Spacecraft Sensors

"There's no way to hide in the black of space. You can only try to confuse or blind the other guy long enough to give you the upper hand."

--Able Crewman Maria Velasquez
TASA Space Corps

There are many forms of detection and identification systems used in space, some familiar to terrestrial battlefields and navigation, others more unique to the horizonless vacuum of space. Sensors can be divided into two main categories: active and passive. Passive sensors collect light, heat, radio waves, and other emissions given off naturally by objects in space. They do not give away the sensor's position or the fact that an object is being observed. Active sensors send out "pings" to search for and identify an object, such as through microwaves (RADAR) or lasers (LiDAR) which are bounced off the target and reflected back into the sensor to gain information. The downside is these signals work both ways, and can be tracked back to the source by passive means. Active sensors are often shorter range due to the inverse square law (which is applied twice for reflected signals) but far higher fidelity for what they can see.
Warships are usually equipped with a number of passive and active sensor systems that work in tandem to develop a clear operational picture, and most ships are able to detect and positively identify any ship long before it is close enough to be a threat. The number of different sensory systems also gives a degree of redundancy, in case one system is jammed, damaged, or unfeasible (for instance turning on radar when the enemy has radar homing missiles).

Passive Sensors

Multispectral "Sky Scan" Camera Array

Spacecraft usually detect and engage each other at distances well beyond when the naked eye would be able to make out a speck of light against the black of space. Not that an unaided eye would ever be used to spot an enemy ship, as spacecraft are devoid of windows or portholes. Nevertheless, visual detection means are the most fundamental aspect of detecting spacecraft. Each ship is equipped with a set of cameras that take a mosaic picture of the entire sky surrounding the ship (excluding whatever is occluded by the ship's drive plume). These cameras stitch together the mosaic to provide the ship's computer and sensors analysts with a nearly full sphere of awareness of any light, infrared, or ultraviolet emissions toward them. The infrared emissions produced by any heat generating object or an object absorbing solar radiation can be seen against the cold background of space at extreme ranges. Fusion powered ships with their drives active are like setting off a sparkler in a dark room, making detection of a ship under thrust trivial, even at distances well beyond the solar system.
The Sky Scan is capable of taking a mosaic snapshot of the visible universe every few seconds. The key bottleneck is in analysis time. Depending on the amount of processing power dedicated to analyzing the Sky Scan mosaic and the resolution of the images taken, updates can be provided anywhere between a few minutes and a few hours. The Sky Scan on most ships usually take constant snapshots, but analyze only a small sample depending on how quickly they can conduct the analysis. By keeping the cache of images, analysts can retroactively analyze specific lengths of time for a more detailed picture of what occurred then.
Once the ship's computer has finished processing the data, it compares all IR sources with the latest star charts, the previous mosaic snapshot, and the transponder locations for all friendly, verified neutral, and known enemy ships (a process known as Comparative Analysis or CompAn). Anything that does not match a previous source is identified as a bogie and marked for further inspection by the computer or human analysts. Objects that appear to make a drastic change between snapshots are also flagged, as they are either close enough or moving fast enough to be of concern.
Sky Scan is primarily a detection system, giving the relative direction of any object. If a ship is able to communicate with friendly ships or stationary sensor arrays, their Sky Scans can be used in tandem to confirm readings and give a parallax to determine the distance of an object. The vector, velocity, and probable mass of an object can also be determined with Comparative Analysis, but other systems can give this information with higher fidelity.
As the Sky Scan is an array of optical sensors, it is limited by line of sight factors. Sky Scan cannot see through or around solid objects like planets or asteroids (without assistance from other ships to cover blind spots). The immense emissions of the Sun essentially blind most visual sensors within 1-2 degrees of the sun, and a ship's own drive generates enough radiating heat and light to obscure anything behind it. Like most sensors, the detection range is theoretically limitless, given a bright enough source, but the probability of detection and fidelity of information decreases with range. At the far extents of detection range, an object might only be a single pixel on the image, or as less than a pixel (appearing colder as the sensors average the temperature of everything in that pixel).
The Sky Scan is also vulnerable to active countermeasures, in particular laser weapons which can be focused directly on the lenses of the Sky Scan cameras to temporarily blind them or even cause permanent damage with enough exposure. Some factions try to hide or decentralize their Sky Scan arrays to prevent laser attacks. More cameras in different locations along the hull of the ship means the lasers would have to target them each individually instead of knocking out the array all at once. "Iris" shutters can be installed to hide these cameras during combat, reopening the apeture once the threat of laser or kinetic damage has passed (at the cost of losing some situational awareness).

