Page 421 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 4. 1 Global positioning technology

either available today or in development to provide that is used in dual-frequency receivers, while the
functionality. quadrifilar helix antenna is used in single frequency
systems.
14.1.4.1 GPS receiver components monopole and dipole antennas are low cost, single
frequency antennas with simple construction and
GPS receivers are composed of three primary compo- relatively small elements.
nents: the antenna, which receives the radio frequency
(RF) broadcasts from the satellites; the downconverter, Systems with an antenna that is separate from the re-
which converts the RF signal into an intermediate fre- ceiver unit, such as a GPS receiver installed in a vehicle
quency (IF) signal; and the baseband processor or corre- with a trunk-mounted antenna, often use an active an-
lator, which uses the IF signal to acquire, track, and receive tenna which includes a low noise pre-amplifier integrated
the navigation message broadcast from each SV in view of into the antenna housing. These amplifiers, which boost
the receiver. In most systems, the output of the correlator the very weak received signal, typically have gains ranging
is then processed by a microprocessor (MPU) or micro- from 20 dB to 36 dB. Active antennas are connected to
controller (MCU), which converts the raw data output the receiver via a coax cable, using a variety of connec-
from the correlator into the positioning information which tors, including MMCX, MCX, BNC, Type N, SMA,
can be understood by a user or another application. SMB, and TNC. Systems that have the antenna inte-
The sections below provide an overview of the three grated directly into the receiver unit (such as a handheld
key components of a GPS receiver, describing in generic GPS device) use passive antennas, which do not include
terms the functionality and capabilities typically found in the integrated pre-amplifier.
these systems. As the capabilities of the MPU or MCU The demand for the integration of positioning tech-
needed to process the correlator output is largely nology into smaller devices is challenging antenna
dependent on the needs of the applications and the development. The industry is already pushing for smaller
particular GPS chip set being considered, MPU/MCU antennas for applications such as a wristwatch with in-
requirements and capabilities are not discussed here. tegrated GPS, which is smaller than most patch antennas
available today. Another demand is for dual-purpose an-
tennas that do double duty in wireless communication
14.1.4.1.1 Antennas
devices, such as in a mobile telephone with an integrated
As with most RF applications, important performance GPS receiver. Inevitably, the future will bring smaller
characteristics to be considered when selecting the an- and more flexible antennas for GPS applications.
tenna for a GPS receiver include impedance, bandwidth,
axial ratio, standing wave ratio, gain pattern, ground plane,
and tolerance to moisture and temperature. In addition, 14.1.4.1.2 Downconverter
the relatively weak signal transmitted by GPS satellites is The function of the downconverter is to step down each
right-hand circularly polarized (RHCP). Therefore, to GPS satellite signal from its broadcast RF frequency to
achieve the maximum signal strength the polarization of an IF signal that can be output to the base-band pro-
the receiving antenna must match the polarization of the cessor. The signal from each SV in view of the antenna
transmitted satellite signal. This restriction limits the (active or passive) is filtered and amplified by a low noise
types of antennas that can be used. Some of the more pre-amplifier, which sets the overall noise of the system,
common antennas used for GPS applications include: and rejects out of band interference. The output of this
pre-amplifier is input into the downconverter, where the
microstrip,or patch, antennas are the most popular conversion to the IF signal is typically made in two stages.
antenna because of their simple, rugged construc- The two-stage mixer is clocked by a fixed-frequency
tion and low profile, but the antenna gain tends to phase-locked loop controlled by an external reference
roll-off near the horizon. This makes it more difficult oscillator that provides frequency and time references
to acquire SVs near the horizon, but it also makes the for the downconverter and base-band processor.
antenna less sensitive to multipath signals. This type The mixer outputs, which are composed of in-phase
of antenna can be used in single or dual frequency (I) and quadraphase (Q) signals, are amplified again and
receivers. latched as the IF input to the base-band processor to be
helix-style antennas have a relatively high profile used for satellite acquisition and tracking. To enable the
compared to the other antennas, maintaining good baseband processor to account for frequency variation
gain near to the horizon. This can provide easier over temperature, an integrated temperature sensor is
acquisition of SVs lower on the horizon, but also often included in the downconverter circuit.
makes it more sensitive to multipath signals that can The downconverter in a GPS receiver is often sus-
contribute to receiver error. The spiral helix antenna ceptible to performance degradation from external RF

428