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!
Radiation kills - IF you get a very, very large amount in a very short period of time.
The fissile material in a modest-sized nuke isn't really very radioactive itself. And most of what is emitted is low energy alpha and medium energy gamma rays. No real problem at all to shield the bomb itself sufficiently to work on it, carry it around (Particularly if inside some sort of "case" to hold the explosives) or "ship" in a van/truck/cargo container.
Problem is in the "junk" left in the workshop and machine shop and shelter to build the thing. Loose "dust" and embedded material there, although not radioactive enough to kill, or even make you sick, is EXTREMELY hard to clean up enough so there are no lingering traces.....
So, if a search team comes in (ever again) into that building they've got specific evidence you're machining and shaping and storing radioactive material of so-and-so density and type and radioactivity --- and you did that machining at so-and-so time before the inspection took place, and you handled this kind of material ... etc..
So building the bomb can't be hid - IF you can get your inspection teams into the right buildings.