Novel Timing Antennas for Improved GNSS ResilienceMarch 2018
Topics: Sensing and Signal Processing, Signal Processing, Geographic Information Systems, Satellite Communications, Global Positioning System, Position, Navigation, and Timing
Global Navigation Satellite System (GNSS) antennas installed at fixed site infrastructure are susceptible to interference, jamming, and spoofing signals incident along the direction of the horizon. In this paper, a set of requirements are derived for GNSS antennas that ensure critical infrastructure timing receivers have access to sufficient satellites to derive resilient time and frequency while placing a null in all polarizations at and below the horizon. Multiple quadrifilar helix antennas that meet these requirements are also presented. The efficacy of the designs is demonstrated with field test results. The salient feature of these antennas is a null in the gain pattern in the direction of the horizon and around all azimuth angles to suppress ground-based interference. Other types of antennas have been developed to minimize interference, such as controlled reception pattern antennas. However, none of these antennas simultaneously have sufficient performance, size, weight, power, and cost for widespread applications in commercial and military installations. The proposed high-performance antennas provide GNSS resilience in a small form factor at a low-cost due to the simple architecture.
The first antenna operates at L1 (1.575 GHz) and employs a novel method of reactive loading along the length of the multi-turn helix. The phase distribution along the helix creates a deep null in the gain pattern at the horizon while maintaining sufficient beamwidth in the zenith direction. The prototype antenna is 7.5 inches tall, 1 inch in diameter, and is mounted on a 7-inch diameter ground plane. Gain pattern measurements exhibit a 4.0 dBiC zenith gain and a zenith-to-horizon gain ratio (i.e. null depth) of 29 dB for right hand circular polarization (RHCP) and 34 dB for left hand circular polarization (LHCP). This horizon null minimizes ground based interference. The half power beamwidth (HPBW) of this antenna is approximately 100°, which is sufficient to have access to the required number of satellites for timing applications at least 99 percent of the time.
The second antenna operates at L1 and achieves a horizon null by varying the pitch of the helix arms along the length of the antenna. The variable pitch antenna prototype is 7.8 inches tall, 1.4 inches in diameter, and is mounted on a 7-inch diameter ground plane. Gain pattern measurements exhibit a zenith gain of 7.5 dBiC and a 30-dB zenith-to-horizon ratio for both RHCP and LHCP. The measured HPBW is 60°, which is sufficient to have access to the required number of satellites for timing at least 95 percent of the time.