## More Force Model Options

The dialog launched by the More Options... button is designed for expert users to control certain settings that influence propagation, although their effects are usually small. The default settings are usually sufficient for accuracy.

### Drag Options

The following options relate to the computation of atmospheric drag:

Option Description
Use Approximate Altitude If this option is checked, the drag model uses an approximate expression to determine altitude, instead of finding the exact altitude, when computing density. The density of the model itself is more uncertain than the difference produced with the two altitude measures, and the approximate expression is faster to evaluate than the exact expression, which uses an iterative procedure.
Use Apparent Sun Position If this option is checked, density models that use the position of the sun as part of their computations will use the apparent position of the sun; otherwise, they will use the true position of the sun. Most density models do not distinguish between True and Apparent sun position, though the apparent position is believed to be more consistent with the physics of the atmosphere.

The following options relate to the computation of solar radiation pressure (SRP):

Option Description
Method to Compute Sun Position This specifies the direction of the Sun for SRP computations. Select one of the following options:
• Apparent Sun to True CB -- takes into account the time required for light to travel from the sun to the central body.
• Apparent -- takes into account the time required for light to travel from the sun to the position of the spacecraft.
• True -- assumes that light from the sun reaches the spacecraft instantaneously.
Atmospheric Altitude for the shape of the central body for Eclipse When computing SRP, STK must account for penumbra and umbra regions of eclipse caused by the central bodies obstruction of the Sun from the vehicleâ€™s point of view. (By default, the surface shape is used, corresponding to an atmospheric altitude of 0.0 km). Thus, attenuation and refraction of solar radiation through the atmosphere is not accounted for. A simple model to account for some measure of attenuation is simply to increase the shape of the central body by some altitude height, often taken to be 23 km for the Earth.

### Satellite Options

Option Description
Satellite Mass

This field lets you enter a value for mass, in the desired units, to be used in atmospheric drag and solar radiation pressure calculations.

Note: When the HPOP propagator is selected, the value entered in this field overrides any value for mass entered on the satellite Mass page.

Include Relativistic Accelerations Select this option to model the effects of general relativity in accordance with IERS Technical Note 32, IERS Conventions (2003).
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).

### Earth Tides

Tidal forces, both solid and ocean, arise from changes to the Earth's geopotential induced by variations in the mass distribution of earth. The primary contribution to the solid tide force arises from the gravitational effect of the Sun and Moon, which deforms the shape of the Earth. Secondary effects result from ocean loading on the crust and wobbles of the mantle and core region. Ocean loading itself is modeled by the ocean tides. Tidal forces are modeled in accordance with IERS Technical Note 32, IERS Conventions (2003). Both solid and ocean tide forces are very small, with the ocean tide force usually smaller (though of the same order of magnitude). Tidal force contributions are most important for LEOs; for MEOs (e.g., 2 revs per day), the tidal forces are two orders of magnitude less than the next larger force contribution, and for GEOs even less.

See the International Earth Rotation and Reference Systems (IERS) Web site for more information.

##### Solid Tides (Earth)

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, as 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 attribute controls whether part 2 is computed as well.

The following options relate to the computation of solid tides:

Option Description
Truncate to Gravity Field Size Excludes the solid tide terms beyond the degree and order selected for the gravity model itself.
Include Time Dependent Terms This option applies only if Solid Tides under Central Body Gravity on the Force Model page is set to Full tide.
Minimum Amplitude Includes only those constituents whose tidal amplitude is sufficiently large by specifying minimum amplitude value to include in the computation. Contributors that are below the minimum amplitude will not be factored into the computation.
##### Ocean Tides (Earth)

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 ..\STKData\CentralBodies\Earth directory. The file contains over 1900 contributions to the geopotential field.

Users may choose to limit the ocean tide model to contributions of a specified maximum degree; additionally, the user may choose to include only those constituents whose tidal amplitude is sufficiently large by specifying the minimum amplitude to include in the computation. Note that there are only about 10 constituents with an amplitude larger than 0.5 mm --- and each is less than 1.0 mm.

The Max Order is the maximum order of geopotential coefficients to be included for ocean tide gravity computations.

The ocean tide model includes the 2,1 contribution to the ocean pole tide, as described in the March 2006 update to the IERS Technical Note 32, IERS Conventions (2003). 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.

### Propagator Plugin

Note: The Propagator Plugin option is unavailable if you are defining a force model vector.

A single force model plugin point is provided for the customization of the accelerations used in the propagation of the satellite trajectory. Typical uses of the force model plugin are to implement satellite specific solar pressure, drag and lift models.

Plugins can be written in Perl, VB script, C++, etc.

#### Using the Plugin Force Model

A plugin must be registered. Register the script by opening the Command Prompt window, navigate to the folder where the plugin is stored and enter `regsvr32 <plugin name>`. For example, if the plugin is named `SatXXXBoxWingPlugin.wsc`, enter:

`regsvr32 SatXXXBoxWingPlugin.wsc`

The plugin interface offers the following options:

Option Description
Use Select to use the Plugin model.
Plugin Name Enter the complete plugin name, e.g. `SatXXXBoxWingPlugin.wsc`.
Plugin Settings Click to open the HPOP Plugin Settings window and set parameters for the selected plugin.

For detailed help on the HPOP force model plugin, navigate to Library Reference -> STK Plugins -> STK Engine Plugins -> AGI AgAsHpopPlugin 10 in the STK Programming Interface help.

Regsvr32.exe requires administrator rights in order to register a plugin. Once registered, it's available to all users on that machine (provided that the actual file location on the disk does not change).

It's sometimes difficult to find a system administrator to register the plugin though. In this case we have provided a VBScript utility "register_wsc_hkcu.vbs" located in your install under <INSTALL DIR>\CodeSamples\Extend (where <INSTALL DIR> is typically C:\Program Files\AGI\ODTK 6 (32-bit Windows) or C:\Program Files (x86)\AGI\ODTK 6 (64-bit Windows)).

The utility will register a Windows Script Component to the HKEY_CURRENT_USER area of the Windows registry rather than HKEY_LOCAL_MACHINE. To run the utility, open up a command prompt and change directories to the location of the utility. Then enter

```cscript register_wsc_hkcu.vbs "Your WSC Name.wsc"```

replacing "Your WSC Name.wsc" with the actual full path to the location of your Windows Script Component (WSC) file. The utility will parse your WSC file and make the appropriate entries into the Windows registry. This utility must be run for each user on the machine if they want to use that plugin.