Computing Radiation Dosages Using SEET and the IRENE Model

STK Premium (Space) or STK Enterprise
You can obtain the necessary licenses for this tutorial by visiting http://licensing.agi.com/stk/evaluation or contacting AGI Support at support@agi.com or 1-800-924-7244.

This lesson requires an internet connection and version 12.9.1 of the STK software or newer to complete in its entirety.

The results of the tutorial may vary depending on the user settings and data enabled (online operations, terrain server, dynamic Earth data, etc.). It is acceptable to have different results.

Capabilities covered

This lesson covers the following capabilities of the Ansys Systems Tool Kit® (STK®) digital mission engineering software:

  • STK Pro
  • Space Environment and Effects Tool (SEET)

Problem statement

Engineers and mission planners require a way to comprehensively model the space environment and the ambient near-Earth environment effects on space vehicles. They need to estimate the trapped energetic electrons and protons and how the thickness of a satellite 's shielding affects radiation dosage for purposes of space system design and mission planning.

Solution

Use the STK Space Environment and Effects Tool (SEET) capability and the International Radiation Environment Near Earth (IRENE) computational model to determine the expected dosage rates and accumulated doses due to energetic electron and proton particle fluxes for a range of shielding thicknesses on a spacecraft. Configure scenario and spacecraft properties as needed to obtain low- and high-resolution dose depth and minimum / maximum flux comparisons.

What you will learn

Upon completion of this tutorial, you will be able to:

  • Use the Fast International Geomagnetic Reference Field coefficients for the geomagnetic field model
  • Apply the local-time-dependent Olson-Pfitzer external field model
  • Use the IRENE computational model
  • Analyze radiation dose depth
  • Analyze radiation flux

Creating a new scenario

First, you must create a new scenario, and then build from there.

  1. Launch the STK application ().
  2. Click Create a Scenario in the Welcome to STK dialog box.
  3. Enter the following in the STK: New Scenario Wizard:
    4. Option Value
    Name SEET_IRENE
    Location Default
    Start Default
    Stop Default
  4. Click OK when you finish.
  5. Click Save () when the scenario loads. The STK application creates a folder with the same name as your scenario for you.
  6. Verify the scenario name and location in the Save As dialog box.
  7. Click Save.

Save () often during this scenario!

Modeling the spacecraft

Insert and propagate a Satellite object to model your spacecraft.

Inserting a Satellite object

Use the Insert STK Object tool to create a new Satellite object using the Orbit Wizard. The Orbit Wizard is a satellite-level tool designed to assist you in creating any one of several standard orbits, or designing your own satellite orbit. The configurable options available will depend on the orbit type selected.

  1. Bring the Insert STK Objects tool () to the front.
  2. Select Satellite () in the Select An Object To Be Inserted list.
  3. Select Orbit Wizard () in the Select A Method list.
  4. Click Insert. . ..

Using the Orbit Designer

With the Orbit Designer option, you can create any orbit you wish.

  1. Open the Type drop-down list in the Orbit Wizard.
  2. Select Orbit Designer.
  3. Enter Test_Sat in the Satellite Name field.
  4. Set the following in the Orbital Elements panel:
    5. Parameter Value
    Semimajor Axis 10000 km
    Eccentricity 0.1
    Inclination 30 deg
    Argument of Perigee 60 deg
    RAAN 90 deg
    True Anomaly 0 deg
  5. Click OK to accept your changes, to propagate the satellite, and to close the Orbit Wizard.

Changing the scenario-level units of measure

Scenario units establish the default settings for all units of measure used in a scenario. These settings are used for display and data input purposes throughout the STK application. Use these settings to change the default Radiation Shield Thickness unit of measure from mils to millimeters.

