HPOP Force Models: Drag

The Drag tab of the HPOP Force Model Properties window comprises options for modeling spacecraft prperties and atmospheric density. HPOP will apply these options in the computation of atmospheric drag accelerations on the satellite. To have HPOP model drag for a trajectory calculation, select the Use Drag check box.

Model

Use the following parameters to specify the vehicle modeling for drag:

Parameter	Description
Type	From the shortcut menu, select one of the following modeling options: Spherical: HPOP assumes a spherical shape for the satellite. This is the default setting. DragModelPlugin: Selecting this enables you to choose from any registered drag plugins.
Cd	If you select Type = Spherical, then enter an atmospheric drag coefficient.
Area/Mass Ratio	If you select Type = Spherical, then enter an area-to-mass ratio in m²/kg.
Name	If you select Type = DragModelPlugin, then select the a plugin model name from the shortcut menu.
Plugin Settings...	If you select Type = DragModelPlugin, then you can click this button to open the Drag Model Pluging Settings dialog box. You can then edit the settings of the plugin you chose.

Atmosphere

HPOP uses the Jacchia-Roberts model as a default atmospheric density model. However, you can use the following options and parameters to define a customized atmosphere model to use in drag calculation.

Parameter	Description
Atmospheric Density Model	Use the shortcut menu to select one of the following models: 1976 Standard is a table look-up model based on the satellite's altitude. It has a valid range of 86 km to 1000 km. CIRA 1972is an empirical model of atmospheric temperature and densities as recommended by the Committee on Space Research (COSPAR). It is similar to the Jacchia 1971 model but uses numeric integration rather than interpolating polynomials for some quantities. The lower altitude boundary is 90 km. DTM 2012 is the Drag Temperature Model (DTM), 2012 version, a semi-empirical model that computes the temperature, density, and composition of the thermosphere. It was developed at CNES. It has a valid range of 120 km to 1500 km. DTM 2020. The Drag Temperature Model (DTM), 2020 version, is developed and maintained by CNES. This is the operational format of the model, which relies on F10.7 and Kp for solar and geomagnetic indices. Harris-Priester takes into account a 10.7 cm solar flux level and diurnal bulge. It has a valid range of 0 km to 1000 km. Jacchia 1960 is an earlier model by Jacchia that uses the solar cycle to predict a value for the F10.7 cm flux and accounts for the effects of the diurnal bulge. The lower altitude boundary is 0 km. Jacchia 1970 is the predecessor to the Jacchia 1971 model. It has a valid range of 90 km to 2500 km. Jacchia 1971 computes atmospheric density based on the composition of the atmosphere, which depends on the satellite's altitude as well as a divisional and seasonal variation. It has a valid range of 100 km to 2500 km. Jacchia-Roberts is similar to Jacchia 1971 but uses analytical methods to improve performance. The lower altitude boundary is 90 km. Mars GRAM models: Mars-GRAM 3.7, Mars-GRAM 2000, Mars-GRAM 2001, Mars-GRAM 2005, Mars-GRAM 2010. Refer to the Notes on atmospheric density models section of this topic for specific information about these models. MSIS models: MSIS 1986, MSISE 1990, NRLMSISE 2000. Refer to the Notes on atmospheric density models section of this topic for specific information about these models. DenistyModelPlugin: If you registered any density model plugins, then the list of Atmospheric Density Models will contain DensityModelPlugin as a choice. All the density models appear as a choice, but only those models with a central body name the same as the central body of the force model can actually be used. After chosen, a shortcut menu appears that contains the list of those registered plugins, using the DisplayName set in the XML registration file for the plugin. If the plugin has registered its own settings, then you can click Plugin Settings... to view and edit those settings. For missile objects, you can only select atmosphere models that are valid to the ground. These are: Harris-Priester, NRLMSISE 2000, MSISE 1990, and Jacchia 1960.
Low Altitude Density Model	Use this shortcut menu to select an alternative atmospheric density model to apply at lower altitudes in combination with any of the upper altitude models. You can select either the MSISE 1990 or NRLMSISE 2000 model if Earth is the central body. Otherwise, you can leave the None default selection in place.
Blending Range	This defines the range of overlap in which HPOP blends the upper and lower altitude atmospheric density models. Valid values are between 0 and 1,000 km.

