  For a team of robots to work collaboratively, it is crucial that each robot
have the ability to determine the position of their neighbors, relative to
themselves, in order to execute tasks autonomously. This letter presents an
algorithm for determining the three-dimensional relative position between two
mobile robots, each using nothing more than a single ultra-wideband
transceiver, an accelerometer, a rate gyro, and a magnetometer. A sliding
window filter estimates the relative position at selected keypoints by
combining the distance measurements with acceleration estimates, which each
agent computes using an on-board attitude estimator. The algorithm is
appropriate for real-time implementation, and has been tested in simulation and
experiment, where it comfortably outperforms standard estimators. A positioning
accuracy of less than 1 meter is achieved with inexpensive sensors.
