Propagator Function Components

The Propagator Functions folder contains components used to define propagation models. Components stored in this folder - or any of its sub-folders - can be embedded into custom propagator components to define their characteristics. In addition, all of them can be duplicated and used in creating new models. A brief description of the component is displayed when you highlight it in the Component Browser. To view the elements of a model (and edit them, if it is a copy), double-click it in the Component Browser, bringing up its Component Edit window.

The Propagator Function sub-folders are:

Atmospheric Models | Gravity Models | Plugins | SRP Models | Third Bodies

Miscellaneous Propagator Functions

In addition to the components found in the sub-folders, the Propagator Functions folder contains two miscellaneous components that can be added to a propagator:

Component Description
General Relativity Models the effects of general relativity in accordance with IERS Technical Note 32, IERS Conventions (2003).
Radiation Pressure Models albedo and thermal radiation pressure of the central body in the force model. If you create a duplicate of this component, you can select to include albedo and/or thermal radiation pressure, and can specify a Ground Reflection Model.
Yarkovsky Effect A proxy for the Yarkovsky effect, which impacts the orbits of comets and asteroids due to the sublimation of gasses. This proxy effect is a means for implementing some of the non-gravitational parameters supplied by the JPL Small Body Database and is consistent with "Cometary Orbit Determination and Nongravitational Forces", D.K. Yeomans, P.W. Chodas, G. Sitarski, S. Szutowicz, M. Krolikowska, in Comets II, University of Arizona Press (2004), pp137-151.

Atmospheric Models

Model Description
Blended Density Combines an upper and lower altitude model with a blended overlap range of between 0 and 1,000 km. The Harris-Priester, Jacchia 1960, MSISE 1990, NRLMSISE 2000, and U.S. Standard Atmosphere models are valid for the lower altitude portion, while any other model, except the exponential models, is valid for the upper altitude portion.
CIRA 72 Empirical model of atmospheric temperature and densities as recommended by the Committee on Space Research (COSPAR). Similar to the Jacchia 1971 model but uses numeric integration rather than interpolating polynomials for some quantities. Lower altitude boundary is 90 km.
Exponential This model uses the following equation to calculate atmospheric density:

atmospheric density equation

where = density at a specified altitude, h = specified altitude, = reference density, h0 = reference altitude, and H = scale altitude.
Harris-Priester Takes into account a 10.7 cm solar flux level and diurnal bulge. Uses density tables. Valid range of 0-1000 km.
Jacchia-Roberts Similar to Jacchia 1971 but uses analytical methods to improve performance. Lower altitude boundary is 90 km.
Jacchia 1960 An outdated atmospheric model provided for making comparisons with other software. Lower altitude boundary is 0 km.
Jacchia 1970 Computes atmospheric density based on the composition of the atmosphere, which depends on altitude as well as seasonal variation. Valid range is 100-2500 km.
Jacchia 1971 Similar to Jacchia 1970, with improved treatment of certain solar effects.
US Standard Atmosphere Standard model with no user-specified parameters.
Mars GRAM 2000

These models, published by NASA, are valid for use with Mars as a central body. The data files for these models are installed with the STK Planetary Data Supplement. Primary information about these models is included with PDF files also installed with the Planetary Data Supplement in the STK installation directory under \STKData\CentralBodies\Mars\MarsGRAM. 

The NASA Space Environments and Effects Program website (https://see.msfc/nasa.gov/model-Marsgram) contains additional information about these models.
Mars GRAM 2001
Mars GRAM 2005
Mars GRAM 2010
Mars GRAM 3.7
MSIS 1986 Empirical density model developed by Hedin based on satellite data. Finds the total density by accounting for the contribution of N2, O, O2, He, Ar, and H. 1986 version, valid range of 90-1000 km.
MSISE 1990 Empirical density model developed by Hedin based on satellite data. Finds the total density by accounting for the contribution of N2, O, O2, He, Ar, and H. 1990 version, valid range of 0-1000 km.
NRLMSISE 2000 Empirical density model developed by the US Naval Research Laboratory based on satellite data. Finds the total density by accounting for the contribution of N, N2, O, O2, He, Ar and H. Includes anomalous oxygen. 2000 version, valid range of 0-1000 km. This implementation always calls the gtd7d routine (in contrast to switching between it and gtd7) per the recommendation of Mike Picone, one of the code authors.
DTM 2012 The Drag Temperature Model (DTM), 2012 version, is a semi-empirical model which computes the temperature, density, and composition of the thermosphere. Developed at CNES. Valid range of 120 – 1500 km.

