Global positioning system: signals, measurements, and performance [Book Review]

This is a superb book that deals with modeling and performance of GPS systems, aimed at analysis, integration, and interfacing with other navigation systems. It will be of primary .interest to system engineers as compared to GPS receiver designers. Experienced GPS engineers will find it as useful as newcomers, for whom this book will be a fine primer. It is very up to date because it shows algorithms and error models without the now-obsolete SIA clock-dither and explains how SIA worked (though there is an occasional reference to the impending end of SIA). Each chapter concludes with a problem set and references. It is available through NavTech in Alexandria, Virginia (www .Navtechgps .com). The initial chapter, History of Radio Navigation, offers good coverage of classical celestial navigation, the 1 %-clock differential ranging systems of 1940-1980, and one-way ranging using precise clocks dating from 1970 to the present. The second chapter, GPS System, describes the spacecraft, control segment, and signals-in-space. The spacecraft description includes Block IIR and IIF. The control segment includes monitoring stations belonging to various US government agencies. The chapter has an overview of receivers, differential GPS (DGPS), and applications for land, marine, air, and space. The authors describe the existing US Coast Guard network of marine differential stations, but say nothing about DGPS nets in the rest of the world. They describe how DGPS is being extended over land in the USA by re-using obsolete Air Force communication stations and a sparser wide-area DGPS system, designed by the US Federal Aviation Administration, whose intent is to detect and annunciate system faults within seconds. The authors have an up-to-date summary of planned modernizations including a new military modulation (M-code) and new civil frequency (L5). The book is up-to-date enough to discuss enhanced-9 1 1 (emergency telephone number throughout the US) as cell-phone operators decide how to report the precise position of each mobile phone that declares an emergency position is known to be somewhere within a cell. The authors then present a fine description of the ECEF, WGS-84, ITRF, and local-level coordinate frames and the transformations among them. It is the first time I’ve seen confocal ellipsoidal coordinates in print since my 1960 Ph.D. thesis. They do an excellent job describing various definitions of time and explain clock stability, which, in my lectures, I consider to be key to GPS. This chapter describes the GPS orbits and it explains the Keplerian orbit elements and the added ephemeris parameters required to predict the orbits within a few meters. I am astonished by such precise orbit determination since I remember when it was difficult to predict one revolution ahead within a mile. The chapter describes the visibility of satellites from points on the Earth (not from orbiting spacecraft): contact. time and number of satellites visible from Massachusetts. I didn’t know that the satellites are distributed unevenly in the orbit planes. I was surprised that the authors did not say that the 55-degree inclined orbit planes are mutually orthogonal. The authors also describe the data-messages in detail. The GPS Measurements chapter describes pseudorange measurements by correlating the known time-shifted code emitted by each satellite with the satellite’s actual signal, to determine the transit time of the radio wave. It describes carrier-phase measurements: integral cycles and fractional cycles. It contains an excellent discussion of propagation errors in the ionosphere (frequency-dependent), in the troposphere (density-dependent), and of predictive algorithms that can subtract most of those errors from pseudoranges. They do not describe the latitude-dependence of these errors, a current topic of interest in arctic nations. They describe receiver noise and multipath generically (more in Chapter 8). They show algorithms for carrier-smoothed code measurements and have a long analysis of differential GPS (DGPS) processing. PositionVelocity-Time Estimates; this meaty chapter describes algorithms for estimating position from pseudoranges, based on linearization around an assumed position (the same method that was used for differential-ranging systems and celestial navigation of the last two generations). It describes the calculation of velocity from pseudorange-rates and the effect of changing satellites geometry on position errors in the form of “dilution of position.”