  We analyse the higher order modes (HOM's) in the 3.9GHz bunch shaping
cavities installed in the FLASH facility at DESY. A suite of finite element
computer codes (including HFSS and ACE3P) and globalised scattering matrix
calculations (GSM) are used to investigate the modes in these cavities. This
study is primarily focused on the dipole component of the multiband expansion
of the wakefield, with the emphasis being on the development of a HOM-based BPM
system for ACC39. Coupled inter-cavity modes are simulated together with a
limited band of trapped modes.
