[WJW]

Most modern military vessels could spot them, but typically the air warning radar has its filters configured such that satellites are excluded (as they are not interesting targets for most naval vessels). This also heavily depends on the exact orbit of course. Low orbit satellites like starlink are quite below the maximum altitude from ballistic missiles, geostationary orbit is usually too far away to still detect within the unambiguous range of the radar system.

[dzhiurgis]

How about RTL-SDR detecting SAR signal?

I just wonder how many SAR sats we don't know about.

[KeplerBoy]

Won't work with a regular RTL-SDR.

Most SAR systems operate in X-Band (~10 Ghz). I guess it's fair to assume that every organization who has ever sent anything to space has also launched SAR satellites. The radar part is comparatively easy to getting things into orbit.