Page 407 - Understanding Automotive Electronics

2735 | CH 11 Page 394 Tuesday, March 10, 1998 1:30 PM

11 FUTURE AUTOMOTIVE ELECTRONIC SYSTEMS

An inertial navigation system locates the vehicle position relative to a
known starting point by integrating acceleration twice with respect to time. For
example, along the x direction, vehicle position at time t is x(t):

t r
xt() = x o + ∫ ∫ ay() y t
d
d
o o
where
x is the initial x position
o
a is the acceleration along x direction
A similar integration is performed along the two orthogonal directions.
An inertial navigation system has position errors due to initial gyro
alignment errors, uncompensated gyro drift, and accelerometer errors. A typical
high-quality commercial navigation system (e.g., Carousel IV) has a position
error of about 3000 feet for each hour of flight. Position errors generated at this
rate in an automotive environment imply a trip of no more than a half hour
before the error exceeds the half-block limit. This error, in combination with
the system’s relatively high cost (about $120,000), renders the inertial
navigation system unfeasible for automotive use for the foreseeable future. On
the other hand, updated measurement of a car’s present position can be
obtained using radio navigation sensors.

Radio Navigation

A radio-based automotive navigation system uses either land- or
satellite-based transmitters and automotive receivers for position location.
Land-based transmitter systems that are potentially applicable include Decca,
Loran-C, VOR, and Omega. The only satellite-based system that is
potentially applicable is the Global Positioning System (GPS). The land-
based systems are primarily intended for aircraft or ship navigation and have
somewhat limited coverage. For example, Loran-C has no coverage for large
portions of the southwestern United States. Nevertheless, research is being
done on the applicability of these land-based systems to automotive
navigation.
The GPS, when fully operational, will have 24 orbiting satellites, of
which there will be a minimum of four within line of sight of any location on
earth. This is sufficient for position location. Figure 11.20 is a block diagram of
a GPS-based automotive navigation system.
In final operational service, there will be two classes of user service
available for GPS: the Precise Positioning Service (PPS), which is available
only to the military, and the Standard Position Service (SPS), which is
available for automotive navigation. Each satellite transmits clock pulses that

394 UNDERSTANDING AUTOMOTIVE ELECTRONICS