Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 501

f 1

ACC 1

L

L

f 2

ACC 2

Figure 9.24 Dual-accelerometer approach to linear and angular acceleration sensing.

treatment [24] attempts to make use of existing accelerometers (e.g., as could be
associated with air bag deployment or the vehicle's ABS) distributed throughout the
car to support an inertial navigation capability. Tests of a prototype system have
demonstrated that the accuracy of measured angular accelerations using accelerom-
eters is nearly equivalent to that provided by low-cost gyro sensors.
Use of single accelerometers aligned with the lateral or longitudinal axis of the
vehicle is an option worthy of consideration. The longitudinal accelerometer mea-
sures vehicle accelerations and decelerations, which, once integrated, could poten-
tially replace use of the vehicle's odometer. The lateral accelerometer could
potentially replace a heading or heading rate sensor, since a lateral acceleration is
generally indicative of a turn: the product of the vehicle's speed and the turn rate is
the lateral acceleration of the vehicle. However, use of single accelerometers has its
drawbacks. As previously discussed, both accelerometers will generally sense a
component of gravity, due either to initial misalignment of the sensor as installed in
the vehicle or the pitch (affecting the longitudinal accelerometer) and roll (affecting
the lateral accelerometer) of the vehicle. Although the pitch and roll of a vehicle dur-
ing normal operation are expected to be small, the error effect, if uncompensated,
can be significant. Relatively high frequency pitch and roll variation, as could be
induced by road or speed bumps, is not as troublesome as a steady offset. A 5º
steady roll angle induced by the crown of the road induces an effective acceleration
error of 0.1 g , or roughly 1 m/s 2 . Without compensation, this will integrate to a
velocity and position error, even when the vehicle is stationary (e.g., in 10 seconds,
roughly 50m of cross track error will develop). In addition, since the lateral acceler-
ometer measures the product of heading rate with the vehicle's speed, heading
changes may be very difficult to detect at low speed. Similarly, a steady climb or
descent on a road will be incorrectly interpreted as an acceleration or deceleration
of the vehicle by the longitudinal axis accelerometer, which, without compensation,
will be integrated into significant along track velocity and position error.
Finally, the use of a low-cost gyro to track the heading changes of the vehicle is
an attractive option used in several of the current navigation systems. The vehicle's