TABLE OF CONTENTS As shown in Chapter 8, satellite-based navigation systems have found application in many fields and even more are expected in the future. Nonetheless, in some situations the user receiver may be unable to track the required minimum number of satellites. This, for instance, may be the case in urban canyons or open-pit mines.
The signal-obstruction problem, however, has been successfully overcome for GPS by integration with other positioning systems or sensors. Augmentation of GNSS is not limited to sensor integration. In fact, systems can be augmented with computer-based tools, such as GIS (see Chapter 8), for efficient data collection analysis. In this context, the word integration is intended as an aid to GNSS to improve its performance and remove some of its intrinsic limitations (the passive integration approach). Even GPS augmentations in the form of barometric altimeter aiding and clock coasting have been reported as a means of improving availability of the navigation RAIM (see Chapter 4) functions.
Integration, however, can also be intended in the opposite way, as an intrinsic aid to other systems to help them improve their performance and effectiveness (the active integration approach). One example is cellular communications systems, which are somehow limited in their ability to determine precisely the location of the origin of a call [1, 2]. This limitation can be critical in emergency situations. For instance, about one-half of all 911 emergency calls in the United States come from cellular phones and in many of these cases the...
