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do not unacceptably degrade the performance of other systems. Primarily this means that compatible systems must provide signals with similar maximum and minimum received power levels.

The internationally accepted definition of interoperability is the “ability of open global and regional satellite navigation and timing services to be used together to provide better capabilities at the user level than would be achieved by relying solely on one service or signal.” As a result of international cooperation, new and modernized GNSS signals will have characteristics that substantially improve interoperability as compared with the original GPS and GLONASS signals.

Some have suggested that the goal should be not only interoperability but also interchangeability, meaning that there should be no discernible differences between the signals from different systems. This paper addresses the problems of achieving full interchangeability of signals while also showing why the remaining differences will not affect users.

IDEAL INTERCHANGEABILITY

It is instructive to realize that the current GPS constellation is populated with four different types of satellites with four significantly different designs from different manufacturers, as shown in Figure 1. These are the GPS Block IIA satellites built by Rockwell International (now Boeing), the GPS Block IIR and the Block IIR-M satellites built by Lockheed Martin, and the GPS Block IIF satellites built by Boeing. Later in this decade a fifth type of satellite, the GPS Block III being designed by

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FIGURE 1 GPS receivers track four different satellite types.



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