EMDAR Overview

Electromagnetic 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. 

EM Spectrum Ranges

Vessels or other objects may generate EM emissions on different parts of the spectrum depending on their design, propulsion system and other technology in use. The EMDAR system divides the EM spectrum into ranges that are likely to represent different sources such as generation, propulsion or communications systems.

Ionising Range

These are gamma rays and x-rays. May be emitted by unknown propulsion systems.

UV Range

These are ultraviolet emissions (including visible light), due to the visible flare of engine exhaust or reflection off a nearby star.

IR Range

The IR range is potentially the most useful as it detects thermal emissions, such as from engine exhaust, thermal radiators, even the interior temperature of a vessel radiating out into space.

Radio Range

Emissions are most likely caused by a vessel’s communications or sensor systems.

Magnetic Range

This isn’t part of the EM spectrum but strong magnetic fields (for example from impulse engines) can be detected by the EMDAR system.

Detection Capabilities

EMDAR uses broadband detection techniques which estimate the range of an emission source by analysing EM dissipation patterns, but are less useful for identifying bearings (direction). As such, a single EMDAR array cannot estimate the speed or course of an object.

Multiple EMDAR arrays with different directional orientations are used to provide additional data for estimating speed and course. The vessel has a forward-facing array and a pair of lateral arrays (port and starboard). Exhaust from the impulse engines prevents the use of an aft array, but a remote array can be launched to provide EMDAR coverage astern.

By comparing the behaviour of an object across multiple EMDAR arrays, conclusions can be drawn on and object’s course and speed.

EMDAR Interface

Example of an EMDAR interface (click for larger). Detected emissions first appear on left or top with time elapsing to the right or downwards (depending on orientation of the array display).Each EMDAR array presents detection data as a grid defined by range and time axes. A contact is represented as a dot, its position indicating estimated range and the time since detection. The colour of the dot represents the EM range of the detected emission and the dot’s size represents the intensity of the emission.

This arrangement of assessing range over time allows patterns to form, typically as a line (or trail) across the display. These patterns can provide hints as to course and speed.


By way of example, sample EMDAR displays are shown in the picture to the right.

This example is simplified in that it shows only static contacts and the vessel itself is not moving. Were either in motion, the waterfall trails would appear to "drift" in the direction of travel. For example, if a contact was getting closer to the vessel, the top (or left side) of the trail representing the most recent detections would have a closer range than the lower (or right) end of the trail which contains less recent detections. When trail drifts are considered across mutiple arrays, an experienced EMDAR operator is able to discern a great deal about a TSMO's relative course and maneuvering.


EMDAR contact board example (click for larger).The crew monitoring EMDAR can declare a possible contact if they believe a track represents a TSMO, passing the coordinates to the EMDAR supervisor who will check the contact. If the supervisor agrees, they will pass the possible contact and it's estimated location to the bridge along with whatever additional information they can provide such as heading, range, maneuvering patterns, etc.

The commanding officer will then choose a designation for the contact from these options:

Simple Designation (Sierra): Not enough is known about the origin, affiliation or configuration of the TSMO to draw a conclusion as to intent or likely posture towards the vessel. This is used to designate a possible contact.

Unaligned (Ultra): The TSMO is thought to represent a neutral affiliation and/or is not displaying any hostile intent.

Threat (Tango): The TSMO is thought to represent a hostile affiliation and/or is displaying hostile intent or aggressive posture.

Friendly (Foxtrot): The TSMO is thought to represent a friendly affiliation and is displaying intent or posture consistent with this.

The contact designation may be changed at any time by the commanding officer. The designation type is also accompanied by a unique identifying number.

For example ”…designate the contact TANGO-ZERO-TWO” would be the order given to designate a second contact on the board, this one thought to have hostile intent.

A designated contact will appear on the Contact Board and the system will automatically track the identified location in space relative to the vessel's own movement. At this stage the system does not have sufficient information to track the object that is emitting the EM, so the operator may need to update the contacts estimated location based on it's EM trails.

Tracked Contacts

Once the operator is confident they have identified the location of the TSMO that is the source of the contact, an additional set of passive sensors which use narrowband detection techniques can be directed to that location to attempt a track.

Narrowband sensors have improved detection capabilities but are limited to scanning within a very limited arc of space and so must be aligned very closely to a TSMO to acquire it. Once acquired, the narrowband array is capable of automtically tracking a TSMO while it remains detectable and the TSMO will be displayed on the Tactical Operating Environment (TOE) grid. The computational and hardware resources required to track a TSMO limit the number of narrowband arrays that can be used simultaneously.