Geometry of the GBT used in Antenna Workbench calculations. SF and PF are the two foci of the ellipsoid of which the subreflector is a segment. PF is also the primary focus of the main parabolic reflector. The “s” coordinate system used for the near-field subreflector patterns is aligned with the z_s axis running along the ellipsoid axis from PF toward SF. The illumination of the primary by these patterns is given in the “p” coordinate system, rotated with respect to “s”, so that the z_p axis runs from PF toward the projected center of the primary. The all-sky far-field response is given in the non-subscripted coordinate system, where the z axis runs from PF in the direction of the main beam. All have y and z in the symmetric plane of the antenna.