Only the Russians ever had active radar satellites (RORSATS). The West used ELINT satellites that could detect electromagnetic emissions (radio etc) - which could be neutralised by following EMCON proceedures (radio silence basically). However, that might still pick up jamming (which tenRAB to work by blocking frequencies with electromagnetic noise). 99% of targets for Photo Reconnaissance satellites are land-based therefore their orbits would designed to maximise their time over target areas, not empty oceans.
But all of this technology is cold war stuff, launched in the 1970s/80s. The threat is has receded no telling how much of the capability has been retained.
The alternatives are then ship based radar (submarines preferably) or maritime patrol aircraft. The former takes time to get on scene if it isn't in place to start with, while right now we have no MPA capability due to the cuts by the last government - they withdrew the Nimrod MRA2 earlier and delayed its replacement by a year at the same time.
Back to Spooks, while it's good to have a new series, suspending belief is becoming ever more difficult. It's a shame when they can get the current affairs aspects spot on but completely ruin it with preposterous technology and mistakes (at various points the ship was far out in the Atlantic (on a Morocco-UK trip presumably?), a 20-odd miles off Plymouth the next minute, still in the middle of the Atlantic on the ship's navigation system but deep in the Bay of Biscay on MI5's tracker!).