  SPICA is a mid to far infra-red space mission to explore the processes that
form galaxies, stars and planets. SPICA/SAFARI is the far infrared spectrometer
that provides near-background limited observations between 34 and 230
micrometers. The core of SAFARI consists of 4 grating modules, dispersing light
onto 5 arrays of TES detectors per module. The grating modules provide low
resolution (250) instantaneous spectra over the entire wavelength range. The
high resolution (1500 to 12000) mode is accomplished by placing a Fourier
Transform Spectrometer (FTS) in front of the gratings. Each grating module
detector sees an interferogram from which the high resolution spectrum can be
constructed. SAFARI data will be a convolution of complex spectral, temporal
and spatial information. Along with spectral calibration accuracy of <1%, a
relative flux calibration of 1% and an absolute flux calibration accuracy of
10% are required. This paper will discuss the calibration strategy and its
impact on the instrument design of SAFARI
