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In this paper we examine a simple method to improve the performance of serial, matched-filter acquisition indirect-sequence spread-spectrum packet radio communications. Each packet transmission includes an acquisition preamble, and the preamble sequence is changed at the boundaries of predefined time epochs based on a pseudorandom sequence generator. It is shown in previous work that the presence of an IF filter and the characteristics of the AGC subsystem, lead to a probability of not acquiring that is a non-monotonic function of the signal-to-noise ratio if the acquisition algorithm uses a threshold-crossing detector with a fixed threshold. The acquisition algorithm presented in this paper employs an estimator to adaptively select the acquisition threshold. It is shown that this technique reduces the severity of the non-monotonicity and substantially improves the acquisition performance.