Not for detecting nuclear blasts, but rather for directing them. The primary motivation for Transit/NNSS, the first satellite navigation system, was to provide

Not for detecting nuclear blasts, but rather for directing them. The primary motivation for Transit/NNSS, the first satellite navigation system, was to provide location information to missile submarines so they could calculate firing solutions for SLBMs. The submarine needs to know where it is so it can tell the missile where to go. Detecting nuclear blasts from space came shortly later with Project Vela. Those satellites didn't depend on something like GPS because the position of each satellite at any given time could be calculated from its known orbital parameters; no need for radio navigation.