Electro-Magnetic Direction and Ranging (EMDAR) is a passive sensor technology that detects emissions along the EM spectrum to identify and track other vessels. The system is passive because it does not emit any EM to make its detection (in the way that RADAR does with radio waves). Instead, the system detects EM radiation falling onto its sensors and calculates likely distance using dispersal patterns, a process very similar to SONAR.

The advantage of the passive system is that it does not provide any EM emission for an opposing vessel to detect. The disadvantage is that passive systems can only achieve relatively weak readings, sometimes difficult to distinguish from background radiation. Dispersal patterns also become increasingly less predictable over distance, making range estimations less certain. 

Vessel Emissions

Major vessel systems such as engines, generation, communications and sensors will generate potentially detectable emissions. The intensity of those emissions is determined by the system and is usually related to system output levels. For example, increasing engine thrust will cause an increase in the IR and magnetic emissions associated with engine output.

The EM emissions of a vessel are constantly calculated depending on system activity. Each system which emits significant EM will have an emissions profile which calculates emissions based on output. These emissions are assigned to one or more of the vessel’s profile planes, where they are aggregated with other emissions in that spectrum range.

Emission Intensity

Emission intensity from vessels for each EM range is managed on a scale of 0-100, representing the detected intensity at the edge of visual range (see below). Based on this scale, maximum emission intensity at source (100) would be detected as 1 at maximum range on the TOE grid (5000 grids).

Visual Range

Visual range is 150,000KM (500 grids). At this point, optical sensors are capable of detecting vessel-sized objects and so EMDAR is at peak effectiveness. Detected emission intensity therefore remains constant at distances less than visual range.

Function

Emission intensity diminishes over distance, described as an inverse square function.

Where I is the intensity in Wm², x is the current range from emission source to the detection point (in grid units), I_m is the intensity of maximum source emission detectable at the TOE edge (a system variable) and R_m is the range of the TOE edge (in grid units).

When x is below 500 grid units, I is constant at the value of x = 500.

Detection

Detected emission intensity will determine how accurately the source’s location is reported. The system will offset the source’s actual location when displaying the emission on the EMDAR interface.

Detected emissions are displayed on the EMDAR interface as a blip at their calculated range. Emission intensity is indicated by the size of the blip, with the display differentiating five intensity levels.

Detection Levels

Detected emission intensity is divided into five levels. Each level represents a range of emission intensity and defines what modifiers are applicable to detected emissions falling within that intensity range.

Detection levels are defined individually for each of the five EM spectrum ranges.

For example, this means that IR emission detection will have different response characteristics to visible emission detection.

Modifier

Each level has a modifier defined. This modifier is used to calculate the extent of any position offset (see below).

Cutoff

Each level has a cutoff level defined. When the position offset exceeds this level, the detection is considered corrupted and no detection is displayed.

Position Offset

The accuracy of emission detections is managed using position offsets. The less accurate a detection (typically due to low emission intensity) the greater the offset between the source’s actual position and what will be displayed on the EMDAR interface.

Position offsets are determined randomly. A random number in the range +/-5 is generated for each of the X and Y grid co-ordinates. This is then multiplied by the modifier appropriate to the detected emission intensity level (see above).

Where D is the displayed location of the emissions source (as X and Y co-ordinates), P is the actual location of the emissions source, O is the offset (randomly generated) and M is the detection level modifier.

Display Cutoff Modifier

If the position offset is larger than the defined cutoff for that detection level, D is modified to zero (no detection is displayed).

Relative Position

The module will determine which directional array the detection should appear on based on the detecting vessel's heading (this parameter is supplied along with the detecting vessel's grid position).

The module calculates which array to allocate the detection to based on the bearing from the detecting vessel to the detection. As there are only four directional arrays (forward/aft, port/starboard) a range-based selection is made at +/- 45° of every 90° interval.

This means that the vessel should maintain a heading that is an interval of 90° to maintain the most accurate directional allocation of detections.

AO Masking

If an astronomical object such as a start or planet is between the detecting vessel and the target, then the target's emisisons will be "masked" and no detections will be displayed.

The system determines whether the bearing to the target is within a bearing range that describes the AO's masking effect. This range is calculated using the AO's radius.

XY mask Calculation

The upper end of the bearing range is calculated based on the AO's XY grid position (which is the centre of the AO) plus its radius. The lower end of the bearing range is calculated based on the AO's XY grid position minus its range. If the bearing to the target is within this bearing range, then the target is masked.