  1. Right-click on SEET_IRENE () in the object browser.
  2. Select Properties () in the shortcut menu.
  3. Select the Basic - Units page.
  4. Select the Sort - Alphabetical option in the Dimension List panel.
  5. Scroll down and locate RadiationShieldThickness.
  6. Click the Mils (Mils) cell in the CurrentUnit column.
  7. Open () the drop-down list.
  8. Select Millimeters (mm).
  9. Select the Enter key.
  10. Click OK to accept your selection and to close the Properties Browser.

Utilizing the SEET capability

The STK Space Environment and Effects Tool (SEET) capability evaluates the effects of the space environment on a spacecraft. It calculates spacecraft exposure to ionizing particles, thermal radiation, and space debris throughout the orbit.

Opening the satellite's SEET Environment properties

Configure the satellite's SEET environment on its Basic - SEET Environment properties page.

  1. Open Test_Sat's () Properties ().
  2. Select the Basic - SEET Environment page.

Setting the geomagnetic main field model

SEET uses a highly customizable set of conditions to compute the local magnetic field at the current location. The STK SEET Geomagnetic Field component evaluates the magnetic field and performs field-line tracing using multiple models. Beyond several Earth radii, the Earth's magnetic field is increasingly distorted by the solar wind interacting with the Earth's magnetosphere; the total magnetic field is therefore modeled primarily as a dipole main-field model, plus an external (magnetospheric) field model. This component provides common magnetic field functionality with current AF-GEOSpace magnetic field models, including simply tilted dipole models based on time-interpolated moments of the full International Geomagnetic Reference Field (IGRF) representation, full time-interpolated IGRF, and full IGRF plus Olson-Pfitzer (1977) external field models.

For your analysis, you'll use the Fast IGRF option. The IGRF main field calculations are based on the full harmonic expansion IGRF coefficients. Fast IGRF is a specially tuned version of IGRF model that returns a field strength estimate accurate to within 1% of the full-IGRF option, but it is significantly faster.

  1. Open the Main Field drop-down list in the Magnetic Field Model panel.
  2. Select Fast IGRF.

Setting the external magnetic field model

Select the optional Olson-Pfitzer external field model for SEET. This model is based on Olson and Pfitzer's 1977 quantitative model of the magnetospheric field and associated procedures for accurately cataloging charged-particle data out to and beyond geosynchronous orbit. It only depends on the local time of one's position of interest within the field (that is, the position relative to the direction of the Sun).

  1. Open the External Field drop-down list.
  2. Select Olson-Pfitzer.
  3. Click Apply to accept your selections and to keep the Properties Browser open.

Selecting the SEET radiation environment computational mode

The STK SEET Radiation Environment computes the expected dosage rate and accumulated dose (ionizing energy deposition) due to energetic electron and proton particle fluxes for a range of shielding thicknesses, and also computes the energetic proton and electron fluxes for a wide range of particle energies.

The STK SEET Radiation Environment incorporates the following models from the Air Force Research Laboratory’s AF-GEOSpace program version 2.1P: APEXRAD, CRRESRAD, CRRESELE, CRRESPRO, NASAELE, and NASAPRO. For dose computations in combination with the CRRES or NASA flux models, the SHIELDOSE-2 radiation-transport model is used to estimate ionizing energy deposition (dose).

  1. Select the SEET Radiation page.
  2. Open the Computational Mode drop-down list in the Model panel.
  3. Select IRENE.

Using the IRENE computation model

IRENE is a set of models for the fluxes of radiation belt and plasma particles in near-Earth space for use in space system design, mission planning, and other applications of climatological specification.

  1. Enter 3 hr in the Dose Report Step field.

    2. This value specifies the default time step used by data providers reporting radiation-dose output.

  2. Clear the Set magnetic field epoch to Mode’s reference epoch check box.

    3. If selected, the field model used for computing indices into the radiation model databases is set to use the fixed epochs and external field models from which those databases were constructed. Clearing it will also automatically disable the Shift SAA using Mode’s reference epoch option. These two selections are not factored into SEET Radiation reports using the IRENE model.

