A software simulator for the Herschel-SPIRE imaging photometer

SPIRE, the Spectral and Photometric Imaging Receiver, is one of three instruments to be flown on ESA's Herschel Space Observatory. It contains a three-band submillimetre camera and an imaging Fourier transform spectrometer, and uses arrays of feedhorn-coupled bolometric detectors operating at a temperature of 300 mK. Detailed software simulators are being developed for the SPIRE photometer and spectrometer. The photometer simulator is based on an adaptable modular representation of the relevant instrument and telescope subsystems, and is designed to produce highly realistic science and housekeeping data timelines. It will be used for a variety of purposes, including instrument characterisation during ground testing and in orbit, testing and optimisation of operating modes and strategies, evaluation of data reduction software using simulated data streams (derived by "observing" a simulated sky intensity distribution with the simulator), observing time estimation, and diagnostics of instrument systematics. In this paper we present the current status of the photometer simulator and the future development and implementation strategy.