Both analysis and measurements show that chip rate detection of a below-noise direct sequence spread spectrum communications signal can be countered by hopping the chip rate pseudorandomly.

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Journal Electronics Letters
McPherson, W.P., Hill, D.A., Mai, L., & Wight, J. S. (1998). Chip rate hopping provides low probability of detection for direct sequence signals. Electronics Letters, 34(7), 628–629.