First, refer to the Sensor Properties > Refraction page
in the STK help system.
There are three models to choose from, but Effective Radius is not recommended for objects above 10km.
The attached PDF, named ADA585881.pdf, (which also can be found at https://apps.dtic.mil/sti/pdfs/ADA585881.pdf
) augments the STK help where it performs a comparison of the Effective Radius and ITU models in the modeling of a ground radar accessing space objects at low elevation angles; the conclusions are found page 29.