Orbital Diversity for Satellite Navigation

Today, satellite navigation is based on a growing suite of satellites in medium earth orbit (MEO). Amongst these, the Global Positioning System (GPS) is the centerpiece and grandest achievement. GPS began in earnest in the 1970's when the planners predicted that GPS would serve a total of 40,000 military users with some ancillary civil use. Today, the civil community ships approximately forty million GPS receivers per month, and this civil use has generated a remarkable breadth of applications. Attracted by this utility, Russia is rejuvenating their satellite navigation system, called GLONASS. China is expanding their regional system, Beidou (also known as Compass), to include global coverage. Europe has launched their first prototype satellites for the Galileo system. Japan and India have launched satellites for their regional systems. Taken together, all of these systems will constitute a Global Navigation Satellite System (GNSS) with over one hundred satellites in medium Earth orbit (MEO). This multiplicity of satellites will provide path diversity that will be helpful for users in urban areas. In addition, each satellite will broadcast signals on three civil frequencies, so frequency diversity will become available to civil users over the next decade. In this paper, we explore orbital diversity by adding satellites in low earth orbit (LEO) to the GNSS. The MEO satellites, described above, have altitudes of approximately 20,000 km, while LEO satellites have much lower altitudes of 1000 km or so. By virtue of this low altitude, the LEO satellites will have much higher signal powers than the MEO satellites, and they will sweep across the user's sky more quickly. On the other hand, we cannot build a reasonably priced navigation system based on LEO satellites alone -too many would be required. Thus, this paper explores the combination of MEO and LEO. When combined with MEO satellites, the LEO orbit provide benefits that cannot be provided with any number of satellites in MEO. This paper substantiates this view based on first principals.