Acquisition in an HF modem: locating a burst from its preamble

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This article, part 4 of a series on building a MIL-STD-188-110 HF modem in C++, explains how a receiver finds a transmission in noisy audio using symbols alone. Acquisition must decide whether a burst is present, where it begins, which mode it uses, and where its data starts, while also estimating the carrier offset. The method relies on the standard's preamble, a fixed sequence of three-bit symbols that encodes the bit rate, interleave setting, and a countdown of remaining segments. The author supplies a demo program with fourteen steps, each implemented as a function and covered by unit tests, and walks through them on a recorded burst with added noise and a frequency shift. The sections cover correlation across faded and repeated preamble segments, handling bursts joined mid-preamble, rejecting unsupported modes, and deciding when to stop searching.

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