High Fidelity Multispectral Telescope

The Multispec Telescope is the more precise cousin to the Sky Scan array. Rather than a wide angle image of the entire visible universe, the Telescope is aimed at a specific direction to interrogate a small patch of the sky (usually a degree or less). The Telescope returns the same kinds of information, but quicker and at higher resolution. It is often used in tandem with the Sky Scan to provide additional information on specific bogies identified by the Sky Scan.
As the Telescope operates on the same principal as the Sky Scan Array, it has many of the same detection/identification weaknesses and vulnerabilities to laser attack. It also can only ever look in one direction at a time (though it can bounce between several different targets in the time that a Sky Scan would develop a full area picture).

Peripheral Camera Suite

Peripheral Cameras are smaller cameras that line the sides of ships. They take the place of windows and portholes for close range situational awareness. They are lower resolution and usually only visible light or visible light and IR based, so they aren't used for threat detection unless other sensors have been knocked out (to avoid the ship flying completely blind). Peripheral Cameras are often used to aid docking, for external security against boarding, and to simulate windows for crew morale. Peripheral Cameras not in use by the ship's Sensors Operators can often be controlled by any crewmember or authorized passenger to look at planets, space stations, or stellar phenomenon on internal screens.

Astronavigational Positioning System (APS)

The Astronavigational Positioning System is an internationally recognized navigational system that tracks the movement of all registered spacecraft in the Solar System. APS satellites determine the position of ships based on the light delay, much like terrestrial Global Positioning Satellites. The signals sent by APS transponders can also give additional data about ships such as class, destination, owning organization, type of drive, etc.
APS data on every ship is broadcast to the public so that civilian ships do not need expensive sensor suites just to track other traffic in their area. Every ship, civilian or military, has both an APS Transponder and Receiver. Any ship can receive APS data by pulling broadcasts from dedicated APS satellites. Transmitting APS data, on the other hand, can needlessly give the enemy information about a warship, so military ships can turn off their transponders in a time of war (while still passively collecting APS data through their receiver). For civilians, however, it is impossible to turn off the APS transponder without bypassing firewalls and tamper alarms that alert everyone in the solar system (including the nearest security patrol).
In either case, turning off a transponder is telling. It immediately lets everyone, including potential enemies or security forces know that the ship doesn't want to be monitored. This action is often seen as an escalation towards a combat posture (along with turning on active sensors).

Emissions Sniffers

Many ships are equipped with a mast of extremely sensitive equipment designed to detect emissions of electrons, gamma rays, x-rays, microwaves, etc. The Sniffer suite is an important warning system for radiological threats and dangerous stellar phenomenon, alerting crews to sudden changes in readings. The systems that detect photonic emissions and microwaves can be used to backtrack and pinpoint enemy radar and LiDAR pulses.

Radio Intercept Suite

Some ships, usually dedicated intelligence collection ships, long range patrol ships, and some of the larger capital ships, will carry a specialized radio receiver that is tuned to pick up enemy communications, both encrypted and unencrypted. These systems then attempt to decipher the encryption, providing valuable intelligence on enemy activities when successful. The radio interceptors can also determine a line of bearing towards the signal's point of origin. By working in tandem with another Radio Interceptor on another ship, the transmission source can be triangulated to a specific point in space.
There have been attempts to develop a similar intercept system for laser communication, but properly tuned encrypted laser transmitters and receivers leave very little backscatter for a third party to detect and observe. Some attempts involved scattering reflective particulates into a region of space, but all tests resulted in a particulate field that was either too small or too spread out to be of any practical use. Multispectral Telescopes can at best tell that a ship within its line of sight is being hit with a communications laser based on subtle light fluctuations, but even in this best case scenario, they cannot make out the contents of the high bandwidth broadcast. For now, encrypted laser communication remains the safest way to carry on private conversations in space unless conditions favor radio usage instead (for instance line of sight occlusion, communicating with multiple receivers simultaneously, etc.).

