The grazing angle describes how high one object appears above the edge (or limb) of the Earth relative to the satellite. It is measured, relative to the satellite, as the angle between the Earth limb and the other object. In STK, you can view and modify the grazing angle constraint of a satellite by right-clicking the satellite and selecting Properties > Constraints > Advanced. You can use this to prevent a sensitive optical sensor, such as those on the Hubble Space Telescope, from aiming too close to the Earth, which might blind the sensor due to reflections off the surface. The STK help documentation for grazing angle constraint is
here.
To generate a report on the value of the grazing angle, first compute access between your two objects. Then open the Report and Graph Manager and for Object Type select "Access". With your access object selected, click the "Create a new report style" icon. In the New Report properties, under Data Providers, expand the Constraint Data item. You can now add the FromGrazingAngle (originating at the primary object, as in the above picture) and the ToGrazingAngle (originating at the other object). You can then apply the grazing angle constraint to either or both of your objects and these angles, regardless of whether or not you have the constraint turned on in their properties.
Another option is to compute the grazing angle using Analysis Workbench. This enables you to visualize the angle and also set the altitude of the grazing point so that it is not at the planet’s surface. To do this, you first need to create a new point type on your primary vehicle. In the Vector Geometry tab, select your primary vehicle and click the "Create new point" icon. When the "Add Geometry Component" window appears, set the following properties:
• Type: Central Body Grazing
• Central Body: Earth (or whichever body you are working with)
• Reference Point: Primary Object Center
• Direction Vector:
• Other Object, in the To Vector folder (STK 11.2 or newer)
• Create a vector of type Displacement that goes from your Primary Center to your Other Center (STK 11.1 and older)
• Altitude: You choose this (note: an altitude of zero matches this calculation to the access constraint grazing angle calculation)
Next, you will need to create a new vector of type "Displacement" that goes from the Primary Object Center to the grazing point you just created. Lastly, create a new angle of type "Between Vectors" between this grazing point vector and the Primary-to-Other vector used as the Direction Vector when creating the grazing point. If you set your grazing point altitude to zero, this angle should match your FromGrazingAngle from the access Constraint Data. You would need to repeat this process in Analysis Workbench for the Other object to get the ToGrazingAngle.
The attached scenario has all of this set up as a reference!