  3. Click OK to accept your changes and to close the Properties Browser.

Saving multiple scenarios

You are going to use the SEET_IRENE scenario's initial setup and properties for two separate analyses. To make things easier, save two different scenarios from your baseline starter scenario.

Saving a scenario to analyze radiation dose depth

You'll save each unique scenario into its own subfolder in the SEET_IRENE scenario folder. First, save a scenario in which you will analyze the radiation dose depth.

  1. Select SEET_IRENE () in the Object Browser.
  2. Open the File menu.
  3. Select Save As... in the shortcut menu.
  4. Within the SEET_IRENE folder, select New Folder and name the folder "SEET_IRENE_Radiation_Dose_Depth"
  5. Open the newly created folder.
  6. Enter SEET_IRENE_Radiation_Dose_Depth in the File name field in the Save As dialog box.
  7. Click Save to accept your change and to close the Save As dialog box.

Saving another scenario to analyze radiation flux

Now, save another scenario in its own subfolder within the SEET_IRENE scenario folder. You'll use this scenario to analyze radiation flux.

  1. Select SEET_IRENE_Radiation_Dose_Depth () in the Object Browser.
  2. Open the File menu.
  3. Select Save As... in the shortcut menu.
  4. Within the SEET_IRENE folder, select New Folder and name the folder " SEET_IRENE_Radiation_Flux"
  5. Open the newly created folder.
  6. Enter SEET_IRENE_Radiation_Flux in the File name field in the Save As dialog box.
  7. Click Save to accept your change and to close the Save As dialog box.

Analyzing radiation dose depth

This scenario will guide you through customizing shielding thicknesses to create a radiation dose depth report.

  1. Click Open () in the STK toolbar.
  2. Navigate to and select SEET_IRENE_Radiation_Dose_Depth in the Open dialog box.
  3. Click Open.

Customizing the shielding thickness

You can specify up to 70 different Aluminum shielding thicknesses as needed. This option is effective when operating with the CRRES ( Combined Release and Radiation Effects Satellite) and NASA computational modes for arbitrary proton and electron incident spectra and used for dose computations from the SHIELDOSE-2 radiation-transport model.

  1. Open Test_Sat's () Properties ().
  2. Select the SEET Radiation page.
  3. Click Remove All in the Shielding Thicknesses panel.
  4. Click Add.
  5. Enter 0.5 mm in the Value cell.
  6. Select the Enter key.
  7. Enter the following additional values:
    • 1 mm
    • 2 mm
    • 3 mm
    • 5 mm
  8. Click OK to accept your changes and to close the Properties Browser.

Customizing the SEET Radiation Dose Depth report

The SEET Radiation Dose Depth data provider computes the radiation dose over the requested time interval(s), using the step size to perform the numerical integration of dose rate with time. It is valid for vehicles whose central body is earth. The combined dose data provider element is the accumulated total radiation dose from all constituents, at the given time.

  1. Right-click on Test_Sat () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the SEET Radiation Dose Depth () report in the Installed Styles () folder in the Styles panel of the Report & Graph Manager.
  4. Click Duplicate style () in the Styles panel toolbar.

Changing the Shielding thickness report units and options

You want to match your report units to match the scenario units you set earlier. Both data providers are using scientific notation. You can make the report easier to read by setting the values to floating point notation.

  1. Select the Content page.
  2. Select Shielding thickness in the Report Contents list.
  3. Click Units....
  4. Clear Use Defaults in the Units: Section 1, Line 1, Shielding thickness dialog box.
  5. Select Millimeters (mm) in the New Unit Value list.
  6. Click OK to accept your selection and to close the Units: Section 1, Line 1, Shielding thickness dialog box.
  7. Click Options....
  8. Open the Notation drop-down list in the Options: Section 1, Line 1, Shielding thickness dialog box.
  9. Select Floating Point.
  10. Click OK to accept your selection and to close the Options: Section 1, Line 1, Shielding thickness dialog box.

Changing the SEET Radiation Dose Depth - Combined dose report options

You also need to update the units for the Combined Dose to Floating Point.

