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Sensors are components that detect the presence of target objects either by detecting energy emitted by the target or by reflecting energy off them and detecting the reflection. Active Sensors are essential for target acquisition, and may out-range passive sensors for finding large objects with small signatures, but may give away your position. Passive EM and Thermal sensors don't give you as much information as actives, but they could allow you to stay undetected.
Sensor data is shared, hence the sensor range of a fleet depends only on the sensor range of the ship with the longest range. While having some medium-sized sensors on each warship can be useful as a backup, it is more useful to build dedicated surveillance vessels with long-range sensors.
- Sensor efficiency can be improved with research, while ECM jamming and stealth technology can reduce thermal and active sensor detection range.
- Jump shock will render sensors temporarily blind. You can view the active and passive sensors detection radius of your ships and installations in the System Map window by adjusting the Sensors tab values. If you think a 500-strength contact might be out there somewhere, move the slider to 500 and you'll at what distance your sensors would pick it up.
- Every military and commercial ship has intrinsic thermal and EM sensors of strength 1 (not listed as components).
An active sensor is a "radar" component that gathers target data by bouncing gravitational pulses off a target. This is necessary to open fire on a target. Active sensors allow a ship or a missile to detect targets equal to or above its stated resolution (i.e. ship size) within its stated range: the higher the resolution value, the greater the range, but ships smaller than the resolution are more difficult to detect. Active sensor emissions can be detected by passive EM sensors, which can prove a serious tactical vulnerability.
The idea is that as with real-life radar emissions, the signal from your sensor has to travel the distance to a target twice: once to the target, where part of the energy is reflected, and once back to your ship, where an EM sensor picks up the tiny "echo". Therefore your outgoing signal can possibly be detected by other ships with EM sensors much further than your sensor can see those ships.
Actives are designed based on the role intended for that sensor. An area search sensor might be designed with a large resolution to find large enemy ships, while a sensor designed to detect fast attack craft would need a small resolution. Missile detection sensors usually have a resolution of 1, which is the lowest possible. A versatile fleet will employ a multiple sizes/resolution sensors. Design considerations are discussed in-depth here: Active Sensor Design.
Active Sensor target detection capabilities can be increased through background research, while Cloaking technology employed by the enemy will making it harder for active sensors to detect. Active sensors also gather tactical intelligence, i.e. they can tell you the target's size.
Passive sensors emit no energy, instead gather target data by detecting incoming emissions. Thermal sensors pick up the infrared emissions from ships under drive, while EM sensors pick up the emissions from active sensors and shields. Both will detect Colonies, where higher EM to thermal ratio seem to indicate the presence of larger civilian populations and advanced installations.
The stronger the emissions, the farther it can be detected. Passive Sensor detection range is given by
(Sensor Sensitivity x Sensor Size x Emission Power) x 1,000 km. So when designing passive sensors bigger is better, as long as you have the necessary space and resources for it.
A Thermal sensor is a passive sensor sensitive to infrared emissions, and can detect Ship/missile engines or colony activity thermal signature. Not so good at finding anything powered down or moving under minimal power. As engines designed with thermal reduction technology or travelling at less than full speed will emit less heat, and will be harder for Thermal sensors to detect. You can set the speed in the Task Group window.
A few more or less typical thermal signatures, early-mid-game tech:
Size 4 Missile 8 1,000-ton gunboat 88 (no thermal reduction, thus equal to engine power) 38,000-ton freighter 1350 (they use a lot of big engines) 4,000-ton frigate 123 (would have 352 without thermal reduction tech) 6,200-ton alien cruiser 640 (probably not thermally shielded) Mars colony, pop. 67m 7,600 Earth, pop. 1.5 billion 22,500
A strength-28 (sensitivity 14, size 2) Thermal Sensor, small enough to fit on most ships, would detect them at:
Size 4 Missile 224,000 km 1,000-ton gunboat 2.2m km 38,000-ton freighter 37.8m km 4,000-ton frigate 3.4m km 6,200-ton alien cruiser 17.9m km Mars colony, pop. 67m 212.8m km Earth, pop. 1.5 billion 630.0m km
Not ideal, but at least a ship equipped isn't completely blind. And if you doubled the size, you'd double the range. If you equipped a ship with a thermal sensor 10 times that size (size 20, or 1,000 tons), it would spot the cruiser at 179m km, which is pretty sweet. Just remember that it wouldn't tell what the contact is, just how hot it is. And you can't fire at thermal contacts, unless you play around with missiles that have active sensors, but that's a different matter.
And the size-20 sensor would detect the Earth or Mars colonies from anywhere in the solar system.
Compare the emissions from your colonies (first page of the Population Screen):
- a single Deep Space Tracking Station emits only 5 points of thermal radiation and is difficult to detect, except at very short range or with very large sensors.
- a tiny start-up colony of 100,000 people with little industry might give off 300 pts of thermal and EM radiation.
- a large mining colony with 600 auto-mines and some mass drivers might reach 3,000 pts
- a well-developed homeworld with a billion people, thousands of factories and a dozen shipyards could radiate 50,000 thermal pts and 100,000 EM pts.
An EM Sensor is a passive sensor that detects electromagnetic emissions from active sensors, shields and colonies on a planet. Good for locating civilian populations or fully powered military fleets. Not so good at finding a ship under emissions control, which has actives and shields switched off and does not emit any EM at all.
Note that Earth, in the above example game, has an EM signature of 95,000. That means EM sensors are particularly useful for finding enemy colonies.