Parameter

Description

Atmospheric Density Model

Use the shortcut menu to select one of the following models:

1976 Standard is a table look-up model based on the satellite's altitude. It has a valid range of 86 km to 1000 km.
CIRA 1972is an empirical model of atmospheric temperature and densities as recommended by the Committee on Space Research (COSPAR). It is similar to the Jacchia 1971 model but uses numeric integration rather than interpolating polynomials for some quantities. The lower altitude boundary is 90 km.
DTM 2012 is the Drag Temperature Model (DTM), 2012 version, a semi-empirical model that computes the temperature, density, and composition of the thermosphere. It was developed at CNES. It has a valid range of 120 km to 1500 km.
DTM 2020. The Drag Temperature Model (DTM), 2020 version, is developed and maintained by CNES. This is the operational format of the model, which relies on F10.7 and Kp for solar and geomagnetic indices.
Harris-Priester takes into account a 10.7 cm solar flux level and diurnal bulge. It has a valid range of 0 km to 1000 km.
Jacchia 1960 is an earlier model by Jacchia that uses the solar cycle to predict a value for the F10.7 cm flux and accounts for the effects of the diurnal bulge. The lower altitude boundary is 0 km.
Jacchia 1970 is the predecessor to the Jacchia 1971 model. It has a valid range of 90 km to 2500 km.
Jacchia 1971 computes atmospheric density based on the composition of the atmosphere, which depends on the satellite's altitude as well as a divisional and seasonal variation. It has a valid range of 100 km to 2500 km.
Jacchia-Roberts is similar to Jacchia 1971 but uses analytical methods to improve performance. The lower altitude boundary is 90 km.
Mars GRAM models: Mars-GRAM 3.7, Mars-GRAM 2000, Mars-GRAM 2001, Mars-GRAM 2005, Mars-GRAM 2010. Refer to the Notes on atmospheric density models section of this topic for specific information about these models.
MSIS models: MSIS 1986, MSISE 1990, NRLMSISE 2000. Refer to the Notes on atmospheric density models section of this topic for specific information about these models.
DenistyModelPlugin: If you registered any density model plugins, then the list of Atmospheric Density Models will contain DensityModelPlugin as a choice. All the density models appear as a choice, but only those models with a central body name the same as the central body of the force model can actually be used. After chosen, a shortcut menu appears that contains the list of those registered plugins, using the DisplayName set in the XML registration file for the plugin. If the plugin has registered its own settings, then you can click Plugin Settings... to view and edit those settings.

For missile objects, you can only select atmosphere models that are valid to the ground. These are: Harris-Priester, NRLMSISE 2000, MSISE 1990, and Jacchia 1960.

Low Altitude Density Model

Use this shortcut menu to select an alternative atmospheric density model to apply at lower altitudes in combination with any of the upper altitude models. You can select either the MSISE 1990 or NRLMSISE 2000 model if Earth is the central body. Otherwise, you can leave the None default selection in place.

Blending Range

This defines the range of overlap in which HPOP blends the upper and lower altitude atmospheric density models. Valid values are between 0 and 1,000 km.

SolarFlux / GeoMag

The solar flux and geomagnetic properties available for drag modeling are dependent on the currently selected Atmospheric Density model. From the shortcut menu, select Enter Manually or Use File to specify the solar flux and geomagnetic property values.

Manually specified solar flux and geomagnetic property values

Property	Description
Daily F10.7	The daily Ottawa 10.7 cm solar flux value
Average F10.7	The 81-day averaged Ottawa 10.7 cm solar flux value
Geomagnetic Index	Planetary geomagnetic flux index, Kp

Parameters that are not available for a given atmospheric density model are grayed out when you select that model.

Flux file properties

If you use a flux file to compute atmospheric density for drag, specify the following:

Property	Description
Flux/Ap File	Click the ellipsis button to browse to the path and name of a flux file.
Geomag Update Rate	From the shortcut menu, select an update rate from the following options: 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 Interp 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. These updates cause (small) discontinuous changes in the drag perturbation force. If you select the 3-Hourly or 3-Hourly Interp 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. Also, STK does not use interpolation with MSIS models, so selecting the 3-Hourly Interp update rate for an MSIS model will produce the same result as selecting the 3-Hourly rate. The MSIS models use a slightly different algorithm when you select to use Daily versus the 3-Hourly options.
Geomagnetic Flux Source	Use this shortcut menu to specify whether to use the kp or ap data from the flux file; CSSI predicts files always use ap data.

Property

Description

Flux/Ap File

Click the ellipsis button to browse to the path and name of a flux file.

Geomag Update Rate

From the shortcut menu, select an update rate from the following options:

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 Interp 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.

