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The High Frequency (HF) radio band, commonly taken to be that portion of the electromagnetic spectrum lying between approximately 3 MHz and 30 MHz, remains a popular and often cost-effective alternative to communications satellites and terrestrial microwave links for low data rate signals such as teletype, and at sea or in underdeveloped areas. HF radio wave propagation is governed by reasonably well-understood, but not fully predictable, atmospheric phenomena. Determining the location of an HF transmitter is important since many vessels lack satellite terminals, and maritime distress signals must often be sent by HF, and requires both that a viable propagation path exists between the transmitter and receiver(s), and that the receiving station(s) be listening on the same frequency as the transmitter. This report reviews some of the fundamentals of HF propagation and investigates the relevance of historical information about which frequencies a given transmitter has used in the past under one set of atmospheric conditions to the question of what frequencies that same transmitter will use under a different, but known, set of atmospheric conditions.