  1. Select SEET Radiation Dose Depth - Combined Dose in the Report Contents list.
  2. Click Options....
  3. Open the Notation drop-down list in the Options: Section 1, Line 1, SEET Radiation Dose Depth - Combined dose dialog box.
  4. Select Floating Point.
  5. Click OK to accept your selection and to close the Options: Section 1, Line 1, SEET Radiation Dose Depth - Combined dose dialog box.
  6. Click OK to accept your selections and to close the Properties Browser.

Renaming the custom report

Although it's not required, it's a good idea to rename custom reports duplicated for an installed style.

  1. Right-click on the SEET Radiation Dose Depth () report in the My Styles () folder in the Styles panel.
  2. Select Rename in the shortcut menu.
  3. Rename the SEET Radiation Dose Depth () report to Custom SEET Radiation Dose Depth.
  4. Select the Custom SEET Radiation Dose Depth () report.

Generating the custom report

With your report configured, create and review the report.

  1. Click Generate....
  2. Examine the data in the report.

    3. It's very obvious from the report that the thicker the shielding, the lower the combined dose.

  3. Close the report and the Report & Graph Manager when finished.
  4. Save () your work.

Analyzing radiation flux

This scenario will guide you through defining custom electron and proton flux models and creating radiation flux reports.

Opening the Radiation Flux scenario

Open the SEET_IRENE_Radiation_Flux Scenario file you saved previously and build on it from there.

  1. Click Open () in the STK toolbar.
  2. Navigate to and select SEET_IRENE_Radiation_Flux in the Open dialog box.
  3. Click Open.

Defining custom energies

When using custom NASA energies, levels between 0.04 and 7.0 MeV (particle energy) may be specified for the NASA electron-flux model and levels between 0.1 and 400 MeV for the NASA proton-flux model.

  1. Open SEET_IRENE_Radiation_Flux's () Properties ().
  2. Select the Basic - SEET Radiation page.
  3. Open the NASA Energies drop-down list.
  4. Select Custom.

Entering custom electron and proton values

Although it is a bit time consuming, can will manually enter custom electron and proton energy values.

  1. Click Remove All in the Electron Values (MeV) panel.
  2. Add the following Electron Values (MeV):
    3. Electron Values (MeV)
    0.04
    0.07
    0.10
    0.25
    0.50
    0.75
    1.00
    1.50
    2.00
    2.50
    3.00
    3.50
    4.00
    4.50
    5.00
    5.50
    6.00
    6.50
    7.00
  3. Click Remove All in the Proton Values (MeV) panel.
  4. Add the following Proton Values (MeV):
    5. Proton Values (MeV)
    0.10
    0.20
    0.40
    0.60
    0.80
    1.00
    2.00
    4.00
    6.00
    8.00
    10.0
    15.0
    20.0
    30.0
    50.0
    60.0
    80.0
    100
    150
    200
    300
    400
  5. Click OK to accept your changes and to close the Properties Browser.

Increasing the satellite's step size

The step size is used during ephemeris generation. In order to speed up your analysis, you can increase the step size to six hours on the satellite's Basic - Orbit page.

  1. Open Test_Sat's () Properties ().
  2. Select the Basic - Orbit page.
  3. Enter 6 hr in the Step Size field.
  4. Click OK to accept your change and to close the Properties Browser.

Generating a SEET Radiation Model report

The SEET Radiation Model report at the scenario level uses the SEET Radiation Environment Model data provider and the Definition data provider element to provide details on the scenario's radiation environment model parameters such as the electron and proton energy levels for the NASA and CRRES models.

  1. Right-click on SEET_IRENE_Radiation_Flux () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the SEET Radiation Model () report in the Installed Styles () folder in the Report & Graph Manager Styles panel.
  4. Click Generate....

