Some common antennas used for GPS applications

GPS receivers are composed of three primary components: the antenna, which receives the radio frequency (RF) broadcasts from the satellites; the downconverter, which converts the RF signal into an intermediate frequency (IF) signal; and the baseband processor or correlator, which uses the IF signal to acquire, track, and receive the navigation message broadcast from each SV in view of the receiver. In most systems, the output of the correlator is then processed by a microprocessor (MPU) or microcontroller (MCU), which converts the raw data output from the correlator into the positioning information which can be understood by a user or another application. The sections below provide an overview of the three key components of a GPS receiver, describing in generic terms the functionality and capabilities typically found in these systems. As the capabilities of the MPU or MCU needed to process the correlator output is largely dependent on the needs of the applications and the particular tracking device chip set being considered, MPU/MCU requirements and capabilities are not discussed here.

Antennas

As with most RF applications, important performance characteristics to be considered when selecting the antenna for a GPS receiver include impedance, bandwidth, axial ratio, standing wave ratio, gain pattern, ground plane, and tolerance to moisture and temperature. In addition, the relatively weak signal transmitted by GPS satellites is right-hand circularly polarized (RHCP). Therefore, to achieve the maximum signal strength the polarization of the receiving antenna must match the polarization of the transmitted satellite signal. This restriction limits the types of antennas that can be used. Some of the more common antennas used for GPS applications include:

microstrip, or patch, antennas are the most popular antenna because of their simple, rugged construction and low profile, but the antenna gain tends to roll-off near the horizon. This makes it more difficult to acquire SVs near the horizon, but it also makes the antenna less sensitive to multipath signals. This type of antenna can be used in single or dual frequency receivers.

helix-style antennas have a relatively high profile compared to the other antennas, maintaining good gain near to the horizon. This can provide easier acquisition of SVs lower on the horizon, but also makes it more sensitive to multipath signals that can contribute to receiver error. The spiral helix antenna is used in dual-frequency receivers, while the quadrifilar helix antenna is used in single frequency systems.

monopole and dipole antennas are low cost, single frequency antennas with simple construction and relatively small elements.

Systems with an antenna that is separate from the receiver unit, such as a vehicle GPS locator installed in a vehicle with a trunk-mounted antenna, often use an active antenna which includes a low noise pre-amplifier integrated into the antenna housing. These amplifiers, which boost the very weak received signal, typically have gains ranging from 20 dB to 36 dB. Active antennas are connected to the receiver via a coax cable, using a variety of connectors, including MMCX, MCX, BNC, Type N, SMA, SMB, and TNC. Systems that have the antenna integrated directly into the receiver unit (such as a portable GPS tracking ) use passive antennas, which do not include the integrated pre-amplifier.