Antenna-based Multipath and Interference Mitigation for Aeronautical Applications: Present and Future

Kurzfassung

Vulnerability to radio interference and multipath is a wellknown drawback of satellite-based navigation systems. This is widely accepted as a critical issue for safety-of-life applications such as precision approach operations in aviation. Interference and multipath signals enter the navigation receiver through the antenna which, in order to minimise such signals, can be used as a spatial- and frequency-domain filter provided that some critical antenna parameters are controlled. The most important parameters are the shape of the antenna pattern with associated cross-polar levels and polarisation purity as well as an appropriate in and out-of-band frequency response. This paper aims at giving an overview of existing and future antenna technologies to be used with satellite navigation receivers. Also, we will consider some auxiliary technologies like resistive or high impedance ground planes which can be used in combination with antenna methods for both the above mentioned categories. Firstly, interference and multipath encountered by an aviation receiver of a Global Navigation Satellite System (GNSS) are briefly discussed. The objective is to set the scene and to identify the most significant sources of radio interference, like DME (Distance Measuerment Equipment) and TACAN (Tactical Air Navigation) systems, and multipath for which some antenna solutions may be provided. The critical antenna parameters for each type of interference will be identified. Secondly, existing antenna solutions and techniques used nowadays with an airborne GPS receiver are reviewed. The majority of existing GNSS antenna solutions are single element based, which usually provide low costs and low implementation complexity. We summarise the main technologies used with single antennas including helical antennas, patch antennas, stacked patch antennas and cavity backed antennas. The compatibility of the reviewed structures with the requirements to critical antenna parameters determined above will be discussed in detail. Advanced antenna systems including antenna arrays and auxiliary technologies will be presented in the paper. Such technologies are based on the use of absorbing materials or borrow from the concept of choked antennas commonly used in ground based geodesy applications. These will be discussed and assessed in view of combating interference and multipath. Array system methodologies currently used in GNSS anti-jam systems will be described and assessed. Array GNSS systems offer advanced capabilities to overcome interference at the expense of size, cost and manufacturing complexity. They are mostly used in military applications to combat intentional interference by shaping the antenna radiation pattern and placing nulls in the direction of interference. These types of antennas are commonly called CRPA´s (Controlled Radiation Pattern Arrays) due to their abilities to tailor their radiation patterns as a function of the interference threats. In this paper, we will analyse conformal array geometries with rectangular and circular grids adjusted to the airframe curvature.