Short Range Energy Passive Sensors
These sensors have an operational range of 1 light year but can only detect items that generate energy in the form of heat, radio waves or radiation. The advantage of this type of sensor is that it does not require the scanning ship to send out any sort of pulse or signal and is therefore useful when trying to hide the ship (in a nebula for example).
The range that objects can be detected at will vary with the amount of energy they are emitting. For example, an asteroid which is only reflecting heat from a nearby star may only be detected at relatively close range, where as the star itself will show up clearly on passive scans.
A ship travelling at warp will show up clearly within the 1 light year range with its warp signature clearly readable (and comparable to the Starfleet database). However a ship that is powered down and drifting may register as an asteroid on the scanners. Weapons fire, ships with active shields and photon torpedoes always show up due to the large amount of energy being generated.
These passive scans can be carried out very quickly and the ship can scan 360° in all axis in a matter of seconds. They also draw very little power and for this reason the short range passive sensors are almost always left on all of the time.
The short range energy scanners can be used to target both phasers and photon torpedoes, however the accuracy/lock will vary depending on how good a reading the sensor has. For example, locking on to a ship broadcasting a message would be easy, but trying to lock on to a drifting ship with no power would be much harder. The preferred method of aiming phasers and photon torpedoes is the Short Range Active Sensors.
Power Requirements
The short range energy passive sensors only draw 1MW of power when they are active.
Upgrades
Research can be used to increase the range of the scanners and to improve the detection strength of each object (so that a positive id can be made on a weaker signal).