A similar question on the functionality of sensors can be found here, but my question goes beyond that (pun intended). What is the range (distance) of the sensors of the Enterprise-D? My understanding (and correct me if I'm wrong here please) is that there are three types of sensors: long-rage, short-range and visual (ie the viewscreen). What exactly is the range of these three types of sensors (as in what is the maximum distance they can each detect something)? I know from several TNG episodes the Enterprise can apparently scan an entire system when it is in it, but that's rather ambiguous I think - I'm looking for a unit of measurement (km - although that's probably an underestimate of the Enterprise-D's abilities I'm sure).
According to the excellent "TNG Technical Manual" (written by Senior Trek Technical Advisors Rick Sternbach and Michael Okuda and foreworded by Gene Roddenberry), the max range for the subspace Long Range Sensors at low resolution and maximum normal power is 17 light years, some 160,836,661,043,596 kilometres.
In "The Wounded" we see Geordi using the Long Range Sensors at a range of approximately 10 light years. The resolution at this range was apparently sufficient to be able to distinguish a Federation vessel.
Going by canonical data... at least 10 light years (from 'The Wounded' episode). For comparative purposes, the USS Voyager was able to scan in a radius of 40 light years when searching for Borg ships.
Voyager's newly developed Astrometric Sensors however had a range of 2500 Ly's (at least).
I would imagine that the Enterprise-D and Voyager would be able to scan in excess of several hundred Ly's with regular sensors.