HPOP Force Models: Drag

The Drag tab of the HPOP Force Model Properties window provides a variety of options for modeling the atmospheric density used in the computation of atmospheric drag accelerations on the spacecraft. To model drag for an HPOP trajectory calculation, select the Use.

Model

By default, the Spherical model will be used. If you have any drag plugin models registered, you can click the Model drop-down menu and select DragModelPlugin. Select the specific plugin model that you want to use from the drop-down menu that appears; click Plugin Settings to configure any parameters that the plugin allows you to set.

If you are using the Spherical drag model, you can define the atmospheric drag coefficient, Cd, and the Area/Mass Ratio - the area to be used in drag calculations (in m²/kg).

Atmosphere

The default atmospheric density model that is used by HPOP is the Jacchia-Roberts model, but you can use the Atmospheric Density Model drop-down menu to select from among any of the following models:

1976 Standard. A table look-up model based on the satellite's altitude. Valid range is 86 km to 1000 km.
CIRA 1972.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.
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 is 120 km to 1500 km.
Harris-Priester. Takes into account a 10.7 cm solar flux level and diurnal bulge. Valid range is 0 km to 1000 km.
Jacchia 1960. 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 dirunal bulge. Lower altitude boundary is 0 km.
Jacchia 1970. The predecessor to the Jacchia 1971 model. Valid range is 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. Valid range is 100 km to 2500 km.
Jacchia-Roberts. Similar to Jacchia 1971 but uses analytical methods to improve performance. Lower altitude boundary is 90 km.
Mars GRAM models. MarsGRAM 3.7, MarsGRAM 2000, MarsGRAM 2001, MarsGRAM 2005, MarsGRAM 2010. (see below)
MSIS models. MSIS 1986, MSISE 1990, NRLMSISE 2000. (see below)

Only atmosphere models that are valid to the ground can be used for missile objects. These are: Harris-Priester, NRLMSISE 2000, MSISE 1990, and Jacchia 1960.

You can use the Low Altitude Density Model drop-down 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, or any of the Mars GRAM atmospheric density models if Mars is the central body. The Blending Range defines the range of overlap in which the upper and lower altitude atmospheric density models will be blended. 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 drop-down menu, select Enter Manually to individually define the solar flux and geomagnetic properties, or Use File to browse to the path and name of a flux file.

Manually Defined Solar Flux and Geomagnetic Properties

Option	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 that model is selected.

Flux File Properties

If you use a flux file to compute density for drag, select an update rate from the Geomag Update Rate drop-down menu.

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. In addition, STK does not use interpolation with MSIS models; selecting the 3-Hourly Interp update rate for an MSIS model will produce the same result as selecting the 3-Hourly rate.

You can use the Geomagnetic Flux Source drop-down 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 page of this help system.

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.

Additional Drag Options

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

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.

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

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.

Notes on Atmospheric Density Models

MSIS Models

Model	Description
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 is 85 to 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 is 0 to 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 is 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

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 is 85 to 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 is 0 to 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 is 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 MSIS models are available at http://ccmc.gsfc.nasa.gov/modelweb/.

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 PDF files also installed with the Planetary Data Supplement in the STK installation directory under \STKData\CentralBodies\Mars\MarsGRAM.

The Astrogator interface for the MarsGRAM atmospheric density models exposes two additional settings for the control of the density computations. The Density Type has four options: High density, Low density, Mean density, and Random perturbed density. The HPOP implementation computes the Mean density. The Astrogator interface also exposes a Namelist File selector that allows for the selection of a custom group of MarsGRAM specific settings. The HPOP implementation loads model-specific, namelist files from the STK installation area 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.