For the distance calculation, the planet is modeled as a spheroid.
If you do not pass in values for the semimajor and invflattening arguments, the computation uses the semimajor axis and the inverse flattening ratio from the World Geodetic System, WGS84. A value of 6,378,137.0 meters is used for the semimajor axis, and a value of 298.257223563 is used for the inverse flattening ratio.
Distance calculations between antipodal or nearly antipodal points using the ST_SpheroidalDistance method may fail to converge, generating a Teradata Database error. For these distance calculations use instead the ST_SphericalDistance method.