Lessons Learnt: The Development of a Robust Multi-Antenna GNSS Receiver

Abstract

In the year 2006 the Institute of Communications and Navigation of the German Aerospace Center started the development of a multi-antenna real-time receiver platform to demonstrate the performance gain of digital beamforming technologies for satellite navigation, especially in the field of safety critical applications. For those safety critical applications a reliable and precise navigation solution is crucial. It was clear at the very beginning that the impact of interference on the system has to be considered at each stage of the receiving chain from the single antenna element to the position solution. For this reason a completely new receiver design was necessary, which provides access to each part in the receiver to take appropriate counter measures against almost any kind of interference and jamming. In the current stage of the receiver robust adaptive beamforming has been developed and implemented. The algorithm manages to reach the theoretical bound of signal power boost which is approx. 7 dB for a square 2x2 antenna array if mutual couplings are taken into account. The direction of arrival of impinging GNSS satellite signals can be estimated within 2-3 degrees of accuracy thanks to a novel calibration method. This is a very powerful and effective tool for detecting and mitigating spoofing, interference and multi path. Due to its real-time capability, the effects of multi path signals, interference and other kind of distortions can be directly detected and assessed online for signal monitoring purposes, e.g. GBAS ground stations. The paper will give detailed insights into the necessary measures and developments for achieving the desired robustness. The first part of this paper will give a brief overview of the receiver hardware architecture. The second part will describe the development of a 2x2 antenna array for dual frequency operation. In this section the perils and tricks for an appropriate antenna array design for satellite navigation applications will be discussed. The next section will present the front end design and point out key technologies to improve the interference robustness. Subsequently a new online calibration method will be introduced which allows tracking the relative phase changes between the antenna array elements and the digital receiver input in real-time. It will be shown that for direction of arrival estimation such continuous phase calibrations are crucial and of high importance. The next section will give a brief overview of the developed and implemented beamforming and DOA algorithms. Finally, the results of laboratory and field measurements will be shown and the performance of the digital beamforming receiver will be assessed. The work intends to provide valuable insights in the development of a safety of life receiver with adaptive antennas and to present the performance gains that are obtained. These insights allow to assess the usability of the proposed safety of life receiver technologies for future applications, e. g. CAT I-III precision approaches in aviation. Also a field measurement campaign in the Galileo test range (GATE) in Berchtesgarden, Germany, is planned for summer 2010. This campaign promises very interesting results of a first multi-antenna combined GPS and Galileo receiver in a real and especially interfered environment.