Electromagnetic Sensors

A standard type of sensors, described sometimes also as thermal sensors. They work more or less similarly to old termovision, detecting heat sources planetwide (mainly due to space being generaly cold unless in proximity to the stars). They make sneaking into the system absolutely impossible, since every single heat sources is literally like a flare announcing that there is something there (for every ship/fortification/planet equipped with EMS).
They also make entering system painfully obvious, since hyperspace is much warmer, thus leaving it (or entering) causes thermal flashes, the bigger and more visible the greater the Hyperspace compression rate in the region. They, however, also have a disadvantage, since while they can locate hostile fleet and properly identify the size, course, and acceleration of hostile unit(s), they can tell next to nothing about configurations and variants of enemy ships.