  The IceCube Neutrino Observatory will be upgraded with more than 700
additional optical sensor modules and new calibration devices. Improved
calibration will enhance IceCube's physics capabilities both at low and high
neutrino energies. An important ingredient for good angular resolution of the
observatory is precise calibration of the positions of optical sensors. Ten
acoustic modules, which are capable of receiving and transmitting acoustic
signals, will be attached to the strings. These signals can additionally be
detected by compact acoustic sensors inside some of the optical sensor modules.
With this system we aim for calibration of the detectors' geometry with a
precision better than 10 cm by means of trilateration of the propagation times
of acoustic signals. This new method will allow for an improved and
complementary geometry calibration with respect to previously used methods
based on optical flashers and drill logging data. The longer attenuation length
of sound compared to light makes the acoustic module a promising candidate for
IceCube-Gen2, which may have optical sensors on strings with twice the current
spacing. We present an overview of the technical design and tests of the system
as well as analytical methods for determining the propagation times of the
acoustic signals.
