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.

Electronics Letters
Department of Electronics

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.