GPS receiver is 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 GPS chip set being considered, PU/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
Downconverter
The function of the downconverter is to step down each GPS satellite signal from its broadcast RF frequency to an IF signal that can be output to the base-band processor. The signal from each SV in view of the antenna (active or passive) is filtered and amplified by a low noise pre-amplifier, which sets the overall noise of the system, and rejects out of band interference. The output of this pre-amplifier is input into the downconverter, where the conversion to the IF signal is typically made in two stages. The two-stage mixer is clocked by a fixed-frequency phase-locked loop controlled by an external reference oscillator that provides frequency and time references for the downconverter and base-band processor.
Correlator/data processor
The correlator component in a GPS receiver performs the high-speed digital signal processing functions on the IF signal necessary to acquire and track each SV in view of the antenna. The IF signal received by the correlator from the downconverter is first integrated to enhance the signal, then the correlator performs further demodulation and despreading to extract each individual SV signal being received. Each signal is then multiplied by a stored replica of the C/A signal from the satellite being received, known as the Gold code for that satellite. The timing of this replica signal is adjusted relative to the received signal until the exact time delay is determined. This adjustment period to calculate the time delay between the local clock and the SV signal is defined as the acquisition mode. Once this time delay is determined, that SV signal is then considered acquired, or locked.