Three types of long range scan are performed:
- Active scan using active sensors.
- Passive scan using passive sensors.
- Tachyon scan using a t-relay in receive mode.
Long range scan is inherently more difficult and demanding than a short range scan.
The three-dimensional volume surrounding the ship expands with range, attenuating the sensor capabilities and the time required to search, analyze and identify a potential detection. Data is subject to increasing interpolation as the time delay between hard data updates increases.
The local surroundings can be quickly and regularly swept; at long range the sensor sweep is slower and less comprehensive. As range increases resolution and accuracy falls away; at significant ranges the sensor time lag becomes a major factor as distances expand from light seconds to minutes or hours.
If a detection is made, it may be minutes or hours old – where the target was, not where it is now. Acquiring and maintaining nav-tracking acquisition is difficult – almost impossible if the detection manoeuvres, but conversely, by manoeuvring the detection becomes (briefly) more visible to passive sensors.
Any detection becomes the central point origin for a cone of probability derived from any data on the velocity and acceleration of the object, any assumptions of its nature and capability and any distortions in the g-field such as planetary gravity wells. The cone of probability allows for deceleration should the ship turn to use its main drive to shed velocity.
It is very easy for a ship to hide in the great expanse of empty space if it does not betray its location by radiating waste heat, drive emissions or active scan.
Long range scan can be improved by using sensor outriders and by multiple ships co-operating to divide up the search volume. Remote sensor platforms can expand the coverage and provide early detection.
Active Long Range Scan
Long range active scan has the penalty of alerting the presence of the ship negating any advantage of stealth. When employed in a long range mode an active sensor transmits an enormous amount of energy, announcing its presence. A detection is made by the reflected energy but only provides the velocity component relative to the transmitter.
The most commonly employed sensor is radar using long pulses with long delays between them. The distance resolution and the characteristics of the received signal to noise ratio depends on the shape of the pulse which can be modulated to enhance performance using pulse compression. The main sensor array emitters are spread across the shipskin and so permit full coverage of the surrounding volume of space.
The quality of the scan can be enhanced or reduced depending on the situation, with a variable number of emitters assigned to long range and others to short range scan. The characteristics of the pulses and delays between them dictate whether an emitter is being operated in short or long range. When a return is detected an emitter can be assigned to track that specific source – until the return moves out of the visible arc of that one emitter node. This reduces the probability of losing the detection and provides near immediate data on any changes in trajectory (subject to the sensor time lag).
Lasers are less useful for long range scans. However, lidar uses a much shorter wavelength of the electromagnetic spectrum; in general it is possible to image an object only about the same size as the wavelength, or larger. Radar frequencies are useful in detecting metallic objects, but non-metallic objects such as rock provide a weaker reflection, some giving no reflection at all (an effect emulated by shipskin). A laser is more effective at detecting small objects: molecules, dust and the inevitable imperfections of shipskin arising from wear and tear, and battle damage.
Passive Long Range Scan
Long range passive scan does not advertise the presence of the ship but has the penalty of taking longer to scan the larger volume – unless there is a detectable active transmission along the electromagnetic spectrum, including waste heat and neutrino emissions. If there is no obliging transmission then the sensors rely on detecting either the anomalous reflection of a natural energy source (such as reflected light), the blocking of radiation from a natural source (such as the silhouette of a ship crossing in front of stars), any disturbances in dust clouds or solar wind, or an anomalous spike or wave in the g-field.
This type of scan is slow and inaccurate. It also requires enormous processing power to analyze, correlate and perform data fusion between the different sensors.
If an apparent detection is made then more passive sensor resources can be utlized to sweep the suspect area up to and including the assignment of a bigeye. If necessary a long range active sensor can then be used to interrogate the detection. This reduces the energy output of the active sensor because it is being used in a dedicated mode rather than scanning the entire volume, and the radar or laser beam is transmitted only in a narrow arc.
Tachyon Long Range Scan
A t-relay is used to listen for coherent transmissions within its operational range.
In this mode the t-relay is effectively being used as a passive sensor to detect the presence of ships and message drones to obtain a t-lock. To counter this, in the alert status of emissions control and above, the t-relay is shut down.
T-lock is subject to a minimal time lag because tachyon particles travel at superluminal velocities and so has the benefit of providing positional data as of now, rather than in the past.