Antenna control systems: From PI to H-infinity

This paper discusses the compensation of antenna-pointing errors following the recent analysis and retrofit of the NASA Deep Space Network antenna-con trol systems. The desired high-frequency communications with spacecraft (at Ka-band) require improved pointing precision over lower-frequency communic ations (at X-band). The quality of the antenna drives (hardware), the contr ol algorithm (software), and the physical structure of the antenna (in term s of thermal deformations, gravity distortions, encoder mounting, and wind gusts) all influence pointing precision, and create the challenging task of remaining within the required pointing-error budget. Three control algorithms - PI (proportional-and-integral), LQG (linear-quad ratic-Gaussian), and H-infinity - are discussed, and their basic properties , tracking precision, and limitations as applied to antenna tracking are ad dressed. The paper shows that the PI algorithm is simple and reliable, but its performance is limited. It also explains how significant improvements i n tracking precision are achieved when implementing the LQG control algorit hm or the H-infinity control algorithm. Still, pointing precision attributa ble to software modification is limited. It is pointed out that an addition al increase of tracking precision requires concurrent improvements in the a ntenna drives.