Page 412 - Automotive Engineering Powertrain Chassis System and Vehicle Body

14.1
Chapter 14.1

Global positioning technology

Ljubo Vlacic and M. Parent

There is no doubt that one of the most important of determining position came inherent limitations.
enabling technologies in the intelligent vehicle space is Boundaries and landmarks change with the passage of
the global positioning system (GPS). Without the ability time, making mapping a continual, time-consuming task.
to accurately determine a vehicle’s position on demand, Positioning via the stars has long been a necessity for
there would be no way to cost-effectively implement mariners, but accurate time keeping and clear skies are at
autonomous or server-based vehicle navigation, nor times elusive. Until the deployment of today’s GPS, the
would the ability to deliver customized, location-based ultimate solution did not exist – an ‘always on, always
services to the vehicle be possible. available’ system for determining an exact position any-
This chapter will provide a brief overview of the GPS, where on the globe.
and how it can be leveraged in intelligent vehicle appli- The constellation of satellites being used for global
cations. This chapter begins with a section describing the positioning today has it roots in the satellite positioning
history of space-based positioning projects that have led and time transfer systems of the early 1960s. Like many
to the current GPS, followed by a detailed description of successful endeavours, the GPS was conceived from
the system as it exists and operates today. This is building blocks of other programmes such as the Navy
followed by a discussion of the science behind the GPS, Navigation Satellite System (NNSS, or Transit), Tima-
and the techniques and components required to accu- tion and Project 621B. It is worthwhile to have a brief
rately and cost-effectively determine a user’s position. understanding of these predecessors of GPS in order
The chapter concludes with some example applications fully to understand and appreciate the complexity of
where GPS is being used in the intelligent vehicle and space-based radio navigation.
related spaces, as well as future services that will be made Transit was conceived to provide positioning capabil-
possible because of GPS-based positioning capabilities. ities for the US submarine fleet, and originally deployed
in 1964. While Transit proved to be a tremendous success
in demonstrating the concept of radio navigation from
14.1.1 History of GPS space, the system was inherently inaccurate and required
long periods of satellite observation in order to provide
Long before the development of the GPS in use today, a user with enough information to calculate a position.
the concept of time transfer and positioning via signals Periods of observation in excess of 90 minutes were not
from space was being researched around the world. uncommon, which limited the system’s effectiveness
These costly research projects were mainly sponsored by for positioning a submarine at sea, since extended
government agencies, to address their long-standing need surface time could leave the vessel vulnerable. In its
to improve techniques for quickly and accurately posi- simplest form, Transit consisted of a small constellation
tioning military vehicles and personnel on or above the of satellites broadcasting signals at 150 MHz and
battlefield. Troops and vehicles of centuries past relied on 400 MHz. The Doppler shift of these signals as measured
maps, charts, the stars and various electronic devices to by observers at sea, coupled with the known positions of
find their location; however, with each improved method the satellites in space, was sufficient to provide range
Intelligent Vehicle Technologies; ISBN: 9780750650939
Copyright Ó 2001 Ljubo Vlacic and M. Parent. All rights of reproduction, in any form, reserved.