GPS World, February 2009

Antenna Technology INNOVATION addition to the legacy GPS bands and the antenna feeding a receiver may need to cover some or all of these bands TABLE 1 presents an overview of the frequencies used by the various GNSS constellations Keep in mind that you may see slightly different numbers published elsewhere depending on how the signal bandwidths are defined As the bandwidth requirement of an antenna increases the antenna becomes harder to design and developing an antenna that covers all of these bands and making it compliant with all of the other requirements is a challenge If small size is also a requirement some level of compromise will be needed Gain Pattern For a transmitting antenna gain is the ratio of the radiation intensity in a given direction to the radiation that would be obtained if the power accepted by the antenna was radiated isotropically For a receiving antenna it is the ratio of the power delivered by the antenna in response to a signal arriving from a given direction compared to that delivered by a hypothetical isotropic reference antenna The spatial variation of an antennas gain is referred to as the radiation pattern or the receiving pattern Actually under the antenna reciprocity theorem these patterns are identical for a given antenna and ignoring losses can simply be referred to as the gain pattern The receiver operates best with only a small difference in power between the signals from the various satellites being tracked and ideally the antenna covers the entire hemisphere above it with no variation in gain This has to do with potential cross correlation problems in the receiver and the simple fact that excessive gain roll off may cause signals from satellites at low elevation angles to drop below the noise floor of the receiver On the other hand optimization for multipath rejection and antenna noise temperature see below require some gain roll off FIGURE 1 shows what a perfect hemispherical gain pattern looks like with a cut through an arbitrary azimuth However such an antenna cannot be built and real world GNSS antennas see a gain roll off of 10 to 20 dB from boresight looking straight up from the antenna to the horizon FIGURE 2 shows what a typical gain pattern looks like as a cross section through an arbitrary azimuth Circular Polarization Spaceborne systems at L Band typically use circular polarization CP signals for transmitting and receiving The changing relative orientation of the transmitting and receiving CP antennas as the satellites orbit the Earth does not cause polarization fading as it does with linearly polarized signals and antennas Furthermore circular polarization does not suffer from the effects of Faraday rotation caused by the ionosphere Faraday rotation results in an electromagnetic wave from space arriving at the Earths surface with a different polarization angle than it would have if the ionosphere was absent This leads to signal fading and potentially poor reception of linearly polarized signals Frequencies MHz System L1 L2 L3 L5 E5 E6 GPS 1563 1588 1215 1240 N A 1164 1189 N A GLONASS 1592 1615 1237 1257 1194 1209 N A N A Galileo 1554 1596 N A N A 1145 1238 1258 1300 Å TABLE 1 GNSS Frequency Allocations 10 08 06 04 02 0 150 100 50 0 50 100 150 Gain linear Theta degees Å FIGURE 1 Theoretical antenna with hemispherical gain pattern Boresight corresponds to 0 10 5 0 5 10 15 20 25 30 35 40 90 60 30 0 30 60 90 Theta degrees Gain dBi Å FIGURE 2 Real world antenna gain pattern Circularly polarized signals may either be right handed or left handed GNSS satellites use right hand circular polarization RHCP and therefore a GNSS antenna receiving the direct signals must also be designed for RHCP Antennas are not perfect and an RHCP antenna will pick up some left hand circular polarization LHCP energy Because GPS and other GNSS use RHCP we refer to the LHCP part as the cross polar component see FIGURE 3 We can describe the quality of the circular polarization by either specifying the ratio of this cross polar component with respect to the co polar component RHCP to LHCP or by specifying the axial ratio AR AR is the measure of the polarization ellipticity of an antenna designed to receive circularly polarized signals An AR close to 1 or 0 dB is best indicating a good circular polarization and the relationship between the co cross polar ratio and axial ratio is shown www gpsworld com February 2009 GPS World 43