Using Analysis Workbench to Report the Subsolar or Sub-observer Points

Information

 
QuestionHow do you report the subsolar or sub-observer point?
Answer

Reporting the Subsolar or Sub-observer Point

The subsolar point is the point on a planet where the sun is perceived to be directly overhead. To display the subsolar point on your central body in STK, go to the 2D Graphics window properties->Lighting page, and enable Show under  Subsolar Point.  This point is displayed on the central body that has been selected for the 2D Graphics window.  If you wanted to report on the detic latitude and longitude of the subsolar point over time though, you would just need to create a couple of components in Analysis Workbench.  The following steps show how to report on the subsolar point for Mars, but can be easily applied to any central body in STK.
  1. Insert a Mars Planet (User-added image) object:
    1. Insert->Default Object->Planet.
    2. In the Planet properties->Basic->Definition, set the Central Body to Mars and select the ephemeris source that is appropriate for your problem.
  2. Open Analysis Workbench (User-added image) and go to the Vector Geometry tab:
    1. Filter by: Planet.
    2. Select the Mars object that you created.
    3. Create a new Point and select the following:
    OptionValue
    Type
    Surface
    Name
    subsolar_pt_mars
    Central Body
    Mars
    Reference Point
    Sun Center (Filter by primary bodies to select the sun)
    Reference Shape Type
    Ellipsoid
    Surface Point Type
    Detic 
    User-added image 
  3. Still in Analysis Workbench with Mars selected, go to the Calculation tab and create a new Parameter Set.
  4. Set the following options:
    OptionValue
    TypeCartographic Trajectory
    Namesubsolar_pt_mars_param
    Locationsubsolar_pt_mars (the point you created in step 2)
    Central BodyMars
     User-added image
    Note:  Step 2 created the point and step 3 assigned it to the central body of Mars, rather than the planet object Mars that you created.
  5. In the Report & Graph Manager select the planet Mars.
  6. Click the Create a new report style (User-added image) button.
  7.  Name the new report Subsolar Mars Lat Long.
  8. Open.the report style properties:
    1. Scroll down to the Parameter Set: Cartographic Trajectory folder and expand it.
    2. Look for a folder for subsolar_pt_mars_param and expand it.
    3. Select the Time, Detic.LLA.Latitude, and Detic.LLA.Longitude.
    4. Generate the report.
  9. To dynamically display this chart so that it animates along with the point in the 2D graphics window, generate it as a Dynamic Display/Strip Chart.
User-added image
The sub-observer point is similar to the subsolar point, except that it is the point on the surface of a planet where a spacecraft appears directly overhead, rather than the Sun.  You can report on this point for a satellite by repeating the above steps for the subsolar point, except in step 2c,  the Reference point will be your satellite center rather than the Sun center.
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