Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 493

digital map. The digital map can also be used to look up the nearest street address, a
process known as reverse geocoding .
There are many wireless technologies that may be used as the data radio, includ-
ing cellular modems, cellular packet data, two-way paging, satellite links, and pri-
vate data networks. Some systems track the vehicles on a continuous basis with
position reports broadcast at certain intervals, while other systems are designed to
record data to be uploaded periodically or on demand. Enterprises that own or
operate fleets of vehicles (e.g., taxis, delivery trucks, or service vehicles) use vehicle
tracking systems to monitor the usage of the vehicles and improve efficiency in
logistics through optimum dispatching. Public safety departments (e.g., police, fire,
or ambulance) use vehicle tracking to reduce call response time and to locate
workers in the case of distress calls.
Individual vehicles can be located in emergency situations using GPS and wire-
less communications. These emergency messaging systems, also known as
telematics systems, are offered by many automobile manufacturers today. A generic
emergency messaging system architecture is shown in Figure 9.20. Typically, these
systems use a cellular phone for wireless data communications because of the
dual-purpose voice and data capabilities, extensive coverage throughout most
developed countries, and relative low cost. These devices are connected to vehicle
systems or to the vehicle bus and can notify a service provider automatically when
an air bag is deployed or some other crash sensor is triggered. The user interface
includes one or more buttons to activate the system, a hands-free voice call capabil-
ity, and possibly a display to indicate status. The GPS position of the vehicle is sent
via the cellular phone so that emergency services or other assistance can be sent to
the exact location of the vehicle. These devices are also used for crash notification,
roadside assistance, theft tracking, and direction assistance.
In navigation, tracking, and emergency location, the availability of an accurate
GPS position fix is essential. In all of these applications, a GPS C/A code receiver

Differential
corrections
(optional)

Wireless
data/voice
connection

GPS
receiver

Auxiliary sensor(s)
• Gyroscope
• Magnetic compass
• Odometer
• Wheel speed sensors
• Accelerometer
• Barometric altimeter

Integrated processor
• Positioning solution
• Communications control
• User interface

User
interface

Crash
sensor(s)

Figure 9.20 Generic emergency messaging system architecture.