  At present a 10\,m prototype interferometer facility is being set up at the
AEI Hannover. One unique feature of the prototype will be the suspension
platform interferometer (SPI). The purpose of the SPI is to monitor and
stabilise the relative motion between three seismically isolated optical
tables. The in-vacuum tables are suspended in an L-shaped configuration with an
arm length of 11.65\,m. The design goal of the SPI is to stabilise longitudinal
differential displacements to a level of 100\,pm/$\sqrt{\mathrm{Hz}}$ between
10\,mHz and 100\,Hz and relative angular noise of 10\,nrad/$\sqrt{\mathrm{Hz}}$
in the same frequency band. This paper covers the design aspects of the SPI,
e.g. cross-coupling between the different degrees of freedom and fibre pointing
noise are investigated. A simulation is presented which shows that with the
chosen optical design of the SPI all degrees of table motion can be sensed in a
fully decoupled way. Furthermore, a proof of principle test of the SPI sensing
scheme is shown.
