Sensors

This chapter covers the theory of operation for sensor systems integrated into the DRONECOM tactical display. All contact data presented on the map display — tracks, threat warnings, classifications — originates from the sensor suite. Operators must understand sensor capabilities, limitations, and the tradeoffs involved in emission management to effectively employ the system.

How Detection Works

Every sensor follows the same fundamental process:

1. Energy propagates — either emitted by the sensor (active) or by the target (passive)
2. Signal attenuates with distance — strength decreases as a function of range
3. Detection occurs when the received signal exceeds the sensor’s sensitivity threshold

The critical distinction is between active and passive sensors:

• Passive detection of natural emissions (IR, visual, passive sonar hearing engine noise) — the sensor listens for energy the target produces naturally: thermal signatures, visible light, machinery vibration. Signal strength falls off with the square of distance (1/R²). This is the shortest-range mode but produces no emissions from the receiving platform.

• Active detection (radar, active sonar) — the sensor emits its own signal and listens for the return echo. The signal makes a round trip, so strength falls off with the fourth power of distance (1/R⁴). Longer range than passive detection of natural emissions, but the transmission itself is detectable by hostile platforms.

• Passive detection of active emissions (RWR hearing a hostile radar, passive sonar hearing a hostile sonar ping) — the sensor detects the powerful transmission of a hostile active sensor. The signal is strong and travels only the one-way path (1/R²), giving this mode the longest detection range of the three. A warning receiver will detect a hostile radar at significantly greater range than that radar can detect a return echo. This asymmetry is the foundation of the active-vs-passive tradeoff covered in the next section.

Active vs Passive — The Core Tradeoff

The central operational tension is situational awareness vs emission security.

When radar is active, detection ranges exceed those of any passive sensor. However, every hostile platform equipped with a Radar Warning Receiver (RWR) will detect those emissions — and because RWR operates on the one-way signal path (1/R²), an RWR can detect a radar transmission from significantly greater range than that radar can detect a return echo.

Operating in passive mode eliminates RWR exposure but limits detection to shorter-range passive sensors — IR, visual, and passive sonar. Coverage is reduced accordingly.

Every engagement involves this decision: activate sensors to establish the tactical picture, or maintain emission security and rely on passive detection.

The Electromagnetic Domain

Four sensor types operate in the electromagnetic spectrum — above the water surface.

Radar — Active. Emits radio energy, detects returns. Provides the longest detection ranges available. Subject to Doppler notching and ground clutter effects (see Radar Effects and Doppler Processing). At sufficient signal strength, radar can resolve contact affiliation via IFF (Identification Friend or Foe).

RWR (Radar Warning Receiver) — Passive. Detects hostile radar emissions, providing bearing to the transmitting platform. As a passive sensor, RWR is always operational — EMCON settings have no effect on it.

IR (Infrared) — Passive. Detects thermal emissions — engine heat, exhaust plumes. Shorter range than radar but produces no emissions. Provides detection without revealing the receiving platform’s position.

Visual — Passive. Detects visible-spectrum signatures. The shortest-range electromagnetic sensor. Produces no emissions.

flowchart LR
    R["Own Radar\n(Active)"]:::friendly -- "Emission 1/R²" --> RWR["Hostile RWR\n(Passive)"]:::hostile
    R -. "Echo return 1/R⁴" .-> T["Hostile\nPlatform"]:::hostile
    T -- "Thermal 1/R²" --> I["Own IR\n(Passive)"]:::friendly
    T -- "Visible 1/R²" --> V["Own Visual\n(Passive)"]:::friendly

    classDef friendly fill:#0a2e1a,stroke:#22c55e,color:#22c55e
    classDef hostile fill:#2e0a0a,stroke:#ef4444,color:#ef4444

Note: Radar emissions detected by a hostile RWR travel the one-way path only — the RWR detects at greater range than the radar can detect a return echo. IR and visual sensors operate independently, detecting the target’s own thermal and visible signatures without producing emissions.

The Acoustic Domain

Two sensor types operate underwater. Electromagnetic sensors cannot penetrate the water surface, so subsurface platforms rely entirely on acoustic detection.

