The gravity tab of the HPOP Force Model Properties window enables you to configure a central body gravity model, solid and ocean tide effects, and third body gravity effects.
Central Body Gravity
The standard Central Body Gravity force model parameters are listed in the following table:
| Option | Description |
|---|---|
| Gravity File | Click the  button to select the gravity file that you want to use for this model. A gravity file is an ASCII file containing the Central Body geopotential model coefficients. You can use one of STK's standard gravity models or import your own user-defined gravity file.
|
| Maximum Degree | The maximum degree of geopotential coefficients to be included for Central Body gravity computations. The range of values is enforced by the central gravity model (.grv). |
| Maximum Order | The maximum order of geopotential coefficients to be included for Central Body gravity computations. The range of values is enforced by the central gravity model (.grv). |
| Include Secular Variations in Gravity Coefficients | If you are using the EGM96 or EGM2008 gravity model, select this option to use the evolution parameters for C20, C21, and S21 in accordance with IERS Technical Note 32, IERS Conventions (2003). | Solid Tides |
This option can be used to include the perturbation of the gravity field caused by the effects of solid tides, providing that the Central Body gravity field has a solid-tide model available.
Additional settings for this parameter are available.
If the selected gravity field for the Earth does not contain its own solid-tide model, the IERS model will be the default solid-tide model. |
Tides (Earth)
Tidal forces - both solid and ocean - arise from changes to the Earth's geopotential that are induced by variations in the mass distribution of earth. The primary contribution to the solid tide force is the gravitational effect of the Sun and Moon. This effect deforms the shape of the Earth. Secondary contributions include ocean loading on the crust and wobbles of the mantle and core region. Ocean loading is modeled by the ocean tides. These forces are modeled in accordance with IERS Technical Note 32, IERS Conventions (2003). Both solid and ocean tide forces are minute, but the ocean tide force is usually smaller (though of the same order of magnitude). Tidal force contributions are most important for LEO satellites. For MEO satellites, the tidal forces are two orders of magnitude less than the next larger force contribution; for GEO satellites, even less.
See the International Earth Rotation and Reference Systems (IERS) website for more information.
Solid Tides (Earth)
From the drop-down menu at the top of this section, select an option for applying the perturbation of the gravity field caused by the effects of solid tides, providing that the Central Body gravity field has a solid-tide model available. This option is only active for Earth gravity files.
- Full tide - include the permanent tide plus all other solid-tide modeling contributions.
- Permanent tide only - include only the permanent (time-independent) tidal contribution of the solid-tide model. By default, gravity fields that do not specify a separable permanent tide are assumed to have the permanent tide included in the gravity field.
- None - exclude solid-tide contributions entirely.
If the gravity field model you selected for the Earth does not contain its own solid-tide model, the IERS model will be the default solid-tide model.
The solid tide contribution is computed in three parts:
Part 1: The primary contribution from the effects of the Sun and Moon.
Part 2: A secondary contribution arising from centripetal acceleration loading caused by the earth's rotation.
Part 3: A secondary contribution from the effects of other loading of the crust and core.
The computation of part 3 can be time-consuming, since it accounts for geopotential variations of degree and order 2 caused by 71 different tide constituents. Because it is time-consuming and represents a secondary contribution to the total solid tide force, it is not included in the computation by default; select Include Time Dependent Terms to account for this effect. Since part 2 is of the same order as part 3, this option controls whether part 2 is computed as well.
Select Truncate to Gravity Field Size to exclude the solid tide terms beyond the degree and order selected for the gravity model itself. Also, you can choose to include only those constituents whose tidal amplitude is sufficiently large by specifying the Minimum Amplitude of a constituent.
Ocean Tides (Earth)
Select Use Ocean Tides to include the perturbation of the gravity field caused by the effects of ocean tides. This option is only active for Earth gravity files.
Like the solid tide contribution, the ocean tide contribution is a time-consuming computation, as it computes geopotential variations of up to degree and order 30, for over 200 tide constituents. Coefficients for the ocean tide model, based on the TOPEX mission, is provided in the ascii file OTIDES.TOPEX_3.0.PURE.ot, located in the <STK install folder>\STKData\CentralBodies\Earth folder. The file contains over 1,900 contributions to the geopotential field.
You can choose to limit the ocean tide model to contributions of a specified Maximum Degree. Also, you can choose to include only those constituents whose tidal amplitude is sufficiently large by specifying the Minimum Amplitude of a constituent.
There are only about 10 constituents with an amplitude larger than 0.5 mm --- and each is less than 1.0 mm.
The Maximum Order is the maximum order of geopotential coefficients to be included for ocean tide gravity computations.
Ocean tides are modeled using the FES2014bv1 model, an updated model based upon the IERS Conventions 2010 (Tech Note 36). Details of that model can be found in the text file <STK install folder>\STKData\CentralBodies\Earth\FES2014bv1OceanTideModel.txt. When ocean tides are on and the Maximum Degree >=2 and The Maximum Order >= 1, then the 2,1 contribution to the ocean pole tide is computed as part of the ocean tide model. The 2,1 term represents about 90% of the ocean pole tide effect.
Third Body Gravity
Besides earth gravity, you can model the effects of gravity from a third body on your satellite's orbit behavior. The Third Body Gravity table is ordered to reflect the maximum possible third body force produced by each central body, when that body is located at its closest to the Earth. Bodies appearing higher in the list can produce a larger third body force than those appearing lower in the list. The actual contribution of a body, however, depends on the distance of that body from the earth during the satellite's time interval. For example, although Jupiter appears in the list right after the Sun and Moon, it is possible for Venus (next in the list) to produce a larger third body force when Venus is near its closest distance to the Earth if Jupiter is near its farthest distance from the Earth.
The ephemeris source for third bodies in HPOP will be inferred from the Gravitational Source setting. Earth, Moon, and Sun always use the Cb file setting for the ephemeris. The setting of JPL DE will cause the central body to use the DE file for its ephemeris (which for the outer planets is barycenter position). If the setting is Cb file, then the barycenter position and system mu value will be used for the outer planets unless the central body is the parent of the satellite central body, in which case the Gm value from the Cb file and the planet centered position is used.
In general, the inclusion of solar and lunar third body gravity contributions for earth-orbiting satellites is enough for accuracy in all but the most demanding applications. The importance of including third bodies in modeling the force environment increases the farther the satellite is away from all central bodies (e.g., deep space spacecraft).
Use the editable grid cells in the Third Body Gravity table to select third body objects for consideration. To consider a body's gravitational pull in computations, set the following: