  A coordinate system is proposed that replaces the usual three-dimensional
Cartesian x,y,z position coordinates, for use in robotic localization
applications. Range, azimuth, and elevation measurement models become greatly
simplified, and, unlike spherical coordinates, the proposed coordinates do not
suffer from the same kinematic singularities and angle wrap-around. When
compared to Cartesian coordinates, the proposed coordinate system results in a
significantly enhanced ability to represent the true distribution of robot
positions, ultimately leading to large improvements in state estimation
consistency.
