  We present a method of extrinsic calibration for a system of multiple
inertial measurement units (IMUs) that estimates the relative pose of each IMU
on a rigid body using only measurements from the IMUs themselves, without the
need to prescribe the trajectory. Our method is based on solving a nonlinear
least-squares problem that penalizes inconsistency between measurements from
pairs of IMUs. We validate our method with experiments both in simulation and
in hardware. In particular, we show that it meets or exceeds the performance --
in terms of error, success rate, and computation time -- of an existing,
state-of-the-art method that does not rely only on IMU measurements and instead
requires the use of a camera and a fiducial marker. We also show that the
performance of our method is largely insensitive to the choice of trajectory
along which IMU measurements are collected.
