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Antennas
| Receivers | |
|---|---|
| Title | Antennas |
| Author(s) | GMV |
| Level | Advanced |
| Year of Publication | 2011 |
From a user prespective, antennas are the main interface between the GNSS Space Segment (the satellite constellations) and the User Segment (GNSS receivers), as they are responsible for capturing the L-band signals transmitted from space. Due to the characteristics of GNSS systems, receiver antennas are typicaly Right-Hand Circularly Polarized[1] (RCHP) antennas, and the spacial reception pattern is hemispherical: this configuration enables user reception of satellite signals in any azimuthal direction, and from zenith to horizon, thus maximizing satellite visibility.
Antenna types
Different applications may require different antenna location and technology[2]. In fact, as show in Figure 1, antennas can range from large roof-mounted antennas (e.g. for a static open sky view) to embedded in receiver boards themselve (e.g. in mobile handheld devices):
- Patch Antennas: Patch antennas...
Characteristics
Satellite signals are received via the antenna, which is right-hand circularly polarized (RHCP) and provides near hemispherical coverage. Typical coverage is 160° with gain variations from about 2.5 dBic at zenith to near unity at an elevation angle of 15°. (The RHCP antenna unity gain also can be expressed as 0 dBic = 0 dB with respect to an isotropic circularly polarized antenna.) Below 15°, the gain is usually negative. An example antenna pattern is shown in Figure 3.24. This pattern was pro duced by a stacked-patch antenna element embedded in a dielectric substrate. This particular antenna is designed to operate at both L1 and L2, but only the L1 pattern has been provided for illustration. Even well-designed GPS antennas will exhibit a small but nonzero cross-polarized left-hand circularly polarized (LHCP) response in addition to the desired RHCP pattern shown in Figure 3.24. It can be observed that the RHCP response is nearly perfect at boresight, but as the elevation angle decreases the response is attenuated (i.e., the antenna gain decreases). This gain decrease is attributed to the horizontal electric field component being attenuated by the conducting ground plane. Therefore, a typical GPS antenna tends to be predominantly vertically polarized for low elevation angles. At zenith, the ratio of the vertical electric field to the horizontal electric field response is near unity. This ratio is referred to as the axial ratio. As the elevation angle decreases, the axial ratio increases. Another GPS antenna design factor is transfer response. So that the signal is undistorted, it is desirable for the magnitude response to be nearly constant as a function of frequency and for the phase response to be linear with frequency within the passband of interest. (GPS signal bandwidths are discussed later as well as in Chapter 4.) Furthermore, when we compute position with a GPS receiver, we are truly estimating the position of the electrical phase center of the antenna. There is both a physical and an electrical realization of this phase center. The physical realization is just that. One can actually use a ruler to measure the physical center of the antenna. However, the electrical phase center is often not collocated with the physical phase center and may vary with the direction of arrival of the received signal. The electrical and physical phase centers for survey-grade GPS antennas may vary by centimeters. Calibration data describing this difference may be required for high-accuracy applications. Finally, a low-noise amplifier may be embedded in the antenna housing (or radome) in some GPS antennas. This is referred to as an active antenna. The purpose of this is to maintain a low-noise figure within the receiver. One must note that the amplifier requires power, which is usually supplied by the receiver front end thru the RF coaxial cable. The antenna (and receiver front end) must have sufficient bandwidth to pass the signals of interest. Typically, the bandwidth of a GPS patch or helix antenna ranges from 1% to 2% of the center frequency. Two percent bandwidths for L1, L2, and L5 center frequencies are 31.5 MHz, 24.6 MHz, and 23.5 MHz, respectively. GPS receivers that track P(Y) code on both L1 and L2 need to accommodate on the order of 20.46-MHz bandwidths on both frequencies. If the set only tracks C/A code or L1C on L1, the antenna (and receiver) need to accommodate bandwidths of approximately 2.046 and 4.092 MHz, respectively. It should be noted that the receiver’s antenna/front-end bandwidth is directly proportional to the accuracy required for the specific application of the receiver. That is, the more frequency content of the received satellite signal that is processed, the better the accuracy performance will be. For example, a survey receiver antenna/front end will most likely be designed to pass the full 20.46 MHz of the P(Y) code. Whereas, a low-cost hiking receiver designed for C/A code may only have a front-end bandwidth of 1.7 MHz instead of the full 2.046 MHz. (Further elaboration on bandwidth and accuracy performance is contained in Chapter 5.) New civil signals L2C and L5 have null-to-null bandwidths of 2.046 MHz and 20.46 MHz, respectively. The military M code can be processed within the existing L1 and L2 24-MHz bandwidths. Since M code signal power is defined within a 30.69-MHz band around the center frequency, approximately 92% of this power is within the 24-MHz band. (GPS signal characteristics are contained in Chapter 4.) The addition of new signals (M code, L1C, L2C, and L5) will require new antennas for some users. For example, those utilizing L1 C/A code and L2C will need a dual-band antenna. (Dual frequency measurements enable determination of the ionospheric delay and provide robustness to interference. Ionospheric delay determination and compensation are discussed in Chapter 7.) SOL signal users that require operation in the ARNS bands will need antennas to receive C/A code on L1 and the L5 signal on L5. At the time of this writing, RTCA was developing aviation standards for a dual-band L1/L5 antenna. Some receivers may be tri-band. That is, they will receive and process the signals broadcast on all three GPS frequencies, L1, L2, and L5, which will require a tri-band antenna. Reference [33] provides details on one approach for a tri-band (L1/L2 M code and L5) antenna design. Antenna designs vary from helical coils to thin microstrip (i.e., patch) antennas. High-dynamic aircraft prefer low-profile, low–air resistance patch antennas, whereas land vehicles can tolerate a larger antenna. Antenna selection requires evaluation of such parameters as antenna gain pattern, available mounting area, aerodynamic performance, multipath performance, and stability of the electrical phase center of the antenna [34]. Another issue regarding antenna selection is the need for resistance to interference. (In the context of this discussion, any electronic emission, whether friendly or hostile, that interferes with the reception and processing of GPS signals is considered an interferer.) Some military aircraft employ antenna arrays to form a null in the direction of the interferer. Another technique to mitigate the effects of interference is to employ a beam-steering array. Beam-steering techniques electronically concentrate the antenna gain in the direction of the satellites to maximize link margin. Finally, beam forming combines both nulling and beam steering for interferer mitigation. (References [35–37] provide detailed descriptions of the theory and practical applications of nulling, beam steering, and beam forming.)
USER vs GROUND SEGMENT ANTENNAS (figure)?
Front end connection
- Distance / placement vs embedded - Coax cable losses (table)?
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References
- ^ http://en.wikipedia.org/wiki/Circular_polarization
- ^ Grewal, M. et al, "Global Positioning Systems, Inertial Navigation, and Integration", chapter 4, section 4.3.1