Characterisation of exoplanet spectra by transit spectroscopy requires observational precision of the order of 10^-5 or better. In addition to photon noise from the source and astrophysical backgrounds, such observations will be subject to various other experimental uncertainties, including detector and electronic noise, telescope thermal emission, spacecraft jitter and pointing strategy, and other systematics, the ensemble total of which may be difficult to determine analytically. We are developing an observation simulator – ExoSim – that incorporates models of the
astrophysical scene, the instrument and the detector, as well as multiple noise sources. This is intended as a generic application that can be applied to multiple different instruments and used for various purposes. It can be used predictively for future exoplanet instruments and observations to determine the instrument requirements and observation strategy needed to observe all kinds of transiting planets, including those in the habitable zones of late type stars. Retrospectively it can be used to evaluate results from previous or existing instruments, by simulating the observation, and repeating this many times to obtain a direct determination of the uncertainties, and after data reduction obtain new error bars on the emergent spectrum. We apply ExoSim to two test cases, a predictive case, the proposed Ariel
mission, and retrospective case, Hubble WFC3, and demonstrate its capability to produce simulated spectral images and noise for both.