  An international team is currently investigating the best way to use Higher
Order Modes (HOM) for beam diagnostics in 3.9 GHz cavities. HOMs are excited by
charged particles when passing through an accelerating structure. Third
harmonic cavities working at 3.9 GHz have been installed in FLASH to linearize
the bunch energy profile. A proof-of-principle of using HOMs for beam
monitoring has been made at FLASH in the TESLA 1.3 GHz cavities. Since the
wakefields generated in the 3.9 GHz cavities are significantly larger, their
impact on the beam should be carefully minimized. Therefore our target is to
monitor HOMs and minimize them by aligning the beam on the cavity axis. The
difficulty is that, in comparison to the 1.3 GHz cavities, the HOM-spectrum is
dense, making it difficult to identify individual modes. Also, most modes
propagate through the whole cryo-module containing several cavities, making it
difficult to measure local beam properties. In this paper the options for the
HOM-based beam position monitors are discussed.
