Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 491

antenna element, to optimize the weighting applied to each element. Electronics
used for currently available CRPA antennas also include the first down conversion
and AGC electronics of the GPS receiver (see Figure 6.1). This allows the power
detector of the AGC circuitry to be included in the AE.
In some implementations, a time delay line with M taps are added to each of the
N antenna elements. The combiner then weights the M × N delay line taps. This
technique, referred to as space-time adaptive processing (STAP), can significantly
improve nulling and beamforming performance. STAP improves performance by
allowing a null to be created in a certain direction only for a certain band of fre-
quencies, which is sufficient to suppress the interference while possibly allowing
enough desired signal power to remain so that the receiver can track a satellite visi-
ble in the same direction.
As mentioned earlier, to implement beam steering, the AE must know the LOS
direction to the satellites being utilized by the GPS receiver. This is accomplished by
means of a serial interface between the navigation processor and AE. Satellite azi-
muth and elevation relative to the antenna and usually the vehicle are periodically
sent to the AE to use in optimizing the gain toward the satellites.

9.3 Sensor Integration in Land Vehicle Systems

This section examines integrated positioning systems found in vehicle systems,
automotive electronics, and mobile consumer electronics. Low-cost sensors and
methods used to augment GPS solutions are presented, and example systems are
discussed.

9.3.1 Introduction
Ever since GPS was first conceived, it was envisaged that receivers would be used for
positioning in motor vehicles. By the early 1990s, GPS receiver technology had
advanced to the point where GPS products functioned reliably in automotive envi-
ronments and costs had dropped to a point where widespread use was possible. GPS
is now used in automotive systems for locating vehicles, tracking vehicles, and pro-
viding navigation assistance to drivers.
Vehicle navigation systems are available on dozens of vehicle models in the
market today. The purpose of these systems is quite simply to help a driver get to a
destination. A generic vehicle navigation system architecture is depicted in Figure
9.18. Major components include a user interface to enter a destination, a GPS
receiver to determine the absolute position of the vehicle, possible auxiliary sensors
for augmenting the positioning solution, access to a digital map database for plan-
ning routes and determining maneuvers, and means to present the directions to the
driver by voice, graphics, or both via the user interface. Access to digital map data is
essential for route planning and guidance, and when available in the vehicle may
also be used to improve the positioning, as will be discussed in this chapter. GPS is
used for positioning in virtually every vehicle navigation system on the market. Dif-
ferential GPS corrections may be provided and applied to improve the positioning
accuracy of the solution.