These updates cause (small) discontinuous changes in the drag perturbation force. If you select the 3-Hourly or 3-Hourly Interp 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. Also, STK does not use interpolation with MSIS models, so selecting the 3-Hourly Interp update rate for an MSIS model will produce the same result as selecting the 3-Hourly rate. The MSIS models use a slightly different algorithm when you select to use Daily versus the 3-Hourly options.

Geomagnetic Flux Source

Use this shortcut menu to specify whether to use the kp or ap data from the flux file; CSSI predicts files always use ap data.

About flux files

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 file may follow the format of CSSI predicts (.dat) files, SpaceWeather (.txt) files, or the stkFluxGeoMag (.fxm) files. For more information about the format of these file types, see the Solar Flux Files topic of this Help.

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. Also, 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 get observed F10.7 values.

All three files can be reported using the Geo Mag Flux Tool under the Scenario menu.

Use Approximate Altitude

If you select this check box, 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 test than the exact expression, which uses an iterative procedure.

For technical notes on this option, see Approximate Altitude Computation in STK.

Use Apparent Sun Position

If you select this check box, 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.

Notes on atmospheric density models

MSIS models

Model	Description
MSIS 1986	This is an empirical density model developed by Hedin based on satellite data. It finds the total density by accounting for the contribution of N2, O, O2, He, Ar, and H. This 1986 version has a valid range of 85 to 1000 km.
MSISE 1990	This is an empirical density model developed by Hedin based on satellite data. It finds the total density by accounting for the contribution of N2, O, O2, He, Ar, and H. This 1990 version has a valid range of 0 to 1000 km.
NRLMSISE 2000	This is an empirical density model developed by the US Naval Research Laboratory based on satellite data. It finds the total density by accounting for the contribution of N, N2, O, O2, He, Ar, and H, and includes anomalous oxygen. This 2000 version has a valid range of 0 to 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.

Model

Description

MSIS 1986

This is an empirical density model developed by Hedin based on satellite data. It finds the total density by accounting for the contribution of N2, O, O2, He, Ar, and H. This 1986 version has a valid range of 85 to 1000 km.

MSISE 1990

This is an empirical density model developed by Hedin based on satellite data. It finds the total density by accounting for the contribution of N2, O, O2, He, Ar, and H. This 1990 version has a valid range of 0 to 1000 km.

NRLMSISE 2000

This is an empirical density model developed by the US Naval Research Laboratory based on satellite data. It finds the total density by accounting for the contribution of N, N2, O, O2, He, Ar, and H, and includes anomalous oxygen. This 2000 version has a valid range of 0 to 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.

The NRLMSIS model is available at https://ccmc.gsfc.nasa.gov/models/.

Mars GRAM models

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 the following PDF files:

The following table outlines the Mars GRAM-specific fields in the SolarFlux / GeoMag section.

Parameter	Description
Namelist File	The namelist (.nml) file is an input file for Mars GRAM that you can use to set various parameters described in the Mars GRAM user's guide for the specific model that you are using. The default file installed with the STK Planetary Data Supplement, INPUT.nml, sets many of the parameters in such a way that they are compatible with STK's environment; these parameters are indicated with comments in the file. Some parameters that can be specified in the namelist file will naturally be countermanded by their contextual usage in STK. For example, FLAT (initial latitude) will be overridden in favor of the initial STK state, and NPOS (number of points) will be ignored in favor of the number of steps that the numerical integrator requires.
Density Type	Specify the Mars GRAM density value for HPOP to use for trajectory evaluation from the following options: High corresponds to the Mars GRAM DENSHI value. Low corresponds to the Mars GRAM DENSLO value. Mean corresponds to the Mars GRAM DENSITY value. Random perturbed corresponds to the Mars GRAM DENSTOT value.

Parameter

Description

Namelist File

The namelist (.nml) file is an input file for Mars GRAM that you can use to set various parameters described in the Mars GRAM user's guide for the specific model that you are using. The default file installed with the STK Planetary Data Supplement, INPUT.nml, sets many of the parameters in such a way that they are compatible with STK's environment; these parameters are indicated with comments in the file.
Some parameters that can be specified in the namelist file will naturally be countermanded by their contextual usage in STK. For example, FLAT (initial latitude) will be overridden in favor of the initial STK state, and NPOS (number of points) will be ignored in favor of the number of steps that the numerical integrator requires.

Density Type

Specify the Mars GRAM density value for HPOP to use for trajectory evaluation from the following options:

High corresponds to the Mars GRAM DENSHI value.
Low corresponds to the Mars GRAM DENSLO value.
Mean corresponds to the Mars GRAM DENSITY value.
Random perturbed corresponds to the Mars GRAM DENSTOT value.

The Mars GRAM models are available on NASA's website.