The MSIS models are available at http://ccmc.gsfc.nasa.gov/modelweb/.

Atmospheric Model Parameters

The atmospheric models other than the exponential models are comprised of one or more parameters that can be edited in a duplicate model. These parameters, some or all of which may be present in each model, are defined in the table below.

Parameter Description
Use Approximate Altitude Select to have the drag model use 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.
Sun Position Type

Define the direction of the Sun for the model. 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.
Source Select to define Solar and Geomagnetic Flux using Constant Values or Data File. The selection in this field determines the three parameters beneath it:
  • Constant Values - Daily F10.7, Average F10.7, Kp
  • Data File - File, Geomag Update Rate, Geomagnetic Flux Source
Daily F10.7 Defines the daily Ottawa 10.7 cm solar flux value.
Average F10.7 Defines the 81-day averaged Ottawa 10.7 cm solar flux value.
Kp Planetary geomagnetic flux index, Kp.
File

Browse to the path and name of a flux file. A flux file contains flux data (ap, kp, f10.7, and avg f10.7) for each date. The geomagnetic flux data (ap/kp) includes a daily value and eight values measured at three-hour intervals for each date. STK reads both the ap and kp data from a file, and each density model uses the appropriate data natively.

The following file formats are supported:

When reading the flux file, the observation time for F10.7 data is 20:00 UTC (the time at which the value begins to apply for each day listed in the file) and when using F10.7 values from the flux file, the value of F10.7 at any given time is found using linear interpolation of the table of data.

When reading fxm flux files, which contain only adjusted F10.7 values, the values listed in the file are corrected by the Sun-Earth distance to obtain observed F10.7 values.

All three files can be reported using the Geo Mag Flux Tool under Scenario Tools.
Geomag Update Rate Select an update rate from the following options. Note that these updates cause (small) discontinuous changes in the drag perturbation force.
  • Daily - Updates using the daily ap/kp value for the entire day.
  • 3-Hourly - Updates using the eight values measured at three-hour intervals.
  • 3-Hourly Interpolated - Updates by interpolating the three-hour values. The interpolation uses a spline between data points, the average value of which - over a three hour window - is made equal to the ap/kp value for that three-hour value.
  • 3-Hourly Cubic Spline - Updates by interpolating the 3-hourly data using natural cubic splines.

If you select the 3-Hourly update rate when using MSIS models, a special setting of the MSIS model, (SW[9]=-1), is set for the use of present and past ap values when computing density.
Geomagnetic Flux Source Specify whether to use the Kp or Ap data from the flux file (CSSI predicts files always use ap data).
Drag Model Select to calculate drag using the installed Spherical model or a Plugin model. If you select Plugin, click the button to select a registered plugin to use. Click Plugin Settings to configure any parameters that the plugin allows you to set.

Gravity Models

Component Description
Gravitational Force This model provides a complex gravitational force calculation, optionally including solid and ocean tide effects.
TwoBody Force This model is a standard point mass model.

Gravity Model Parameters

The gravity models are comprised of a set of parameters that can be edited in a duplicate model. These parameters are defined in the tables below.

TwoBody Force

Parameter Description
Gravitational Parameter Source

Select the source from which the gravitational value is being supplied:

  • Cb File - gravitational value from editable central body file shipped with STK.
  • JPL DE (Sun, Moon, planets, and Pluto) - Gravitational Parameter used to compute JPL's DE 405 ephemerides. The inner planets are body centered and the outer planets are barycentered.
  • User Specified - enter the desired gravity value in the field provided.
Gravitational Parameter Enter the gravitational parameter to be used for purposes of this gravity model in the selected distance unit cubed per selected time unit squared.
Modify gravity model below this percentage of central body surface Specifies the percentage of the central body's minimum radius at which a modified force model will be used - provided there is no altitude stopping condition. The modified force model will use only the two-body force for the part of the central body still below the satellite. The acceleration becomes , where is the gravitational parameter, r is the position vector, and R is the reference radius of the central body.

