The far field pattern degradation of space fed phased arrays suitable for a space based radar is examined. The effects considered are: Structural; (1) Axial lens surface distortions, (2) Uniform radial thermal expansion, and (3) Axial and lateral feed displacements; Electrical; (1) Element phase and amplitude excitation errors, and (2) Failed elements. An introductory section discusses the size, cost, and weight penalties of low sidelobe designs. The final section presents a method of phase compensation or coherence of large axial lens distortions.