    5. Note that the default electron and proton energies for both the CRRES and NASA models are displayed in the report.

  5. Close the report when finished.

Generating a SEET Radiation Flux report

The SEET Radiation Flux report uses the SEET Radiation Flux by Energy data provider and the Flux data provider element. SEET Radiation Flux by Energy computes the radiation flux at each electron and proton energy level. Flux is the radiation flux at the given time.

  1. Return to the Report & Graph Manager.
  2. Open the Object Type drop-down list.
  3. Select Satellite.
  4. Select Test_Sat () in the Object Type list.
  5. Select the SEET Radiation Flux () report in the Installed Styles () folder in the Report & Graph Manager Styles panel.
  6. Click Generate....
  7. Scroll through the report to obtain electron and proton flux values.

    8. The first half of the report shows the electron flux values at the custom electron energy levels you specified earlier. The second half of the report shows proton flux values at the custom proton energy values you created.

  8. When finished, close the report.

Creating a custom SEET Radiation Integral Flux report

Though it is not preinstalled, you can easily create an radiation integral flux report. Start by creating a new report style.

  1. Return to the Report & Graph Manager.
  2. Select the My Styles () folder in the Styles panel.
  3. Click Create new report style () in the Styles panel toolbar.
  4. Enter SEET Radiation Integral Flux as the report name.
  5. Select the Enter key.

Selecting data providers and elements

You will use the SEET Radiation Integral Flux data provider and all of the Electron Flux data provider elements. The SEET Radiation Integral Flux data provider computes the integral radiation flux at each electron and proton energy level. Electron flux is the radiation flux due to electrons for the energy level indicated, at the given time. Proton flux is the radiation flux due to protons for the energy level indicated, at the given time.

  1. Ensure the Content page is selected.
  2. Expand () the SEET Radiation Integral Flux () data provider.
  3. Select the Time () data provider element.
  4. Hold down the shift key and select the Proton Flux (400.000 MeV) () data provider element to multi-select all of the Electron Flux and Proton Flux elements.
  5. Click Insert ().

    6. The Time data provider element and all of the custom Electron and Proton Flux values should be in the Report Contents list.

  6. Click OK to accept your changes and to close the Properties Browser.
  7. Select the SEET Radiation Integral Flux () report in the My Styles () folder.
  8. Click Generate....

    9. The report details fluxes for each custom electron and proton energy level at a 6-hour interval.

  9. When finished, close the report.

Creating a custom SEET Radiation Integral Flux by Energy report

Create another custom report to analyze the radiation integral flux by energy.

  1. Return to the Report & Graph Manager.
  2. Select the My Styles () folder in the Styles panel.
  3. Click Create new report style () in the Styles panel toolbar.
  4. Enter SEET Radiation Integral Flux by Energy as the report name.
  5. Select the Enter key.

Selecting data providers and elements

You will use the SEET Radiation Integral Flux by Energy data provider and the Flux data provider element. The SEET Radiation Integral Flux by Energy data provider computes the integral radiation flux at each electron and proton energy level. Flux is the radiation flux at the given time.

  1. Ensure the Content page is selected.
  2. Expand () the SEET Radiation Integral Flux by Energy () data provider.
  3. Select both the Time () and Flux () data provider elements.
  4. Click Insert ().
  5. Click OK to accept your changes and to close the Properties Browser.
  6. Select the SEET Radiation Integral Flux by Energy () report in the My Styles () folder.
  7. Click Generate....

The report details fluxes for each custom electron and proton energy level at a 6-hour interval. This report and the SEET Radiation Integral Flux report show the same values. Basically, do you want to read the data horizontally or vertically?

Saving your work

Clean up your workspace and close out your scenario.

  1. Close any open reports and the Report & Graph Manager.
  2. Save () your work.
  3. Close the scenario when finished.

Summary

You created a base scenario setting a satellite's SEET main field model to Fast IGRF, the external field model to Olson-Pfitzer, and the computational mode to IRENE. Your first analysis was to set a range of shielding thickness and running a report that analyzed the absorbed radiation dose. Next, you analyzed radiation flux using custom electron and proton values.