The Effects of Ground-Planes on Rotating GPS Antennas

Avram K. Tetewsky and Jeffery B. Lozow

Abstract:	Time-varying changes in the orientation of a single GPS antenna with respect to the circularly polarized GPS sig-nals in space can produce carrier phase modulation (Refs. [1]-[3], [7]-[9]). However, popular GPS references and software tools (Refs. [4]-[6]) do not include these terms, and typically show only the contribution of translational terms to the GPS group delay and carrier phase measure-ments. With the emerging real-time kinematic (RTK) dif-ferential positioning and attitude fix algorithms requiring cm level accuracy, coupled with arrays of antennas that are not constrained to be coplanar, orientation effects must be modeled. This paper presents a general theory of the phase effects introduced by changing antenna orientation and extends previous work (Refs. [1]-[3], [7]-[9]) by allowing ground plane effects to be modeled. Although computer models will ultimately be needed in order to account for ground-plane effects, by working with some simplified coordinate frames and motion models, additional factoring of the polarization functions into a pure orientation term plus a small residual spin modulation function yields valuable insights into the physics and interpretation of the polari-zation terms. A brief review of the phase wrap-up problem and the original derivation based on the Hertzian (infinitesimal) dipole approximation without a ground-plane is presented. Next, directivity patterns for a small micro-patch antenna with ground planes are evaluated, and the phase wrap-up and spin modulation polarization effects are calculated. Because the directivity patterns hold only for the upper half-space, care must be used when calculating element responses so as to exclude contributions from beneath the ground plane. Due to this additional complexity, only the results for the antenna’s boresight aligned with the spin axis will be covered. Although the technique is general, other geometries introduce significant amounts of algebra, and results are not yet available. The impact that the po-larization phase has on raw (pseudorange and carrier phase) measurements, navigation fixes, and general re-ceiver operation are also discussed.
Published in:	Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999) June 27 - 30, 1999 Royal Sonesta Hotel Cambridge, MA
Pages:	289 - 300
Cite this article:	Tetewsky, Avram K., Lozow, Jeffery B., "The Effects of Ground-Planes on Rotating GPS Antennas," Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999), Cambridge, MA, June 1999, pp. 289-300.
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