Page 135 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 8

orientation estimate of an inertial navigation system is being regularly updated with
beacon or GPS position fixes, then it becomes live reckoning. The distinction be-
tween the two is therefore largely a matter of frequency, but it is an important
distinction.

To avoid the confusion we will avoid the term dead reckoning. The process of regis-
tering relative motion on a continual basis will be called odometry instead.

Relative vs. absolute

At this point, we must differentiate between relative navigation information and
absolute information. Odometry is relative information that tells us how much we
have moved, but not where we are. If we know where we started, then we can add
the relative movement to the starting position and obtain an estimate of our current
position.
Odometry gives us very fast feedback, but because of accumulated errors, it is subject
to drift. Other navigation techniques will be needed to correct the odometry estimate
periodically. Some of these may correct all axes, and some may correct one or two.

These principles have been used for much of recorded history. Late in the Battle of
Britain in World War II, the Luftwaffe decided to switch from day bombing to night
bombing to reduce their rather severe losses from Royal Air Force fighters. Since
England was under a strict blackout, there were no natural landmarks from which
German pilots could correct their dead reckoning navigation, especially on cloudy or
moonless nights.
Thus, the Luftwaffe developed a ground-based beam system code named “Crooked
Leg.” The bombers “rode” a primary pair of beams that provided an indication showing
whether they were straying to the left or right of their intended path to the target.
On the left the pilot would hear Morse code dots, and on the right dashes. In the
middle, these signals would combine to provide a continuous tone. A second beam
pair intersected the first just before the target at the IP (initial point), telling the
1
crew to prepare to drop their bombs .

1 The British quickly detected the beam system and added a signal of their own to cause the
indicated path to be deflected off of the target. The history of WWII is rife with fascinating
examples of technologies and counter measures being developed in rapid succession.

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