Chapter 8 - Live Reckoning

Abstract

Live reckoning is the core of any good navigation system, providing continuous position updates among fixes. This position estimate is then used as a basis for evaluating the incoming navigation data. If a platform provides good odometry, then it would have even better live reckoning. If a robot is navigating from a global positioning system (GPS) and suddenly passes under a highway overpass, good odometry can keep it rolling until the GPS signal is restored or the vehicle receives some other kind of navigational data. The quality of a mobile platform's inherent odometry is dependent on the drive-system configuration. A robot will normally use two coordinate systems: one for global reference and one for local reference. The global reference system is concerned with where the robot is in its environment, and it has no inherent center. For this reason, global systems are almost always Cartesian. The local reference system, on the other hand, is used to integrate data from sensors into the robot's sense of things. This process involves vector math and is inherently centered on the robot. For speeding up the calculations, it is necessary to know how fast the processor can perform these calculations. The two most common ways to increase the speed of such calculations are by adding a coprocessor or by using lookup tables for the geometric functions. Live reckoning is achieved when odometry begins interacting tightly with the other processes of navigation. The position estimates produced are used by sensor systems to orient the data that they are processing. These sensor systems may in turn correct one or more of the baseline position and heading estimates.