GNSS Antennas and Front Ends - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 54

sources to mitigate for GNSS is multipath. When the GNSS signal is reflected
off an object, an undesirable situation for a system attempting to measure time-
of-flight, the polarization will flip to left-hand circular polarization (LHCP). An
RHCP antenna is quite effective in suppressing the LHCP reflection and mini-
mizing this error source. Of course, a second reflection will reestablish the RHCP
polarization, but the signal power is also likely diminished as a result of the multi-
ple reflections. Thus, the polarization of the GNSS antenna provides a significant
level of suppression from erroneous multipath reflection.
The antenna pattern describes the directivity of the antenna. The most basic
idea for the antenna pattern would be one that receives signals equally from all
directions—this is known as an isotropic antenna. However, such a uniform gain
pattern does not make sense for GNSS. Since the signal source, GNSS satellites,
are overhead for most applications the preferred antenna pattern would be hemi-
spherical, designed to receive signal from only positive elevation angles from all
azimuth directions. Given the problem of multipath and that most multipath rays
arrive from low elevation angles, the antenna pattern could be crafted such that it
was designed to receive signals only above 10 ◦ -20 ◦ elevation. Such an approach
is definitely bound to further reduce multipath reflections, but as a consequence,
the low elevation satellite signals would also be neglected, decreasing the avail-
ability of satellite measurements. A promising research area within GNSS an-
tennas is that of antenna arrays, or a combination of individual antenna elements
combined in such a way to shape distinct antenna pattern beams and nulls. Such an
implementation should provide significant performance enhancement for GNSS.
Probably two of the most popular GNSS L1 antenna implementations are the
patch and helix approaches but others also exist. These refer to the actual con-
struction of the antenna element itself. Yet the parameters above should provide a
measure of comparable performance between antennas.
The last topic in regard to antennas refers to the choice of an active or passive
antenna. An antenna will often be integrated with other front-end components
that improve their performance or are necessitated by the environment in which
the antennas will operate.
One important parameter in front-end design is the overall noise figure F n of
the system. This parameter quantifies the noise added as a result of the analog
signal conditioning. Of course, any additive noise or decrease in signal-to-noise
ratio (SNR) is undesirable and should be minimized.
Denoting the resulting system noise figure by F system , the noise figure F n of the
n th element in cascade, and the gain of the n th element in cascade G n the formula
for noise figure is

F 2 − 1 F 3 − 1

G 1 G 2 +

1
G 1 G 2 G 3 +···+

F 4 − 1

G 1 G 2 G 3 ···

F n −

F system = F 1 + +

G n − 1 .

(4.2)

G 1

What this equation indicates is that the first element in the RF cascade dominates
the resulting noise figure for the system. This indicates that all passive compo-
nents (cables and filters) prior to the first amplifier will have a negative impact