Z Mask Calculation

If an XY mask is detected, then another check as made based on the vector to the target. A vector range is calculated based on the AO's Z gid position plus/minus its range (similar to the XY Mask Calculation). If the vector to the traget falls within this range, the target mask is confirmed.

Background Radiation

Background radiation represents cosmic radiation drifting randomly in space, or radiation generated by a star or astronomical phenomenon.  

Cosmic Radiation

Cosmic radiation appears randomly. A number of random detections will be displayed on each EMDAR array per system cycle. Random numbers generated between 0 and 10 will determine:

Detection range

A random number between 3 and 50 will be generated. Multiplied by 100, this gives the X or Y co-ordinate representing the detection range.

EM spectrum

A random number between 0 and 10 will determine which EM spectrum is displayed. Some EM detections are more likely, so detection is determined by this table:

Intensity

A random number between 0 and 10 will determine the intensity of the detection. Lower-intensity cosmic radiation detections are more likely, so intensity is determined by this table:

Detection Hold

In order to simulate the appearance of cosmic radiation beyond a single detection instance, occasionally a cosmic radiation detection will be held and repeated in the next cycle. This allows the display of cosmic radiation as a possible track, adding complexity to the EMDAR operator’s role.

This will occur if the detection range falls between 40 and 50 inclusive.

A held detection will appear with the same parameters originally generated (the same detection range, EM spectrum range and intensity).

A random number between 0 and 10 will determine if a held detection will continue to be displayed in subsequent cycles. If below 6 (inclusive), the detection will be held. If the result is 7 or above, the detection is cleared and will not be displayed further.

Forced Cosmic Radiation

Mission Operations may add a cosmic radiation detection and display it as long as required.

This is achieved by entering the detection range, EM spectrum and intensity. These base parameters will be modified randomly before display.

[ADJUSTMENT TABLE HERE, +/- ONE LEVEL]

The base parameters may be adjusted at any time to produce the desired effect. This is not suitable for displaying complex tracks – a TSMO should be used for this purpose.

Proximate-Source Radiation

This type of background radiation is generated by a star or other astronomical phenomenon. As this type of radiation has an identifiable source, the EMDAR system is able to measure and filter it out so it is not displayed. Instead, it modifies the intensity of detected emissions from an object.

If either the source TSMO or the detecting vessel are located within the affected area, any emissions detected from that TSMO will be modified.

Radiation Area Definition

The area materially affected by radiation from a proximate source is defined as a sphere with the astronomical phenomenon as its origin (centre). Emission intensity at the source is defined, with the edge (the surface of the sphere) typically defined as 1 Wm².

Each Radiation Area Definition (RAD) applies to a single EM spectrum range. If a proximate-source outputs background radiation across multiple EM spectrum ranges, then multiple RADs will need to be made.

Proximate Source Radiation Modifier

Emission intensity at any point within the proximate-source radiation sphere is calculated using the Emission Intensity function (see above), with the sphere’s origin as the emission source. The calculated background radiation intensity is subtracted from the detected emission intensity prior to detection level modification.

Module Details

Script: ../view/nav/EMDAR/detect.js

Functions

• refreshDetections(vesselID)

Input Parameters

• Vessel ID: The submitting vessel or TSMO's UID (as stored in the nav_objects DB table)
• EM Profile: Each EM band has a profile describing the pattern dissipation characteristics of that band. There are five levels in a profile based on the intensity of the detection. Each level defines a different reponse, displaying detections ore accurately as intensity increases.

  Profiles are stored as applet data JSON (one for each EM Band):

  {

     level1: {

        detect: 1,  Defines the detection intensity that applies at this level

        modify: 501, Defines the modifier (shift from actual position)  that applies at this level

        cutoff: 300  Defines the cutoff (when a detection is not displayed) that applies at this level

     },

     level2: {

        detect: 2,

        modify: 502,

        cutoff: 1000

     },

     level3: {

        detect: 4,

        modify: 503,

        cutoff: 600

     },

     level4: {

        detect: 8,

        modify: 504,

        cutoff: 200

     },

     level5: {

        detect: 100,

        modify: 505,

        cutoff: 400

    }

  }

Output Parameters

The following parameters are output as an array

• Array: Which directional array this detection should be displayed on.
  Options are: sbd, prt, fwd, aft
• EM Band: Which EM Band the detection is within
• Range: The range from the detection ship (in grid units)
• Intensity: The inetnsity of the detection (on a 0-100 scale)