Page 425 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 4. 1 Global positioning technology

first time after leaving the factory, or in other situations being used to provide location-based services in vehicles
where the device has not been powered up or used for today, and how the richness and complexity of those
long periods of time. services will increase in the near future.
Many GPS receivers allow the user to enter time, date
and even current position information, which can reduce
the TTFF in a cold start situation down close to that of 14.1.5.1 Basic positioning applications
a warm start.
The most basic applications for the positioning capabil-
ities provided by the GPS are those providing the user/
14.1.4.3.3 Reacquisition time
operator with information regarding their current loca-
Reacquisition time is the amount of time required by tion, where they have been, and more recently, where they
a GPS receiver to regain a position fix when the satellite are going. These systems include today’s wide assortment
signal is temporarily disrupted due to a loss of visibility of of handheld devices, which provide the user with their
one or more satellites. This condition can occur when the current location, speed and elevation, as well as track their
receiver is operating in areas of dense foliage, or in urban most recent movements. More complex devices integrate
canyons, or anywhere that the satellite views may be navigation capabilities, which map the user’s position in-
intermittently blocked. Most GPS receivers should have formation onto a map database, and use that information
reacquisition times of five seconds or less. This is an to provide the user with text-based or graphical directions
important parameter for assessing the capability of GPS to their destination. Each type of system is described in
receivers for intelligent vehicle applications, since navi- more detail in the following sections.
gation systems must routinely operate in locations, such
as downtown areas, where reception can be intermittent. 14.1.5.1.1 Handheld GPS system
A handheld GPS device, while not strictly an intelligent
14.1.4.3.4 Receiver sensitivity
vehicle system, provides a good example of an applica-
The sensitivity to satellite transmissions is another tion utilizing the most basic positioning services provided
measure of performance of a GPS receiver. This is basi- by GPS technology. These units range from less than one
cally an assessment of how many satellites a receiver can hundred dollars to a few thousand dollars, depending
detect under varying conditions. Because operating upon their features and capabilities. These devices are
conditions for GPS receivers in intelligent vehicle ap- battery operated, have a small LCD display and basic,
plications can range from high elevations with an un- menu-driven user interface. Most of the newer models
obstructed view of the sky to locations inside or between can detect at least 6–8 satellites concurrently, with many
buildings where reception can be more difficult, it is now offering the ability to track up to 12 satellites at
important for the application developer to understand once. The more complex units also provide a variety of
under what conditions the GPS receiver can detect the configuration options, such as the choice of localized
SV signals, and under what conditions alternate methods datums or different information display formats. Because
of positioning must be relied upon. these devices are small enough to be easily carried in
a briefcase or purse, users can take them as a positioning
14.1.5 Applications for GPS aid when travelling, whether on foot, or by private or
public transport. The basic functional blocks of a hand-
technology held GPS receiver are illustrated in Fig. 14.1-9.
All of these devices provide the user with their current
There are a variety of uses for GPS technology today, location information, usually in the form of a latitude and
from basic positioning applications which might provide longitude reading. Most also provide the user with addi-
a traveller with their current location, speed, and tional positioning information, including the current local
direction to their destination, to highly complex appli- time, elevation above sea level, and velocity, provided
cations where the user’s position information is feed into enough satellites can be detected by the device. Many of
a system that provides location-specific features and today’s handheld devices also provide some tracking
services tailored for that user. What follows is some services, enabling the user to store the location of points
examples of how GPS technology is being used to en- they have previously reached, allowing them to return
hance the capabilities of intelligent vehicle platforms. easily to their starting location. This can be a very useful
The initial examples illustrate some of the more tradi- feature to those travelling in unfamiliar areas, whether in
tional positioning applications, such as basic location and a wilderness area or in an unfamiliar town or city.
autonomous navigation systems, which are already seeing Some of today’s newer handheld systems now have
widespread use today. This is followed by examples larger displays and removable memory devices that
describing how GPS-derived positioning information is enable the unit to plot graphically the user’s current

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