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B Multipath Elimination Technology
64 MiLLennium Command Descriptions Manual
Figure B-3 Illustration of Quadrifilar vs. Microstrip Patch Antennae
Antenna Ground Planes
Nearby objects can influence the radiation pattern of an antenna. Thus, one of the roles of the antenna ground plane
is to create a stabilizing artificial environment on which the antenna rests and which becomes a part of the antenna
structure and its resultant radiation pattern.
A small ground plane (relative to one wavelength at the operating frequency) may have minimal stabilizing effect,
whereas a large ground plane (multiple wavelengths in size) will have a highly stabilizing effect.
Large ground planes also exhibit a shielding effect against
RF signal reflections originating below the antenna’s
radiation pattern horizon. This can be a very effective low angle shield when the antenna is elevated on a hill or
other structure above other reflecting surfaces such as vehicles, railway tracks, soil with high moisture content,
water bodies, etc.
One of the drawbacks of a "flat plate" ground plane is that they do provide an above horizon reflective surface for
low angle
GPS signals. This means that the flat plate is also a multipath generating surface. For pseudorange code
measurements, these multipath signals are too close to cause any significant range errors. However, for carrier
phase measurements, the flat plate can cause significant biases. Even if carrier phase is not being used for range
measurements, the flat plate reflections could be substantial enough to cause signal fades and drop-outs due to
carrier phase reversals from the flat plate reflections (keeping in mind that these problems are most substantial for
low angle signals). It should also be kept in mind that low profile antennas such as the patch antenna will obviously
be less susceptible to this phenomenon than higher profile quadrifilar and bifilar helix antennas.
The most effective type of multipath reduction ground plane structure is the "choke ring" ground plane. Due to its
surface cavity construction, surface reflections are essentially trapped, thus minimizing the problems encountered
by flat plate ground planes. This is what makes NovAtel’s
GPSAntenna model 501 so successful when used with
the NovAtel
GPSAntenna Choke Ring Ground Plane.
Figure B-4 Example of GPSAntenna on a Flat Plate vs. Choke Ring Ground Plane
Quadrifilar Elements
Radome
Antenna Patch
Dielectric
Patch Ground Plane
Quadrifilar Helix Antenna Microstrip Patch Antenna
Flat plate
Choke Ring