Helical antennas have been developed that feature a cutoff pattern and are suitable for practical positioning at millimeter level. The antennas are in the form of a tube with 3 cm in diameter and 40 cm in height. The antennas provide 20-dB suppression of the reflections from the earth surface, starting from low elevations. Open sky tests have demonstrated that the multipath contribution to the real-time positioning error falls below thermal noise. By smoothing the noise, these antennas achieve a positioning accuracy of 1 mm RMS in the vertical coordinate and 0.7 mm in the horizontal in almost real time.

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Common choke ring ground planes are known to contribute to undesirable antenna pattern narrowing in the elevation plane which is associated with difficulties of tracking low elevation satellites. Also known is the comparatively narrow frequency bandwidth of the choke grooves structure. As an alternative, using a convex impedance ground plane has been suggested for fullspectrum GNSS applications. With such ground planes a pin structure is utilized instead of choke grooves to allow a frequency bandwidth increase. A semi-spherical shape of the ground plane is shown to provide increased antenna gain for low elevation angles. Theoretical performance estimates along with experimental test data have been provided.

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