Active Sonar — Emits acoustic pings and listens for echoes. Fourth-power signal falloff, same as radar. The ping itself is detectable by hostile passive sonar — the same tradeoff as radar and RWR, applied to the subsurface domain.

Passive Sonar — Listens for engine noise, machinery vibration, and active sonar pings from other platforms. Square-law falloff. Produces no emissions — the subsurface equivalent of passive electromagnetic operation.

Underwater propagation is further shaped by ocean conditions. Temperature layers, water depth, and convergence zones all modify effective detection ranges beyond the basic signal falloff — a platform operating at optimal depth in favorable conditions may hold contacts that are invisible to one operating identically at the wrong depth. See Acoustic Effects for detail.

Medium Boundaries

The water surface is an absolute boundary for sensor propagation:

• Electromagnetic sensors (radar, RWR, IR, visual) cannot propagate through water. A submerged platform is undetectable by radar regardless of range.
• Acoustic sensors (active and passive sonar) cannot propagate through air. An airborne platform is undetectable by sonar.

This creates two distinct operational domains. A submarine operating silently below the surface exists in a separate detection environment from the air picture above. Maintaining awareness across both domains requires assets in each.

Reading Your Contacts

When sensors detect a target, it appears on the tactical display with the following data:

Track Code — A unique identifier assigned on initial detection (e.g., “T-001”). The track code persists even if the contact is temporarily lost and re-acquired — it is the contact’s permanent identity for the duration of the engagement.

NTDS Class — Platform classification based on sensor data and signature analysis. Examples: Air , Surface , Subsurface , Command Ship , Missile , Torpedo . See Symbology for the full reference.

Affiliation — At long range, contacts are classified as Unknown . As signal strength increases within IFF identification range, affiliation resolves to Friendly , Hostile , or Neutral . Allied platforms share contact data via datalink — if an allied sensor identifies a contact, that identification propagates to your display.

Detection Mode — Indicates how the contact was most recently detected:

• Active return — your active sensor (radar or sonar) illuminated the target and received an echo
• Active emission — you detected the target’s own active sensor emissions (their radar or sonar)
• Passive — you detected the target’s natural emissions (thermal, visible, acoustic)

Contact Lifecycle

Contacts progress through three states:

• Active — Currently held by at least one sensor. Position updates continuously.
• Lost — All sensors have lost the contact. Last known position is displayed, decaying over time.
• Expired — The contact has been lost beyond the stale timeout and is removed from the display.

A lost contact can be re-acquired if any sensor regains detection before expiry.

stateDiagram-v2
    direction LR
    [*] --> Active : New detection
    Active --> Lost : All sensors lose contact
    Lost --> Active : Re-acquired
    Lost --> Expired : Stale timeout
    Expired --> [*]

    classDef success fill:#0a2e1a,stroke:#22c55e,color:#22c55e
    classDef warning fill:#2e2a0a,stroke:#eab308,color:#eab308
    classDef error fill:#2e0a0a,stroke:#ef4444,color:#ef4444

    class Active success
    class Lost warning
    class Expired error

EMCON: Emission Control

EMCON (Emission Control) is the primary tool for managing the active-vs-passive tradeoff at the platform level.

Active — All active sensors enabled. Maximum detection capability. The platform is emitting and detectable by hostile passive sensors.

Passive — All active sensors silenced. Detection limited to passive sensors only. The platform produces no sensor emissions.

Per-sensor toggle — Fine-grained control: radar can be silenced while active sonar remains operational, or vice versa. Only active sensors can be toggled — passive sensors (RWR, passive sonar, IR, visual) are always operational.

EMCON is set per-platform. Setting the carrier to Passive disables its radar but has no effect on embarked or deployed assets — each platform manages its own emission state independently.

The Horizon

Earth’s curvature limits the effective range of electromagnetic sensors. A sensor can only detect targets above its geometric horizon — beyond that distance, the curvature of the earth blocks the line of sight.

All EM sensors — radar, RWR, IR, and visual — are subject to horizon limitations. Sonar propagation follows different physical principles and is not horizon-limited.

Three factors determine horizon range:

Sensor altitude — Higher altitude extends the horizon. An airborne sensor platform can detect beyond the horizon that limits a surface-mounted radar. This is a primary motivation for deploying airborne surveillance assets — they extend the detection horizon significantly.

