Introduction - Understanding GPS: Principles and Applications - page 1

CHAPTER 1
Introduction

Elliott D. Kaplan
The MITRE Corporation

1.1 Introduction

Navigation is defined as the science of getting a craft or person from one place to
another. Each of us conducts some form of navigation in our daily lives. Driving to
work or walking to a store requires that we employ fundamental navigation skills.
For most of us, these skills require utilizing our eyes, common sense, and land-
marks. However, in some cases where a more accurate knowledge of our position,
intended course, or transit time to a desired destination is required, navigation aids
other than landmarks are used. These may be in the form of a simple clock to deter-
mine the velocity over a known distance or the odometer in our car to keep track of
the distance traveled. Some other navigation aids transmit electronic signals and
therefore are more complex. These are referred to as radionavigation aids .
Signals from one or more radionavigation aids enable a person (herein referred
to as the user ) to compute their position. (Some radionavigation aids provide the
capability for velocity determination and time dissemination as well.) It is impor-
tant to note that it is the user's radionavigation receiver that processes these signals
and computes the position fix. The receiver performs the necessary computations
(e.g., range, bearing, and estimated time of arrival) for the user to navigate to a
desired location. In some applications, the receiver may only partially process the
received signals, with the navigation computations performed at another location.
Various types of radionavigation aids exist, and for the purposes of this text
they are categorized as either ground-based or space-based. For the most part, the
accuracy of ground-based radionavigation aids is proportional to their operating
frequency. Highly accurate systems generally transmit at relatively short wave-
lengths, and the user must remain within line of sight (LOS), whereas systems
broadcasting at lower frequencies (longer wavelengths) are not limited to LOS but
are less accurate. Early spaced-based systems (namely, the U.S. Navy Navigation
Satellite System—referred to as Transit—and the Russian Tsikada system) 1 pro-
vided a two-dimensional high-accuracy positioning service. However, the fre-
quency of obtaining a position fix is dependent on the user's latitude. Theoretically,

1. Transit was decommissioned on December 31, 1996, by the U.S. government. At the time of this writing,

Tsikada was still operational.

1