Propagator Function Components

This is a proxy for the Yarkovsky effect, which impacts the orbits of comets and asteroids due to the sublimation of gases. This proxy effect is a way to implement some of the nongravitational parameters supplied by the JPL Small Body Database.

This model incorporates augmented space weather data. For detailed information, visit sol.spacenvironment.net/jb2008/introduction.html.

Sample files have been included with STK, but these files only provide historical data. You can download the required historical indices from sol.spacenvironment.net/~JB2008/indices.html. Contact Space Environment Technologies for real time and forecast data.

These models, published by NASA, are valid for use with Mars as a central body. The data files for these models are included in the STK Planetary Data Supplement. You can read primary information about these models in the following PDF files:

• Mars Environment
• MarsGRAM2000
• MarsGRAM2001
• MarsGRAM2010

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

This model, published by NASA, is valid for use with Venus as a central body. You can access the data files for this model in the STK Planetary Data Supplement. The VenusGRAM model does not support HPOP satellites nor the Force vector in Analysis Workbench. For more information on VenusGRAM and to request the software, see the NASA VenusGRAM website.

You can use a plugin atmospheric density model to incorporate augmented space weather data. The associated data are the same as the data in the Jacchia-Bowman atmospheric density models. Sample files have been included with STK, but these files only provide historical data. You can download the required historical indices from sol.spacenvironment.net/~JB2008/indices.html. Contact Space Environment Technologies for real time and forecast data.

Read from File - Solar and geomagnetic data are provided in separate files at sol.spacenvironment.net/~JB2008/indices.html. STK reads in these files by their respective provided formats.

The F10 and F10bar solar flux indices are included in the Jacchia-Bowman specific files and the solar and magnetic files described above. The reference time of day for the daily provided value may be different. When the Jacchia-Bowman 2008 atmospheric density model is used, the F10 and F10bar values from the Jacchia-Bowman specific files are used.

Static Values - Instead of reading data from a file, you can enter specific values for F10, F10bar, M10, M10bar, S10, S10bar, Y10, Y10bar, Ap, and DstDTc.

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.

Click to specify the directory where the Mars GRAM data files reside. These files include binary and text files that the Mars GRAM model uses to determine density and other values for an area and time. The standard STK installation path for the Mars GRAM model is given by default.

Define the Mars GRAM density value that you want to use for trajectory evaluation. You can choose 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.

• Constant Values - Daily F10.7, Average F10.7, Kp
• Data File - File, Geomag Update Rate, Geomagnetic Flux Source

Browse to 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.

STK supports the following file formats:

• Flux (*.fxm) file
• CSSI predict (*.dat) file (also used by the Lifetime tool)
• Space Weather flux file available at www.celestrak.com/SpaceData and sponsored by CSSI and AGI

When reading the flux file, the observation time for F10.7 data is 20:00 UTC. This is the time when the value begins to apply for each day listed in the file. When using F10.7 values from the flux file, STK determines the value of F10.7 at a particular time using linear interpolation of data in the table.

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.

The Geo Mag Flux Tool under Scenario Tools can report on all three files.

• 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 three-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.

If you select Variable Area, you can specify a Variable Area History File. If you select N-Plate, you can specify an N-Plate Definition file.

Select the source of the gravitational value:

• Cb File - A central body file included with STK
• User Specified - The value that you enter in the Gravitational Parameter field

Specify the maximum degree of geopotential coefficients to include for central body gravity acceleration for state (position, velocity) propagation and point-wise evaluation. The valid range starts at 0 and has an upper bound dependent on the gravity model.

Specify the maximum order of geopotential coefficients to include for central body gravity acceleration for state (position, velocity) propagation and point-wise evaluation. The valid range starts at 0 and has an upper bound that depends on the gravity model and the State Propagation: Degree value.

Select one of the following options for including the perturbation of the gravity field caused by the effects of solid tides. The central body gravity field must have a solid-tide model available.

• None - Exclude solid-tide contributions.
• 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 and 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, since it accounts for geopotential variations of degree and order caused by 71 distinct 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.

Enter a value for Minimum Amplitude to exclude constituents whose tidal amplitude is smaller than the threshold you want to consider.

Ocean tides are modeled using the FES2014bv1 model, an updated model based upon the IERS Conventions 2010 (Tech Note 36). Details of that model can be found in the text file <STK install folder>\STKData\CentralBodies\Earth\FES2014bv1OceanTideModel.txt.

• 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 be excluded from computation.

The secondary body must be a child of the central body for the propagator that this force model is applied to. The secondary body should have enough mass to recover the CR3BP mass parameter , to revolve on a circular orbit with a semimajor axis that is consistent with the CR3BP characteristic distance, and with the appropriate mean motion.

The shape model and attitude definition of the secondary body's fixed frame are optional parameters.

• None - No model. STK will not consider shadowing of the spacecraft.
• Cylindrical - The cylindrical model assumes that the Sun is at an infinite distance from the satellite. All light coming from the Sun moves in a direction parallel to the Sun to spacecraft 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 SRP during penumbra.

• Apparent Sun to True CB - The time required for light to travel from the sun to the central body
• Apparent - 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

This is a simple model that accounts for attenuation and refraction of solar radiation through the atmosphere.

When computing SRP, STK accounts for penumbra and umbra regions of eclipse. These regions are the result of the Earth's obstruction of the Sun from the spacecraft’s position.

By default, STK uses the Earth’s surface shape, which corresponds to an atmospheric altitude of 0.0 km. Thus, attenuation and refraction of solar radiation through the atmosphere is not accounted for. You can increase the shape of the earth by its altitude height. Enter the altitude that you want to apply. The most common altitude used for this kind of modeling is 23 km.

Luminosity and mean flux apply to the Solar Radiation computation.

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 and enter a value for Solar Radius.

Use this to select a method for interpolating Tabulated Area Vector data. The options are:

• Bilinear Cartesian: Interpolates Cartesian area vectors using bilinear interpolation.
• Bilinear MagDir: This decomposes area vectors into magnitude and direction components and interpolates them using bilinear interpolation. It then reconstructs the resultant area vector from the interpolated magnitude and direction. This method can produce accurate results on a coarse grid when used for nearly spherical objects.

In all cases, you need a sufficiently dense table of area vector data for interpolated results to provide an accurate model of the acceleration.