Sensors for use in Space Combat and Defence
From the early Interplanetary period in Sol system onward spacecraft have found themselves under attack in war and in peacetime; soon after the first undefended vessels were destroyed by the AML in 71 A.T. various systems of detection were introduced to provide warning of hostile action. These systems were at first extensions of the near earth object detection radar, but as the sophistication of the attacks increased, many new technologies came into play. Most sensor systems were destroyed in the Nanoplague years, but a mesh of new systems were soon in place as the fragmented Sol system struggled to recover.
Each new colony world was well advised to erect sensor systems to detect new arrivals, possible rival company ships or actively hostile rogue AI; the colony on Zeta 2 Reticuli was lost to the entity 'Ziggy' because of inadequate detection. Real interstellar combat however did not occur on a large scale until the devastating Empire Wars, and soon every civilised system had sophisticated networks of sensors in place.
Active Sensors To detect small, widely spaced objects at a distance in three-dimensional space it is most efficient to emit a pulse of radiation and record the resulting reflections. Active sensors are phased-array radars or lidar systems (easiest and middle tech), hadron, meson or neutrino, (ultra tech.) They are used for hi-res target identification. Active sensors are used to track vessels whose drives are not active, and so are the primary sensors system at shorter ranges. The decision to 'go active' is a major tactical choice as the sensor system becomes visible at a considerably greater range that that at which it can detect targets. To avoid damage to important locations active centres should be bistatic- the expendable transmitters separated from the vital receivers and command centres.
Radar systems using microwave electromagnetic radiation require much less accuracy for aperture synthesis than optical sensors, so a common strategy is to deploy the emitters and detectors on sensor drones. This provides much greater resolution and also keeps the bright radio emission away from the controlling ship.
Lidar systems use shorter, often visible wavelengths and can be used to produce a holographic recording of all objects within a volume, say a solar system; the shorter wavelength the better, to detect dust-like nanoswarms- these can usually be distinguished by their pattern of movement. Doppler effects can determine relative velocity of objects.
Hadron or meson scanning, while of greater resolution again, require very high-energy output and ultratech equipment, and can easily be detected by an intelligent hull.
Neutrino scanning is stealthier and of lower energy, but generally of very poor resolution, and certainly not exempt from detection. Ultratech neutrino sensors are capable of reasonably good resolution, but are not widely available. Receivers for neutrino scanning can be bulky, although may be hidden effectively on or in moons or planets.
Passive Sensors Active sensor emitters can be the target of EMP or other beam weapons such as lasers; it is much more difficult to locate and destroy a widely spread cloud of passive sensors. In a civilised system, the chances are you won't be all that far away from a sensor array of some sort; any polity or entity which has any interest in defending itself and the resources of a solar system will have the outer system seeded with millions (or billions or trillions) of passive sensor platforms. These could be relatively cheap, stealthy and very good at spotting a ship that is radiating at all in any frequency.
The angular resolution of a sensor is dependent on the wavelength of the observed radiation and the diameter of the sensor's aperture. The longer the wavelength, the larger the diameter of the sensor's aperture must be to resolve detail.
Also important is the amount of radiation gathered by the sensor- an object can be detected by reflected or emitted radiation as a point source even if it cannot be resolved. So larger sensors are desirable in many ways. Unfortunately the larger they are the more easy they are to locate.
Passive sensor clouds for use in defended systems effectively rule out the use of active detection systems by exploring or hostile spacecraft. A spacecraft's passive sensor array often consists of a set of Telescopes or focal field arrays operating in the infrared to visible range. A global set of low-resolution, low-sensitivity scopes is used to track all possible targets (typically such an array can detect the waste heat of a fusion reactor several AUs distant). Target tracking and identification is carried out by a smaller number of larger telescopes (usually in the 50cm size range, but as large as 2m for major vessels; even larger for habitat cylinders).
An OASIS (Optical Aperture Synthesis Imaging System) or other form of interferometer array will increase the range at which detailed target identification and imaging may be carried out. However vibrations caused by operating drive systems limits the use of interferometry to ships that are not accelerating. Spacecraft may deploy passive sensor clouds, but they are liable to be detected by accidental radiation from their data communications or any attitude adjustments they might make.
