Calibration of Adaptive Antennas in Satellite Navigation Receivers

Kurzfassung

Adaptive antennas with digital beamforming represent a promising technology for mitigation of radio interference and multipath effects in receivers of Global Satellite Navigation Systems (GNSS, e.g. GPS, Galileo). Due to the mitigation of interfering signals in spatial domain, they allow for better performance in terms of signal to noise plus interference ratio (SNIR). However, the adaptive antenna techniques may also introduce deviation of the array phase centre from its nominal position, which results in erroneous pseudo-range and carrier-phase measurements of the GNSS array receiver. The deviation of the array phase centre manifests itself through some additional carrier phase shifts and signal group delays caused by the adaptive antenna to the received signals. In order to control such deviations and to produce the corresponding corrections, the magnitude and phase response of the adaptive array system described by a system transfer function must be known for the directions of arrival of the satellite signals. The aim of the paper is to explain the challenge and the need of stabilizing phase centres of adaptive GNSS antennas and to detail the novel development of suitable means for controlling the phase centres. The problem of correcting for the phase centre deviation is tightly related to the calibration problem of the antenna array hardware. Indeed, in both problems the phase and amplitude distortions of an ideal array response due to the transfer characteristics of the array antenna elements and RF front-ends should be determined. Therefore, the solution of the calibration problem can be also considered as the solution for correcting for the phase centre deviations. The focus of this paper is on practical aspects of the array calibration in GNSS adaptive antenna systems. After introducing the system transfer function of an adaptive antenna, a model for the error propagation from the domain of code- and carrier-based pseudo-range observations of a GNSS receiver into the positioning error domain will be shortly outlined. This model will be further used to show the effect of phase calibration errors and different radiation patterns of the array elements on the GNSS receiver performance defined in term of user positioning and timing errors. A practical implementation of the adaptive GNSS antenna system that includes a real-time calibration capability will be presented. The performance of the array calibration will be demonstrated by using the field test results for beamforming and direction-of-arrival estimation as well as positioning by using code and carrier measurements.