Don't know about satellite tracking but the detection problem is one of the mitigating factors about "suitcase" nukes. Nuclear weapons work because the fissionable material is radioactive and this radioactivity is detectable. The more radioactive it is, the easier it is to detect and the more difficult it is to hide.
A quantity of high grade uranium or plutonium sufficient for a decent nuke or radiologic bomb is going to make any detector scream at quite a distance unless shielded. Of course, if unshielded, the carrier gets sick/dead quick! So, bringing together a sufficient quantity means moving large shielded containers with a ready-made excuse for eye-popping weights or multiple small parcels which has its own set of risks.
All-in-all this makes me think that the greatest logic for a explosive nuke would be a freighter that makes a suicide rush towards the North California Coast carrying a very dirty bomb set to explode before any can land on board and defuse it. The explosion and fallout cloud, along with the vapor from the seawater, would then drift with the wind currents across the country!