Page 422 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Global positioning technology C HAPTER 14.1

interference from both narrowband and wideband 14.1.4.2 GPS receiver solutions
sources. Common sources of narrowband interference
include transmitter harmonics from Citizens Band (CB) When access to the GPS first became available for mili-
radios and AM and FM transmitters. Sources of wide- tary and commercial use, only a few companies had the
band interference can include broadcast frequency har- technology and expertise to develop reliable, accurate
monics from microwave and television transmitters. In GPS receivers. Application developers who needed GPS
mobile GPS applications such as in intelligent vehicle services would simply purchase a board level solution
systems, the GPS receiver will often encounter this type from a GPS supplier, and integrate it into their design.
of interference, and must rely on the antenna and More recently, the demand for putting GPS capabil-
downconverter design to attenuate the effects. ities into customized packaging has grown dramatically.
To meet that demand a variety of solutions are now
available, ranging from traditional board-level solutions
14.1.4.1.3 Correlator/data processor
that connect to an application via a serial interface, to
The correlator component in a GPS receiver performs integrated circuit (IC) chip sets, which application
the high-speed digital signal processing functions on the developers can embed directly into their designs. The
IF signal necessary to acquire and track each SV in view sections below will give a brief overview of the types of
of the antenna. The IF signal received by the correlator solutions available on the market today.
from the downconverter is first integrated to enhance
the signal, then the correlator performs further de-
modulation and despreading to extract each individual 14.1.4.2.1 System level solutions
SV signal being received. Each signal is then multiplied The first commercially available GPS receivers were
by a stored replica of the C/A signal from the satellite designed as either standalone units with connectors for
being received, known as the Gold code for that satel- power, an antenna, and a serial interface to a computer or
lite. The timing of this replica signal is adjusted relative other device, or as more basic board-level solutions,
to the received signal until the exact time delay is which could be integrated into an application enclosure,
determined. This adjustment period to calculate the but which still required an external antenna connection
time delay between the local clock and the SV signal is and serial network interface. These units were entirely
defined as the acquisition mode. Once this time delay is self-contained, with the RF interface, downconverter and
determined, that SV signal is then considered acquired, baseband processing done entirely independent of the
or locked. application. With this type of solution, the PVT in-
After acquisition is achieved, the receiver transitions formation was transmitted out of the serial port, to be
into tracking mode, where the PRN is removed. There- displayed or used as appropriate depending upon the
after, only small adjustments must be made to the local application. In some cases, the user could provide some
reference clock to maintain correlation of the signal. At configuration data to the system, such as the choice of
this point, the extraction of the satellite timing and a local datum, and in that way ‘customize’ the resulting
ephemeris data from the navigation message is done. This positioning information for their needs. This type of
raw data and the known pseudoranges are then used to solution is still widely available, and for many appli-
calculate the location of the GPS receiver. This in- cations provides a cost-effective way of adding GPS
formation is then displayed for the user, or otherwise positioning or timing services to an existing design.
made available to other applications, either through an One variation of the board-level solution that is be-
external port (for remote applications) or through coming more popular today is to supply the RF section of
a software API (for integrated applications). the GPS receiver, including the discrete RF interface and
In the past, GPS correlators were designed with downconverter, as a self-contained module, along with
a single channel, which was multiplexed between each a standalone correlator ASIC or an MCU with an in-
SV signal being received. This resulted in a very slow tegrated correlator and software to perform the baseband
process for calculating a position solution. Today, systems processing of the IF signal. Typically, the RF section of
come with up to 12 channels, allowing the correlator to a GPS receiver is the most challenging portion of the
process multiple SV signals in parallel, achieving a posi- design because of the sensitivity to component layout
tion solution in a fraction of the time. Also, while the and extraneous signals, and many of the RF circuits
correlator functionality is sometimes performed in soft- that exist today were designed with a combination of
ware using a high-performance digital signal processor technical know-how and trial-and-error experience that
(DSP), the real-time processing requirements and re- few application developers can afford. By comparison,
petitive high rate signals involved make a hardware cor- designing the hardware layout for the baseband processor
relator solution ideal, from both a cost and throughput and interface to the RF module is a relatively minor task,
standpoint. which is what has made this an attractive solution for

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