T HE GPS satellite constellation is selected to satisfy many different conditions to provide worldwide three-dimensional navigation. The satellite constellation has been placed in an inclined orbit at 55 deg (formerly 63-deg inclination for earlier Block I satellites) to provide full Earth coverage, whereas a purely equatorial orbit cannot provide coverage above approximately 72° latitude at the GPS orbit altitude. The requirement for a minimum of four simultaneous pseudorange measurements with good geometry directly leads to a constellation of many satellites. A nongeosynchronous orbit was selected to permit the use of carrier phase/Doppler measurement profiles in addition to pseudorange measurements of code phase/delay. This objective was one of the reasons for the selection of the 12 sidereal hour orbit. This chapter discusses the operational satellite constellation of 24 satellites termed the GPS-24 (closely related to the previous Primary 21) constellation. The specific characteristics of this constellation are discussed in terms of the statistics of the numbers of satellites visible at different user latitudes and various elevation angles, the satellite ground tracks, and the signal Doppler shifts. The potential for the future augmentation of this GPS constellation by adding satellites in either the same planes or as geostationary satellites is also discussed. The chapter then continues with an analysis of the geometric dilution of precision (GDOP) and the related DOPs: PDOP, HDOP, VDOP, TDOP for position, horizontal, vertical, and time dilutions of precision. The bounds on the minimum value of these DOPs are analyzed for various numbers of satellites for both twoand three-dimensional problems with various constraints on elevation angle and user clock stability. Further discussion and development of the DOP concept is found in Chapter 11, this volume. The chapter concludes by describing the GDOP for the GPS-24 satellite constellation with a single satellite outage.
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