Gravity

The gravity model is selected at the scenario level. For the selected model, the following parameters can be set for each satellite in the scenario:

Gravity Model
Parameter Description
GMVal GM (μ) value of the central body. This is a read only parameter that is provided for display purposes and for access by scripting.
Degree & Order Enter a number representing the Degree and Order of the gravity model.
Tides Set SolidTides and/or OceanTides to true and enter appropriate values for additional attributes that appear.
GeneralRelativityCorrection For the Earth, Set to true to model the effects of general relativity in accordance with IERS Technical Note 21, IERS Conventions (1996). For other central bodies, the gravitational parameter and angular momentum values from the IERS conventions are replaced by values appropriate for the current central body.
VariationalEquations

Use the VariationalEquations -> Degree and Order attributes to specify the maximum degree and order of the geopotential used in the computation of the variational equations that are used during the propagation of the state error covariance matrix. The order must be set to be less than or equal to the degree. This setting affects filter and least squares operations if the OrbitErrorTransitionMethod attribute is set to use VariationalEquations.

Note: OD performance will improve for GEO and HEO satellites if the Order is set to 2 or greater.

ProcessNoise

Specify whether process noise is always to be used (set Use to Yes), always to be ignored (set Use to No), or to be used depending on the satellite's orbit class (set Use to Based on Orbit Class). A read-only field - WillUseProcessNoise - indicates whether process noise will be used, given the user's selection and, if relevant, the orbit class of the satellite. If the satellite is in orbit about the Earth and the orbit class is LEO, MEO, GNSS, HEO, HIeHEO or LOeHEO, the gravity process noise model is applied unless the user sets the Use option (see above) to No (i.e., if the user selects Based on Orbit Class or Yes). If the satellite is in orbit about the Earth and the orbit class is anything else, the gravity process noise is not applied unless the user sets the Use option to Yes (i.e., if the user selects Based on Orbit Class or No). See below for further discussion.

The application of gravity process noise may be customized to specify whether errors of omission and/or commission are to be modeled (set the Enabled property to true or false in each case) and to set the scale for each (enter a positive real number where a value of 1 represents the application of process noise based on the formal uncertainty published with the gravity field). Errors of omission represent gravity modeling errors resulting from use of a truncated gravitational potential while errors of commission account for uncertainty in the coefficients of the gravitational potential model.

The integration of radial acceleration errors due to gravity leads to the accumulation of in-track uncertainty. In many cases, this build up of uncertainty is not realized in terms of actual orbit estimate errors due to favorable aliasing of the radial gravity errors into a small error in the semi-major axis estimate that tends to offset the accumulation of in-track error. The MeanMotionUncertaintyScaling setting (enter a real number between 0 and 1) can be used to reduce the impact of the gravity process on in-track uncertainty growth to account for favorable aliasing. Use of this setting is typically only needed when gravity uncertainty is large (central bodies other than the Earth).

The gravity process noise will be computed based on model inputs in the selected gravity file for the central body about which the satellite is in orbit. The inputs for the process noise model are described in the gravity file format description. The generation of these inputs requires significant off-line processing of the variances associated with the gravitational potential function. If process noise is requested and no process noise information is contained in the selected gravity file, the default gravity process noise model based on JGM2 will be used. This should provide a conservative estimate of the gravity process noise for modern geopotential functions.

For satellite orbiting central bodies other than the Earth, gravity process noise is applied for all closed orbits but only if supporting process noise information is included in the gravity file.

ThirdBodies - Settings ODTK gives you the option of modeling the gravitational acceleration from the Sun, Moon or any number of planets. Add selections from a list of available solar system bodies to include their point mass gravitational effects during orbit propagation. The list shows the following three elements:
  • Name - a string containing the name of the third body (e.g., "Moon", "Mars")


  • GM Source - choices are:


    • "Cb File" indicates the GM Value will be obtained from the third body's .cb file, e.g., C:\Program Files\AGI\ODTK 6\STKData\CentralBodies\Jupiter\Jupiter.cb
    • "JPL DE" indicates the GM Value will be read from the JPL DE file.
    • "User Specified" lets the user enter the value manually.
    • "HPOP Historical" indicates that GM Value will be the internal HPOP value (available for the Sun and Moon only)

  • GM Value - shows the value of GM based on the GM Source, in units of m^3/s^2. This value may change as the GM Source changes.

Note: The Satellite and GNSSSatellite object's ThirdBodies.Planets attribute has been replaced with the ThirdBodies.Settings attribute. Backwards compatibility is supported for scripts written for any release previous to 6.0 that may be updating the ThirdBodies.Sun, ThirdBodies.Moon, or ThirdBodies.Planets attributes.

ThirdBodies - UseInVariationalEquations Set to true to have third body accelerations included in the variational equations for the propagation of covariance. It is recommended that this flag be set to true for high altitude satellites.

Note on Gravity Process Noise

By modelling gravity process noise, you are accounting for errors associated with integrating through the gravity field. According to Vallado: "Process noise...is the error in the mathematical modeling of the system dynamics."

REFERENCE: Vallado, David A., Fundamentals of Astrodynamics and Applications, McGraw Hill (1997), p. 665.

The gravity process noise model used here allows the user to account for errors of commission (errors in the gravity field coefficients) and/or errors of omission (errors due to truncating the gravity field). The effect of adding gravity process noise is an increase in the orbit covariance.

ODTK 6.5