Jacchia Roberts Density Model

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The Jacchia-Roberts atmospheric density model uses monthly mean smoothed values of solar radiation along with weighted planetary geomagnetic index (Ap) to produce atmospheric density values. An input file is used to provide definitive and/or predicted values for exospheric temperature and geomagnetic index.


By default, FreeFlyer will use the Jacchia-Roberts drag file included in the installation, but you can specify a different file (such as a Schatten Solar Flux file) or even specify your own custom drag model using a User-Created Atmospheric Drag File. This is useful for performing analysis using different predicted magnitudes and phases of the solar cycle.


It is important to note that the Solar Flux Predictor file is limited to a certain date range - see the Jacchia-Roberts Drag File page for information on the date range of the predictor file packaged with FreeFlyer. For simulations before the first date in the predictor file, FreeFlyer will use the first data entry in the file. For simulations after the last date in the file, FreeFlyer will use the last data entry in the file. This means that, outside the range of the file, the density will not be affected by large solar cycle variations; however, seasonal affects resulting from the changing position of the Sun with respect to the Earth over the course of a year will still be modeled.


This page is divided into the following sections:

Enabling Jacchia-Roberts Drag Modeling

Setting the Solar Flux Predictor File



Enabling Jacchia-Roberts Drag Modeling

The default atmospheric density model used for modeling the Earth's atmosphere is the Analytic Density Model. To enable the Jacchia-Roberts Density Model for a Spacecraft, edit the Spacecraft, proceed to the Force Model page, and select Earth from the list of celestial bodies as shown in the image below.


Then ensure that "Jacchia Roberts" is selected in the Density Model drop-down menu.



To enable Jacchia-Roberts atmospheric density modeling via FreeFlyer script, use the following syntax:


Alias fm = (Spacecraft1.Propagator AsType RK89).ForceModel; // This alias is used to shorten the script examples below


fm.AtmosphericDensityModel = 3;


Where the AtmosphericDensityModel can be one of the following:

1 for Analytic density

2 for Harris-Priester density model

3 for Jacchia-Roberts density model

4 for MSIS-2000

5 for MSIS-90

6 for MSIS-86


You can access the associated Jacchia-Roberts drag model properties via the ForceModel object, using the following syntax:


Report fm.Kp;               // Report Geomagnetic Index (Kp) value

Report fm.Ap;               // Report Planetary Index (Ap) value

Report fm.Tc;               // Report Exospheric Temperature in Kelvin

Report fm.F107Index;        // Report current F10.7 index

Report fm.F107IndexAverage; // Report 81-day average F10.7 index value


Note: You can find additional information about these properties on their object pages: ForceModel.Kp, ForceModel.Ap, ForceModel.Tc, ForceModel.F107Index, and ForceModel.F107IndexAverage.



Setting the Solar Flux Predictor File

By default, FreeFlyer uses the "Jacchia.dat" data file for Jacchia-Roberts atmospheric modeling, but you can specify a different file using the "Solar Flux Predictor File" file browser shown in the image above or via FreeFlyer script using this syntax:


fm.SolarFluxFileType = 2; // ASCII

fm.SolarFluxFilename = "JRdragfile.dat";


Where "JRdragfile.dat" specifies the path and filename of the custom Jacchia-Roberts drag file.


For example, when working with Schatten Solar Flux files, you can specify the desired Schatten file in the file browser or specify it via FreeForm:


fm.SolarFluxFilename = "Meannom.dat";



See Also

ForceModel Properties and Methods

Solar System settings that affect Spacecraft Propagation