Sensor array

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Contents

Introduction

A sensor array consists of:

  • A distributed set of active active sensor emitters and receiver nodes.
  • A distributed set of active passive sensor receiver nodes.
  • The combination of active and passive sensors, with the resulting information combined through data fusion to present maximum coverage of the surrounding spatial volume. Data from outriders and other external sources can be included, subject to the transmission time delay.

Sensor Detection

A ship exists within spheres of concentric data time lag. Realtime data exists only in close proximity. With every light second distant the received data becomes increasingly stale.
Two threats are detected four light seconds distant.</br></br>Passive sensor data when received from one is four seconds old.</br></br>Active sensor data requires a directed sensor sweep, with its energy taking four seconds to reach the target, and another four seconds for the return to be detected.

Phased Array

An individual active sensor array can be enhanced by using it as a phased array where the relative phases of the respective signals feeding the emitters are varied so that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in other directions. The phased array may be used to point a fixed radiation pattern in a dedicated direction, or to scan the surrounding volume rapidly in azimuth and elevation.

Sensor Fusion

The emitters and nodes of the array can be utilized in combination with the other components of the array or individually. This means that the degree of cover afforded a particular area can be managed to increase or reduce the coverage and resolution according to the threat environment.

  • The local volume is usually scanned more frequently than the outer volume. The greater the distance, the longer it takes to provide a thorough scan to provide a reasonable probability of detection.
    • In a threat-well detection data becomes increasingly stale with every light second distant and will be marked with increasing levels of interpolation. Even the use of outriders and other external data sources does not reduce this -- the data will have aged unless sent via a t-relay.
    • Ship emissions (such as the flare of a main drive or waste heat) can be easily observed. The sensitivity of sensor scans can be altered to perform a quick low quality scan (to swiftly detect powerful burst emissions) and a slower more sensitive search.
  • When a number of ships are co-operating, this means that the search patterns can be co-opted to reduce the total long range scan time and provide greater in-depth coverage. Each ship also contributes its own local scan.
  • The usage of active and passive sensors can be managed to increase effectiveness. Different sensors have different degrees of accuracy and range:
    • G-sensors can detect a hyperspace transit across the expanse of a solar system but are of limited use for other detections given the natural distortions to the g-field in a system resulting from the presence of mass ranging from planets to dust.
    • Neutrino detectors can locate engine and reactor emissions across a solar system. Shielding will reduce detection quality.
    • Electromagnetic spectrum (EMS) sensors have a potential long range, but are limited by seethrough conditions, sensor shadows (behind objects such as moons and planets) and the time and energy output required to scan the entire search volume.
  • A detection by one sensor can cause other sensors to be employed to provide a more detailed search pattern. In some instances a larger scale sensor system can be assigned, such as a bigeye.
  • Multi-spectral data from a variety of sensors can be fused to provide a composite image.

Detection Uncertainty

Sensor data quality is a function of the time lag. If a target is at half a light second distant:

  • An active sensor has a one second delay: half a second for the transmitted waveform to reach the target and another for the reflection to return.
  • A passive sensor has a half second delay: the time taken for target emission to travel to the sensor.

The time lag means that it is more difficult to predict track trajectory as the data is effectively looking into the past. In the time taken for the signal to be received the target has had opportunity to change its trajectory, resulting in a probability cone that grows with distance.

The volume to be searched by scanning sensors increases with distance, meaning that either sensor resolution decreases with range, or the scan time increases. A ship is effectively surrounded by expanding spheres of data uncertainty.

Weapon Direction

The sensor array can be used for detection and tracking, and weapon guidance and direction.

  • A missile can be directed by tightbeam point-to-point transmissions from the array to relay guidance commands.
  • The reflection of beam weapon discharges on a target can also be used by passive sensors to enhance the detective and to provide data to improve beam focus and target lock.

Countermeasures

  • The quality of an active sensor reflection will be reduced by the shipskin.
  • Orientate to provide a minimal geometric cross-section in the direction of hostile active and passive sensors. The shipskin will reduce the reflection by absorbing or scattering it. Active sensor reflected energy will inevitably disipate on its return.
  • Heat emissions are reduced by the heat sump.
  • ECM can be employed to confuse sensors.
  • Drones can be employed to act as decoys, providing emissions and reflections to emulate a larger ship.
  • Maintaining distance ensures that the probability of detection is reduced. The time lag means that it is more difficult to predict track trajectory.
  • A ship passing in front of a distant star may be detected by passive sensors, but in contrast close proximity to a sun means that enemy sensors will be overloaded by the radiation output and less likely to distinguish ship emissions. The classic attack-out-of-the-sun strategy. The upper atmosphere of a Jovian gas giant can also provide a hiding place.

Maximum effective range for directed missiles and beam weapons in space engagements is no more than a few hundred thousand kilometers; the time lag becomes increasingly significant, providing more time for the target to maneuver. If the target is small or agile then this effective range is considerably decreased. An incoming missile will adopt numerous strategies to minimize its detection and maximize kill-probability:

  • Launch phase:
    • An incoming missile will be launched using a maglev to induce an initial velocity. This ensures that the missile engine flare does not betray the position of the launcher.
    • The missile sprints up to attack velocity at long range.
    • The sprint booster may be detached at the end of the acceleration phase; residual heat continues to broadcast its position after engine shutdown. The booster then acts as another detection to seduce sensors and point-defense away from the real threat.
  • Mid-flight phase:
    • The missile may employ its own passive sensors to maintain lock on its target, or be guided by a point-to-point data transmission. Under some circumstances the firer may be illuminating the target with beam weapons.
    • The missile will orientate itself to present its smallest cross-section towards the target.
    • The missile adjusts its trajectory using low emission propulsion techniques (such as maneuvering jets expelling gaseous nitrogen and reaction wheels to rotate the missile around its center of mass) to minimize detection. It drifts towards its target.
    • The missile may deploy decoys on diverging trajectories.
    • The missile may spin on its long axis to confuse sensor reflections and to reduce the effectiveness of beam weapons focused on it. If acquired by the defender the missile may detonate prematurely to blind their sensors to other missiles.
  • Terminal phase:
    • The missile may activate its own active sensors to accurately acquire its target.
    • The missile accelerates at short range to minimize the window where point-defense can be effectively employed against it. This reduces the reaction time of the defending system and potential beam weapon dwell time.
    • The missile may perform extreme course changes to hinder tracking and modify its predicted intercept footprint. The classic maneuver is known as a dog-leg; this is a 90° high-load factor turn in one or more axes to radically alter the trajectory requiring extreme acceleration.
    • If a secondary booster is employed this may detach from the warhead. Defending sensors may retain lock on the booster and ignore the warhead as a secondary threat.
    • If the warhead is a nuclear-pumped gamma ray laser it will now discharge.
    • Multiple warheads may be deployed. A stream attack may overwhelm point-defense ensuring that at least one warhead penetrates the defenses.
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