Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 496

expensive augmentation sensors. Systems integrators are finding ways to use lower
grade (and lower cost) sensors and still achieve complete coverage and improved
accuracy over the performance provided by GPS alone.
In recent years, small, low-cost GPS receivers have been integrated into cellular
phones for emergency location in 911 calls (E-911). These cellular phones are in
high demand for personal navigation, tracking workers, friends and family mem-
bers, and finding nearby services (gas stations, restaurants, shops, and such). As the
cost and size of auxiliary sensors decreases, it is likely that these too will be inte-
grated into cellular phones, further increasing the performance and utility of these
systems.

9.3.2 Review of Available Sensor Technology
9.3.2.1 Inertial Systems and Sensors
The use of inertial and various automotive sensors to augment GPS performance in
automotive applications is often termed dead reckoning . Since this term may appear
strange to the reader, and since there is some controversy associated with its origin,
some explanation is in order. The term is much broader than automotive in its appli-
cation and in fact originated long before automobiles were invented. A popularly
held belief is that it derives from “deduced reckoning,” and it is often abbreviated as
“ded. reckoning,” consistent with this interpretation. Certainly, this view is consis-
tent with its meaning (i.e., to deduce one's current position by applying course and
distance traveled to a previously determined position). However, according to the
Oxford English Dictionary , the phrase dead reckoning dates from Elizabethan
times, in 1605-1615. At that time, it applied to navigation in ships in the absence of
stellar observations. With stellar observations, navigation was viewed as navigating
“live,” working with the stars and the motion of the Earth; in comparison, navigat-
ing without sky visibility, by using logs (the process of determining speed by timing
the transit of a log dropped in the water from bow to stern), compasses, and clocks,
was viewed as navigating “dead,” and hence dead reckoning. So either expression is
valid; both are consistent with the modern day application, and both are consistent
with the abbreviation DR.
The use of inertial sensors to augment GPS in automotive applications offers
several advantages over approaches based on measuring tire rotation. The quality of
inertial sensor information does not vary with tire wear or road conditions, whereas
measures of distance traveled certainly do, as their performance will vary with tire
wear and tire slipping or skidding due to nonideal road conditions. However, iner-
tial sensors useful for navigation aiding generally are not preinstalled in the car. In
addition, very-low-cost inertial sensors require nearly continuous calibration: large
bias and scale factor errors are typical, as are high sensitivities to temperature
variations.
In terms of their usage in automotive and other land vehicle applications, the
following inertial system options have emerged as attractive alternatives, with vary-
ing limits of practicality:
• Three orthogonal gyros and three orthogonal accelerometers;
•

Three orthogonal gyros and two level axis orthogonal accelerometers;