Specify the file path of a space weather file for ODTK to use for Ap information when you set ApSource to SpaceWeatherFile. The space weather data file became the default source of Ap data for the IRI ionosphere model in ODTK version 7.4. Attempts to compute ionosphere effects using the IRI model beyond the time span of the data results in a computed ionosphere effect of zero. Data files that support the IRI model are available for update, along with other timely data files, through the Data Update Utility. Scripts built prior to ODTK 6.5.2 that set IRI2007DataDirectory will need to be changed to IRI20xxDataDirectory. For example, this VBScript statement works with ODTK 6.5.1 or earlier: `ODTK.Scenario1.EarthDefinition.Ionosphere.IRI2007DataDirectory = "C:\Documents and Settings\All Users\Application Data\AGI\ODTK 6\DynamicEarthData\IRI2007ModelData\ "` For ODTK 6.5.2 and later, change IRI2007 to IRI20xx: `ODTK.Scenario1.EarthDefinition.Ionosphere.IRI20xxDataDirectory = "C:\Documents and Settings\All Users\Application Data\AGI\ODTK 7\DynamicEarthData\IRI2016ModelData\ "`
DSNMediaCalibrationData	This is a list of JPL DSN media correction data files for ionosphere and troposphere as described in 820-013 Deep Space Network External Interface Specification JPL D-16765 TRK-2-23 Media Calibration Interface. Selected files contain power or trigonometric series representations of tropospheric or ionospheric delay corrections for one or more DSN complexes. Delays due to the troposphere represent corrections to a baseline seasonal correction model from C.C. Chao. This baseline model is described in JPL Publication 94-24, “A Comparative Survey of Current and Proposed Tropospheric Refraction-Delay Models for DSN Radio Metric Data Calibration”, 1994 by Estefan and Sovers.
LeapSecondFilename	This is the name of the file from which ODTK will read the history of leap seconds.

Gravity models

ODTK makes the following gravity models available:

Gravity Models
Model	Central Body	Description
WGS84	Earth	World Geodetic System 1984 - It is an earth-fixed reference frame, including an earth model, with primary parameters defining the shape of an earth ellipsoid. The ellipsoid reference includes its angular velocity and the earth mass, as well as secondary parameters defining a detailed gravity model of Earth. WGS84 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 - This is a geopotential model of 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 - This is a geopotential model of Earth consisting of spherical harmonic coefficients complete to degree and order 360. It was 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. The EGM2008 file contains coefficients to degree and order 100.
EGM2008_200x200	Earth	Earth Gravity Model 2008 - This is a geopotential model of Earth consisting of spherical harmonic coefficients complete to degree and order 360. It was 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. The EGM2008_200x200 file contains coefficients to degree and order 200.
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 - This is the first version of a 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 - This is the second version of a 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 - This is the third version of a 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 - This is 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 - This is 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	This 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. AGI recommends not using this model 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	This is the old version of WGS84. It 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 you can download from https://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. ODTK uses these parameters 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 five years worth) and about one year of predicted data. This file is updated with every release of STK, and so the actual dates spanned by this file have changed with every release. AGI includes older versions of the EOP.dat file in this directory for maintaining backward compatibility to previous versions of ODTK. Version identifiers are included in the filename. If you choose to use the more recent file (EOP.dat), you may notice small differences over the previous version of STK. Position differences are generally less than a few meters. You can find timely updates of the EOP.dat file on AGI's FTP site:

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

You can access the file EOP.dat.all for more historical data; this file contains the more recent data (just as in the EOP.dat file) as well as 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 enables you to update the EOP data files.

When Earth Orientation Parameters are required at times past the end of the tabulated data in the selected file, ODTK decays the EOP parameter values to zero over a one-day interval following the final time in the file. To change this behavior to retain the final values in the file, duplicate the last line and change the Modified Julian Date field in the new final line to a large value (such as 75000) to extend the applicability of those values far out into the future.

Notes

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

The transformation from TOD to 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, 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.