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chapter 8

GPS and a Real-Time

Situational Display

Introduction

In this chapter, I will discuss a GPS-based location system that is easily carried aloft by an
Elev-8 or other quadcopters with similar lifting capacity. The system will transmit its data to
a ground station where the quadcopter’s position, speed, course, and altitude will be visible
on a small display. The coordinates could also be entered into a laptop in order to display the
quadcopter’s position in the Google Earth application. This system, together with the First-
Person Video system described in the next chapter will be used to accurately determine the
quadcopter’s location and to view the environment in real time.

GPS Basics

We will begin with a short history of the Global Positioning System (GPS), and follow that with
a detailed explanation of how GPS systems generally function. Then I will focus on the
quadcopter’s GPS receiver and the development of a real-time display.
Brief GPS history
The GPS is a satellite system that was initially deployed in the early 1970s by the U.S.
Department of Defense (DoD) to provide military users with precise location and time
synchronization services. Civilian users could also access the system, but its services to
civilian users were purposefully degraded by the DoD to avoid any risk that it could be of
help to the country’s enemies. This purposeful degradation was lifted by order of President
Regan in the 1980s to allow civilians full and accurate GPS services.
The current GPS system has 32 satellites in high orbits over the earth. Figure 8.1 shows
a representative diagram of the satellite constellation. The satellite orbits have been carefully
designed to allow for a minimum of six satellites to be in the instantaneous field of view of
a GPS user who is located anywhere on the surface of the earth. A minimum of four satellites
must be viewed in order to obtain a location fix, as you will learn in the GPS basics section.
Several other GPS systems are also deployed:

GLONASS—The Russian GPS
Galileo—The European GPS

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