An Introduction to Wireless Technology, Second Edition

radio bands to become available. These can be divided up into one-way or

half-duplex connections:

•

CB Radio

•

Walkie-talkies

•

Baby alarms and monitors

•

Remote control cars and boats

and two-way or full-duplex connections:

•

Mobile phones

•

Cordless phones

1.3.6 Automobile Communications

There are several ways in which information can be passed to or from a car in

motion.

In the UK a system has been developed for interrupting the vehicle′s sound

system to transmit traffic information, and most new vehicles will have the

system fitted to the car entertainment system. The driver may be listening to a

national radio broadcast or even tape or CD. When a local radio station traffic

report is broadcast, the vehicle system switches over to receive the traffic report

at a predetermined volume level. At the end of the traffic report the system

switches back to the original program. This is accomplished by a high frequency

audio tone at the start and end of each traffic report. The vehicle′s receiver can

detect this on all of the local radio broadcast frequencies. The system is known

as the Radio Data System (RDS).

A system known as Intelligent Vehicle Highway System (IVHS) or Intelligent

Transportation System (ITS) has been tested in Europe and Japan. An Advanced

Traveler Information System (ATIS) which provides electronic maps with

navigational information is available in Japan; about 400,000 of these units are

presently in use. In California, AVIS is testing a satellite guidance system to

show the current position for a vehicle on an electronic map on a display.

Also in use are tollbooths that can read a bar code or RF identification tag on

vehicles as they pass to determine toll charges, and road signs that give drivers

real-time traffic information.

1.3.7 Satellite Applications

Satellite navigation systems can be used to show the position of a vehicle

anywhere on the Earth′s surface.

1.3.7.1 Global Positioning System

The Global Positioning System (GPS) was developed for the US Military, but can

be used to provide positional information for commercial and even leisure

applications. The GPS system consists of a bracelet of satellites transmitting

information about their position relative to the Earth, and very accurate timing

information. A small receiver in a vehicle can determine its position on the

surface of the earth by receiving signals from at least three satellites. With three

satellites the position can be determined in two dimensions, but with four or

more signals received the altitude can be measured as well. In open country, a

receiver can normally receive information from five satellites. The positional

information can be calculated by the receiver knowing how long the radio signal

takes to reach it from each satellite (and thus its distance from it) and the

position of each satellite in space. The US Military has built in a random error

16 An Introduction to Wireless Technology