The HF radio channel is very difficult to characterise as it has non-stationary fading, impulsive noise and man-made interference. Furthermore, the “live” HF environment is not stable and hence cannot be used to provide accurate measures of relative HF modem performance. Consequently modem designers rely on HF channel simulators to provide reproducible channels. The simulators can be categorised as either statistical or replay simulators. A statistical simulator offers a wide variety of channels under parameter control while replay simulators offer fewer channels however these correspond to specific measurements and all the artifacts of the channel are reproduced. This paper presents the design of an HF replay simulator in which a novel scheme is used to accurately estimate the continuously time varying impulse response. The design of the replay simulator is outlined and the scheme for accurately estimating the impulse response is explained. An HF replay simulator has been constructed by the Defence Science and Technology Organisation (DSTO). This paper announces its availability.