What is the standard of review applied to a search warrant?

We apply a narrow standard of review to a magistrate's decision to issue a search warrant. United States v. Flores, 679 F.2d 173, 176 (9th Cir.1982), cert. denied, --- U.S. ----, 103 S.Ct. 791, 74 L.Ed.2d 996 (1983). "[T]he duty of a reviewing court is simply to insure that a magistrate had a substantial basis for ... conclud[ing] that probable cause existed." Illinois v. Gates, --- U.S. ----, 103 S.Ct. 2317, 2332, 76 L.Ed.2d 527 (1983); United States v. Seybold, 726 F.2d 502, 503 (9th Cir.1984). This "substantial basis" standard requires the reviewing court to show great deference to the magistrate's determination. Seybold, at 503. We may not reverse unless it is clearly erroneous. Id. at 503.

B. The Retroactivity of Gates