Active Sensors

Radar

Radar arrays send out pulses of microwaves into space, then identify any microwaves that have been bounced back. The simple technology dates back to the 20th century, with improvements in fidelity and utility along the way. The radars used by most military ships can generate a full 360 3d picture of the environment around the ship. Radar systems have a much higher refresh rate than Sky Scan, and the Radar is also not obscured by drive emissions and certain other line of sight obstacles. Unlike Sky Scan, radar can determine an object's distance from a single sensor based on the time it takes for a signal to bounce back.
These benefits over Sky Scan also come with drawbacks, not the least of which being that radar is an active sensor and it creates signals that can be tracked back to the ship by enemy sensors or guided missiles. Radar also has a limited range, based on the inverse square law of signal attenuation. With radar and other active sensors, the inverse square law is applied twice - once for the emission to hit the target, and again after being reflected towards the sensor. Due to light based lag time, a ship that is pinged by radar will know it has been detected twice as fast as the sensor does. At close ranges this lag is negligible, but at interplanetary distances, this can be a precious few minutes to begin making evasive maneuvers. For instance, a ship in orbit of Mars at its closest approach would be detected by an Earth based radar in 6 minutes. The Martian ship would become aware of the radar pass in only 3 minutes. Further distances would result in longer detection delay.
Radar is also useful beyond just detection and location finding. Though not as robust as LiDAR for this purpose, Radar can be used to determine some attributes about a detected object. Based on the number of radar waves returned, a general size and shape can be determined (although a cluster of objects in close proximity could be mistaken as a single mass of objects). A doppler shift in the return signals can also give a relative velocity difference between the sensor and the target (which can be further analyzed into an absolute velocity).
Certain types of Radar are also useful in orbital support operations by scanning planetary surfaces. Radars can be used to survey ground and even subsurface terrain, or to detect aircraft, watercraft, and even ground vehicles beyond the usual horizon limitations of ground or air radar systems. Radar can penetrate cloud cover and smoke that obscures visual sensors and LiDAR.
Radar can be jammed by electronic warfare systems flooding a region of space with waves on the same frequency as the radar system. Ships can also be coated in radar absorbent materials and constructed with deflecting surface angles to reduce their radar signature (though this will not affect their detection by other sensors, and will in fact increase the ship's heat signature, overall weight, and make them more susceptible to heat damage from laser weapons and solar radiation).

LiDAR

LiDAR is an active visual sensor that operates in a similar principle to Radar. Instead of bouncing radio or microwaves, the LiDAR sensor sends a pulse of laser beams at its target, which are reflected back to the sensor. Like Radar, LiDAR is susceptible to the inverse square law, giving it a limited range compared to passive multispectral scans. LiDAR is, however, the most precise detection system, offering high resolution volumetric data on interrogated objects.
In general LiDAR is shorter ranged than Radar due to faster attenuation of the higher frequency beams. It is also susceptible to the same light and line of sight limitations as multispectral passive sensors. LiDAR can also be blinded or damaged by laser weaponry.

Pinpoint LiDAR

Most tactical LiDAR systems are used for high resolution images of single targets or a small sector of space. So-Called Pinpoint LiDAR can be aimed at a target identified by other systems. The relationship between pinpoint LiDAR and wide area Radar is similar to the relationship between Sky Scan and Multispec Telescopes, only with active sensing.
Pinpoint LiDAR is most often used for identifying a specific object with a high degree of fidelity. The individual LiDAR beams have increasing resolution the closer a target object is, often capable of generating a height map based on miniscule differences in reflection time. This can generate a pseudo-3d image of the target for visual identification and analysis of its activity at the time of the scan (for instance, the direction weapons are pointed or the status of missile tube doors). LiDAR cannot see the other side of a ship unless the target turns 180 degrees, or another LiDAR sensor pings the target from the opposite direction. Additional LiDAR references can be combined to generate a true 3d model or increase the resolution of the pseudo-3d model.

Wide Area LiDAR

Wide Area LiDAR is usually used to survey planet or asteroid surfaces, similar to sounding radar. It consists of a lower resolution LiDAR sensor that rotates or sweeps over an area, generally 180 degrees or less (a 360 degree LiDAR would be inefficient compared to other means). Wide Area LiDAR can be used to scan for landing sites or to detect changes in a region's surface (new structures, earthworks, impact craters, battle damage, and large vehicles). LiDAR cannot penetrate cloud cover or smoke, and is susceptible to atmospheric refraction.

Fire Control Lasers

Fire Control Lasers are specialized laser emitters that serve two main purposes. First, the FC Laser acts as a single point range finder, freeing up LiDAR scanners for more specialized analysis. FC Lasers also paint a target with a specifically tuned frequency of laser beam. This can be used to guide missiles or torpedoes onto a target.