On warships, there is a second type of important sensor: the X-ray Telescope. Operating fusion drives discharge plasma at a temperature of around a hundred million kelvin. The initial cooling of this drive plume is by x-ray emission, and x-rays are also emitted from the fusing regions. A pulsed or continuous fusion drive produces terawatts of power, and an amat or conversion drive can produce several orders of magnitude more, much of which ends up as x-rays, or extreme ultraviolet. As well as detecting the high-temperature drive plume, x-ray calorimetry can give information on the drive geometry and reaction conditions. Such data can help to identify the class of ship (or design of drive). The cylindrical mirrors used on x-ray telescopes are of much poorer quality than the mirrors used in near-optical telescopes, so the x-ray detectors are of only limited use for attaining firing solutions.
Neutrino Detectors can detect emissions from fusion, Amat, conversion Drive and some kinds of reactionless drive, but are usually massive structures, often hidden in asteroids or planets. Small shipborne versions are generally of poor resolution and may be confused by decoy neutrino emitter, as at the battle of Pehhpepep.
Forward mass detectors are so called because they were developed by information age guru Dr. Robert Forward, not because they can't look backwards. These detectors are capable of detecting and determining the mass of objects remotely, and are particularly useful for detecting neutronium or certain kinds of reactionless drive craft.
Superconducting Quantum Interference Devices (SQUIDs) can detect many kinds of magnetic and gravitic anomalies, and are particularly useful for planetary scanning. Hostile spacecraft may hide in planetary atmospheres, oceans, ring systems and even inside the photospheres of suns; mass detection and squid technology can often find them.
Gravitational Wave detectors are sensors capable of detecting many kinds of acceleration at long distances, and can detect the peculiar signature of most reactionless drives, although a finely tuned Bias drive craft may escape detection. Objects moving at relativistic speeds which are not accelerating may also be detected by gravity wave sensors. See Gravitational Interferometers
Biosensors and chemosensors can be deployed throughout a system, to detect biological and nanotech activity, which may otherwise go undetected; these small weapons are often deployed against sensor arrays, causing systemic breakdown as a preliminary to attack.
Passive sensors can track targets at a much greater range than the range at which identification becomes possible. The plume from a fusion rocket is so clearly visible that warships spend most of the time on unpowered orbits, or accelerating using a secondary drive at much lower power, or accelerating in short bursts. The fusion/Amat/conversion drive is only used when stealth has ceased to be a significant factor (i.e. when actively engaging enemy vessels).
To make detection more difficult attacking craft can cover their hulls in active stealth materials; all warships will have some sort of mimetic / chameleon hull, but simple black coating is all that is needed, at that distance (the ship being so far away and tiny) they are not likely to occlude any stars in the defender's visual field. Assuming of course that the enemy aren't using passive sensors to look for the thermal signature of your ship, in which case a black coating will make it stand out because it will make it radiate more.
Ships by their very nature are hard to keep cool, their reactors need to have a high output and so will output a lot of heat which you have to radiate or the ship will suffer problems (ranging from systems over heating to the ship actually melting) very quickly, and even with the reactor and all other heat sources shut down you still have the problem of cooling your ship down to the temperature of the cosmic microwave background radiation (CMBR), because even even if your opponents tech is only equivalent to information age tech they will be able to spot even very tiny temperature variations. A range of Chiller technologies are available at various technology levels, some more efficient than others.
Advanced energy management can allow waste heat to be radiated as a single beam away from the defending positions, in interstellar space the beam will not be obvious, but in the dust cloud of a solar system it might be more obvious.
To disable a large array of active or passive sensors a relativistic projectile which released a very cold cloud of slowly expanding particles was sometimes used by the Conver Ambi and later by Metasoft. This fast-moving cloud would precede the attack formation, the whole process taking years or decades to be effective. Sometimes relativistic fleets would arrive after peace had been agreed between the combatants, thereby starting hostilities once again.