Gravitational Force

Parameter Description
Gravity Field Select a gravity file for the central body that you wish to model.
Degree Defines the maximum degree of geopotential coefficients to be included for Central Body gravity computations. Valid range is from 0 to 90, depending on the gravity model.
Order Defines the maximum order of geopotential coefficients to be included for Central Body gravity computations. Valid range is from 0 to 90, depending on the gravity model.
Include Secular Variations 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.

  • None - exclude solid-tide contributions entirely.
  • 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.
  • Full - include the permanent tide plus all other solid-tide modeling contributions.

The solid tide contribution is computed in three parts: (i) the primary contribution from the effects of the Sun and Moon; (ii) a secondary contribution arising from centripetal acceleration loading caused by the earth's rotation; and (iii) a secondary contribution from the effects of other loading of the crust and core. The computation of (iii) can be time-consuming, as it accounts for geopotential variations of degree and order 2 caused by 71 different tide constituents.

Because (iii) 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 Solid Tides to account for this effect. Since (ii) is of the same order as (iii), this attribute controls whether (ii) is computed as well.

To exclude the solid tide terms beyond the degree and order selected for the gravity model itself, select Truncate to Gravity Field Size.

To include only those constituents whose tidal amplitude is sufficiently large by specifying the Minimum Amplitude to include in the computation. Contributors that are below the minimum amplitude will not be factored into the computation.
Ocean Tides

Tidal forces are modeled in accordance with IERS Technical Note 32, IERS Conventions (2003).

  • Maximum Degree - Specify the maximum degree for force contributions that will be included in the computation.
  • Maximum Order - Specify the maximum order for force contributions that will be included in the computation.
  • Minimum Amplitude - Specify the minimum amplitude of force contribution to include in computation. Contributors that are below the minimum amplitude will not be factored into the computation.
Modify gravity model below this percentage of central body surface Specifies the percentage of the central body's minimum radius at which a modified force model will be used - provided there is no altitude stopping condition. The modified force model will turn off geopotential perturbations, and use only the two-body force for the part of the central body still below the satellite. The acceleration becomes , where is the gravitational parameter, r is the position vector, and R is the reference radius of the central body.

Plugins

This folder contains registered force model plugins. Plugins that have been registered for use with HPOP will be available in this folder for use with Astrogator as well.

There is an Astrogator script driver plugin available to run old plugin scripts; refer to Astrogator Plugin Points for more information.

SRP Models

Model Description
AeroT20 SRP Aerospace T20 solar radiation pressure model for GPS block IIA.1
AeroT30 SRP Aerospace T30 solar radiation pressure model for GPS block IIR.1
GSPM 04a-IIA SRP Bar-Sever GPS Solar Pressure Model 04a for block IIA.2
GSPM 04a-IIR SRP Bar-Sever GPS Solar Pressure Model 04a for block IIR.2
GSPM 04ae-IIA SRP Bar-Sever GPS Solar Pressure Model 04ae for block IIA.3
GSPM 04ae-IIR SRP Bar-Sever GPS Solar Pressure Model 04ae for block IIR.3
Spherical SRP Solar radiation pressure model that assumes a spherical spacecraft. The equation used by STK is described in the Solar Radiation technical note.

1. O’Toole, James W., "Mathematical Description of the OMNIS Satellite Orbit Generation Program (OrbGen)", NSWCDD/TR-02/118, May 2004.

Fliegel, H.F., Gallini, T.E., Swift, E.R., "Global Positioning System Radiation Force Model for Geodetic Applications", Journal of Geophysical Research, Volume 97, No. B1, January 1992.

Fliegel, H.F., Gallini, T.E., "Solar Force Modeling of Block IIR Global Positioning System Satellites", Journal of Spacecraft and Rockets, Volume 33, No. 6, November-December 1996.

