Novel Timing Antennas for Improved GNSS Resilience
Ian McMichael and Erik Lundberg, The MITRE Corporation
Location: Regency B
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, multiple quadrifilar helix antennas are presented for the application of GNSS timing with improved resilience from these ground based interference sources. 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, like controlled reception pattern antennas and fixed pattern blocking 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 beam width 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% 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% of the time.
The third antenna operates over L1 (1.575 GHz) and L2 (1.227 GHz) frequencies, which is currently required for some military applications and will find use in civilian applications as the GPS constellation becomes populated with Block IIR-M and later generation satellites. Helix antennas can be nested concentrically for dual band operation in a compact form, where the outer helix typically radiates at a lower frequency than the inner helix due to its larger diameter. However, the coupling between the antennas can affect the patterns of the individual elements. A novel method is presented here to decouple the concentric helices based on distributed trap circuits along the length of the helix arms, which preserves the horizon nulling patterns at both frequencies. The trap circuits are integrated on the outer helix, i.e. L2, in series with the helix arms. The trap circuits’ high impedance resonance is designed to coincide with the resonant frequency of the inner helix, i.e. L1, which creates an open circuit at each trap location. Open circuiting the outer helix arms at multiple locations breaks up the helix into multiple short sections, which significantly reduces coupling to the inner helix. While the short sections of the open circuited outer helix scatter a small amount of energy, the horizon nulling pattern created by the inner helix is minimally distorted. At the resonant frequency of the outer helix, i.e. L2, the trap circuits have a low impedance and the L2 horizon null is preserved. In this way, the single-frequency helix antennas are able to be nested concentrically for dual band operation without degrading the horizon null for either band and without making the helix taller than the individual single frequency elements. Simulations of the dual band concentric helix antenna show a zenith-to-horizon gain ratio of about 35 dB for both frequencies.