By Oliver Leisten
Technical Director, Helix Technologies Ltd.
To attain the 10-centimeter accuracy required for autonomous vehicle positioning within urban multipath propagation conditions, there is a need for a significant upgrade in GNSS antenna performance. The autonomous vehicle application demands excellent antenna performance together with exploitation of the full set of GNSS multi-frequency and multi-constellation system advances to deliver this performance paradigm in the most severe of real-world use scenarios.
Given that an antenna necessarily operates in open fields, it follows that field resonance must be managed to provide predicable performance in diverse use-scenarios. A new antenna developed by Helix Technologies (Figure 1) deploys balanced fields across a cylindrical ceramic dielectric core to constrain the outreach of resonance fields and thereby minimize the interaction with nearby objects. The antenna feed is designed to provide enforcement of balanced operation, which ensures that the antenna resonates predictably and independently of the platform (i.e., the vehicle in the case of autonomous driving). Thus, the operation is not significantly influenced by the mechanical or material properties of the platform or housing. This architecture provides isolation from common-mode signals and protects the GNSS signals from conducted interference.
It is challenging to configure a GNSS antenna operating at many frequencies in which the performance at any one frequency is not impaired by mode interactions. Such impairments can have serious consequences for the position accuracy in an urban environment because they adversely affect the cross-polar discrimination: a parameter which is most important for eliminating multipath positioning errors. The architecture of the hexafilar-turnstile antenna has overcome this problem and delivers the circular polarization pattern characteristics illustrated (simulated data) in Figure 2.
The figure demonstrates that the antenna is forming cardioid patterns at two frequencies. The 3D graphic is intended to show the omni-directionality and the 2D elevation cuts exhibit the signature cardioid shape which characterize a “spinning-dipole” circular polarization antenna.
It is often suggested that patterns of wide beam-width such as these would not be particularly suitable for positioning in urban canyons where the sky can only be seen in a relatively small solid angle. In fact, the ratio of front-to-back gain is strongly associated with the cross-polar discrimination that is important for position accuracy in urban environments. Patterns of this quality can deliver as much as 30-dB of signal-to-interference advantage in favor of the direct-path satellite signals against signals whose polarization has reversed due to multipath reflection.
Helix Technologies is developing antennas which have two-pole frequency responses that provide two frequencies of optimum cross-polar discrimination that are aligned to the two frequencies of maximum spectral density of an M-BOC or Alt-BOC coded signal, as transmitted by the modern GPS and Galileo satellites respectively. These antennas should be available for test and evaluation in Q2 of 2018.