2. Bar-Sever, Y., Kuang, D., "New Empirically Derived Solar Radiation Pressure Model for Global Positioning System Satellites", IPN Progress Report 42-159, November 15, 2004.

3. Bar-Sever, Y., Kuang, D., "New Empirically Derived Solar Radiation Pressure Model for Global Positioning System Satellites During Eclipse Seasons", IPN Progress Report 42-160, February 15, 2005.

SRP Model Parameters

The SRP models are comprised of a set of parameters that can be edited in a duplicate model. These parameters are defined in the table below.

Parameter Description
Shadow Model Select the type of shadow to be used in determining the lighting condition for the spacecraft.
  • None -- Choosing this option turns off all shadowing of the satellite.
  • Cylindrical -- The cylindrical model assumes the Sun to be at infinite distance so that all light coming from the Sun moves in a direction parallel to the Sun to satellite vector.
  • Dual Cone -- The dual cone model uses the actual size and distance of the Sun to model regions of full, partial (penumbra) and zero (umbra) sunlight. The visible fraction of the solar disk is used to compute the acceleration during penumbra.
Sun Position Type 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 Eclipse When computing SRP, STK must account for penumbra and umbra regions of eclipse caused by the Earth's obstruction of the Sun from the vehicle’s point of view. (By default, the Earth’s 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 earth by some altitude height, often taken to be 23 km.
Other Eclipsing Bodies Click to select other bodies that may eclipse SRP in the model.
Include Boundary Mitigation Select to have the state of the satellite after crossing a shadow boundary corrected for errors that may have been introduced by the sudden change in SRP which occurred during the integration step. If shadowing is turned off, this option is unavailable.
Solar Radius
(Aero and GPS Models) 		Enter a value for Solar Radius or select Use value in Sun.cb file to use the value in STK's Sun central body (.cb) file. The solar radius property is used for eclipsing in the solar intensity computation performed by STK.
Solar values
(Spherical Model)		 Select Use values in Sun.cb file to use the values in STK's Sun central body (.cb) file to define the solar radius and solar flux (using luminosity) values. Otherwise, select Luminosity or Mean Flux and enter a value to define solar flux at 1 AU in the Solar Radiation computation and enter a value for Solar Radius. The solar radius property is used for eclipsing in the solar intensity computation performed by STK.

Third Bodies

This folder contains third body functions for each of the central bodies and for the Earth, Jupiter, Saturn, Neptune, Uranus, and Pluto planetary systems. In addition, if you create a central body, a third body function for it will automatically be created in this folder.

Third Body Parameters

The Third Body models are comprised of a set of parameters that is determined by the selected mode.

Click to select the central body to serve as the basis for the Third Body model. In the Mode field, select Gravity Field to define the gravitational effect as a full Gravitational Force model; select Point Mass to define the gravitational effect as a third body point mass effect.

In the Ephemeris Source field, select the source to use for the location of the central body from the drop-down list. The available selections are dependent on the central body:

Source Description
Cb File This selection will use the default ephemeris source for that central body.
DE File Use the location of the body in the DE file as the ephemeris source. DE files are body centered for the inner planets and barycentered for the outer planets.
SPICE Barycenter Use the SPICE file location for the system barycenter as the ephemeris source for the third body effect.
SPICE Body Centered Use the SPICE file location for the body center as the ephemeris source for the third body effect.

If you have selected the Point Mass gravitational effect, you will need to select a source from the drop-down list in the Gravitational Parameter Source field. The available selections are dependent on the central body:

Source Description
Cb File This selection will use the default body-centered gravitational parameter for the central body.
Cb File - System This selection will use the default barycentered gravitational parameter for the central body.
DE File This selection will use the gravitational parameter specified in the DE file. DE files are body centered for the inner planets and barycentered for the outer planets.
User Specified This selection requires you to enter the desired mu value in the Gravitational Parameter field.

You can select a different source for the gravitational effect than you selected for the ephemeris source, but the sources should be matched in terms of whether they are body centered or barycentered; if they do not match, a warning will appear in the window to advise you against it, but you will be allowed to keep the settings. In addition, if you define a barycentered third body effect for a planet, you should not include separate third body effects for any of its moons or your results will be inaccurate.

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