In a method and apparatus for detecting a message in a signal the message begins with a preamble consisting of a fixed sequence of symbols, each having a symbol time T, modulated by repetitions of a higher rate code having low autocorrelation at lags anything other than zero. The repetitive code and the fixed sequence of symbols is known at the detector. The method comprises correlating the signal with the repetitive code; autocorrelating the signal with a copy of the signal, which is delayed substantially a symbol time; correlating the signal with the preamble; and indicating the detection of a message if a value in the signal is greater than a threshold value.
