GPS System Segments - Understanding GPS: Principles and Applications - page 105

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 conduct-
ing ground plane. Therefore, a typical GPS antenna tends to be predominantly verti-
cally 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 esti-
mating 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 electri-
cal and physical phase centers for survey-grade GPS antennas may vary by centime-
ters. 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 pur-
pose 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 con-
tent of the received satellite signal that is processed, the better the accuracy perfor-
mance 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.)