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A passive sensor that detects electromagnetic emissions from an Active Sensor (which emits radiation only if switched on). Being passive, an EM Detection Sensor does not give away your position (unlike an Active Sensor).
An EM Detection Sensor can detect emissions at a range of
EM Sensitivity * 1000km * Signature strength
A passive sensor contact cannot be fired upon. You need an Active Sensor contact for that.
EM sensors excel at finding active sensors. Spotting your opponents actives can give you valuable information about probable weapon ranges. However active sensors can only be detected if they are turned on, so exclusively relying on EM sensors for passives can lead to a fleet being able to sneak up and only turn on their actives with the first volley.
Populations have relatively large EM signatures, making EM sensors useful for spotting colonies. A sense 10 sensor will pick up a strength 5 EM signature at a maximum of 50,000 km, scaling linearly up and down. You can view the passive sensor detection radius of your ships and installations in the System Map's view by adjusting the Sensors tab values.
EM Sensor Sensitivity
Total Sensor Size
Total Sensor Size ranges between 0.1-50 HS. EM Detector Sensitivity is given by
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