  An electron beam entering an accelerator cavity excites higher order modes
(HOM). These are radiated to HOM couplers and subsequently damped. They can
also be used to facilitate beam position monitoring. The modes excited by
off-axis beams are the focus of this study. Results are presented on the first
analysis of HOM dependency on the beam position for the third harmonic cavities
at FLASH. The analysis is based on the beampipe and cavity dipole modes.
Various analysis methods are used and compared to satisfy the needs emerging
from the characteristics of those modes. The electric center of each mode is
observed by looking for the minima of the HOM signals of different beam
positions. A single electron bunch per RF pulse is used.
