Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 460

sible. With network assistance, however, it is possible to determine the location of
the handset. The network can obtain the requisite GPS navigation data from other
GPS receivers with clear sky view or other sources. Further, the network can assist
the handset in a number of other ways, such as the provision of timing and a coarse
position estimate. Such assistance can greatly increase the sensitivity of the GPS
sensor embedded in the handset.
This chapter consists of three major sections beyond this overview. In Section
9.2, we discuss integration of GPS with inertial sensors. The motivations for
GPS/inertial integration are detailed. The Kalman filter is described, as well as an
example of an elementary Kalman filter implementation. Various classes of
GPS/inertial integrations are introduced and discussed.
Section 9.3 addresses sensor integration for land vehicles. The implementation
issues related to a GPS/inertial integration as a navigator for land vehicle applica-
tions are presented. A description of the sensors, their integration with the Kalman
filter, and test data taken during field testing of a practical multisensor system are
presented.
Section 9.4 discusses methods of enhancing GPS performance using network
assistance. This section includes discussions of network assistance techniques, per-
formance, and emerging standards.

9.2 GPS/Inertial Integration

Navigation employing GPS and inertial sensors is a synergistic relationship. The
integration of these two types of sensors not only overcomes performance issues
found in each individual sensor, but also produces a system whose performance
exceeds that of the individual sensors. GPS provides bounded accuracy, while iner-
tial system accuracy degrades with time. Not only does the GPS sensor bound the
navigation errors, but the GPS sensor calibrates the inertial sensor. In navigation
systems, GPS receiver performance issues include susceptibility to interference from
external sources, time to first fix (i.e., first position solution), interruption of the sat-
ellite signal due to blockage, integrity, and signal reacquisition capability. The issues
related to inertial sensors are their poor long-term accuracy without calibration and
cost.
This section first discusses in more detail the relative weaknesses of GPS (Section
9.2.1) and inertial sensors (Section 9.2.2) as outlined previously. Next, an introduc-
tion to Kalman filtering is provided (Section 9.2.3), followed by a description of a
variety of practical GPS/inertial integrations and their performance features

(Sections 9.2.4-9.2.6).

9.2.1 GPS Receiver Performance Issues
One primary concern with using GPS as a stand-alone source for navigation is signal
interruption. Signal interruption can be caused by shading of the GPS antenna by
terrain or manmade structures (e.g., buildings, vehicle structure, and tunnels) or by
interference from an external source. An example of signal interruption is shown in
Figure 9.1. Each vertical line in this figure indicates a period of shading while driving