HPOP Force Models: Additional Models

The Additional tab of the HPOP Force Model Properties window comprises a miscellany of force models that you can use in an HPOP trajectory calculation.

Central body radiation pressure

You can include albedo or thermal radiation pressure from the central body in your HPOP trajectory computation. If you select either of these optional force models, you can customize related properties of the central body for these models.


Select the Include Albedo check box to model the radiation pressure caused by sunlight reflected off the illuminated part of the central body.


Select the Include Thermal check box to model the radiation pressure caused by black-body heat radiation of the central body.

Central body properties

If you select either of the central body radiation pressure options, three central body properties become available. These properties are applied to the computation of the central body radiation pressure models.


This property defines the coefficient of radiation pressure reflectivity of the satellite. A value of 1 indicates full absorption; a value of 2 indicates full reflection.

Area to mass Ratio

This property defines the area exposed to radiation pressure (nominally along the radial direction) divided by the mass.

Ground Reflection Model File

This file is a text file that defines a ground reflection model for albedo and emissivity properties of the central body surface. Click to browse to the file that you want to use.

Satellite options

There are two properties on the Additional tab that can modify the trajectory computations.

Relativistic accelerations

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

Satellite mass

You can use the Satellite Mass field to override the same property on the Mass properties page of the satellite. STK will use the value that you specify here in atmospheric drag and solar radiation pressure calculations with a nonspherical model, such as a GPS-specific solar radiation pressure model or a custom plugin. If you are using a spherical model for drag and solar pressure, then the area-to-mass ratios specified on the Force Model page still apply and the mass specified here only determines the compatible area.

Propagator plugin

If you want to apply a force model that the HPOP propagator doesn't normally consider, you can load a plugin to implement that model. For example, you can provide a solar pressure model that is specific to the satellite that you are modeling. Select the Use Plugin check box to begin.

You cannot use a propagator plugin if you are defining a force model vector.

Register a plugin

You must register a plugin before you can use it with HPOP.

Windows Script Components

A component that is built as a Windows Script Component with a scripting environment (VBScript, JScript, and Python) has the "wsc" file name extension. This format is an XML text file that declares the implementation of the plugin interface. To register the component, take the following steps:

  1. Open a command prompt window.
  2. Browse to the location of the component.
  3. Type regsvr32 your_file and then press Enter. For example:
    regsvr32 VBScript.Example1.Hpop.wsc

Configure a plugin force model

Once you have registered your plugin, you can select it from the Plugin Name menu. If the plugin has any properties that you can customize, click Plugin Settings to edit them in the Hpop Plugin Settings dialog box.

About the HPOP force model plugin point

For detailed information about the HPOP force model plugin point, see STK AgAsHpopPlugin.