Mast height — Surface platform sensors are mounted on masts above the waterline. Greater mast height extends the sensor horizon. The AN/SPY-310 ship radar has a 15 m mast height, giving it a horizon of approximately 9 NM against a sea-level target.

Target altitude — Horizon range depends on both the sensor height and the target height. Two high-altitude platforms can maintain mutual detection at ranges far exceeding what a surface platform achieves against a sea-skimming target.

Approximate horizon ranges for representative altitudes:

Radar Effects

Two effects modify radar detection performance beyond the basic signal falloff. Understanding these effects is essential for effective sensor employment and tactical positioning.

Low-Altitude Clutter

Targets operating at low altitude are more difficult to detect — radar returns from the terrain or sea surface below contaminate the target echo. Below a ceiling altitude, detection capability is progressively degraded. Above the ceiling, the target is in clear air.

Sea-skimming missiles and low-altitude platforms exploit low-altitude clutter to reduce their radar detectability. Detection may not occur until the target has closed to short range.

Look-Down Clutter

Look-down clutter is similar to low-altitude clutter, but instead of a fixed ceiling altitude, the clutter region is defined by the earth’s horizon. When radar looks below the geometric horizon — the point where the line of sight meets the curvature of the earth — terrain returns compete with target returns. Attenuation increases linearly with the depression angle below the horizon, reaching maximum degradation (approximately 20 dB for the AN/SPY-310) at 10° below the horizon. Beyond that angle, attenuation remains at maximum.

Higher-altitude sensor platforms have their geometric horizon further below horizontal, providing more clear sky before clutter effects begin. This partially offsets clutter degradation for elevated sensor positions.

Acoustic Effects

Three environmental factors modify sonar detection performance and will vary across operating areas.

Thermocline

A thermocline is a sharp temperature gradient at depth — warmer, lighter water above a boundary layer, colder and denser water below. The acoustic velocity gradient at this boundary refracts sound rays: energy propagating at shallow angles bends away from the layer and stays within the upper water column, while steeper angles cross through.

This refraction creates a shadow zone below the thermocline. A platform operating above the layer and searching for a contact below it — or vice versa — faces significant path loss beyond the basic geometric falloff. The signal must cross the boundary twice to return as an active echo, so active sonar is doubly penalized: signal strength degrades on both outbound and inbound legs.

The geometry is asymmetric. A deep platform has steeper angles to the thermocline at any given horizontal range, allowing it to maintain cross-layer paths where a shallow platform at the same range cannot. In practice, a deep submarine may hold a contact on the far side of the layer that a surface ship’s sonar cannot detect at all.

The principal tactical response is depth management. A platform operating below the thermocline is largely concealed from surface sonar — the layer acts as an acoustic screen. Conversely, a platform searching for deep targets should consider operating at depth to close the angular disadvantage.

Shallow Water Attenuation

In open ocean, acoustic energy propagates with relatively little boundary interaction. In shallow water, the sound channel is bounded above by the surface and below by the seabed, and every reflection at either boundary incurs loss. As depth decreases, the channel height decreases and reflection frequency increases — the signal encounters more boundaries per unit range, accumulating more loss per nautical mile.

When a thermocline is present, the effective channel for below-layer propagation narrows further: the acoustic energy is confined to the region between the thermocline and the seabed rather than the full water column. This narrower channel produces higher attenuation rates than the same depth without a layer.

The practical consequence is that shallow water reduces detection ranges across the board. Platforms operating in deep water benefit from longer channels with fewer reflections; the same platform transiting into a shallower operating area will hold contacts at progressively shorter ranges.

Convergence Zones

In deep water, the temperature and pressure structure of the water column creates a sound velocity minimum at mid-depth — the SOFAR channel. Acoustic energy refracted downward below this minimum is bent back upward by increasing pressure, and refracted upward above it by increasing temperature. Both ray families curve back toward the minimum depth, and when they refocus at the surface they form convergence zones: annular rings of enhanced detection at predictable stand-off ranges, typically 20–35 NM from the source depending on local conditions.

The zones repeat at approximately equal intervals as successive ray families refocus. Detection within a convergence zone can exceed what range-geometry alone would predict — the signal arrives having propagated through the low-loss deep channel rather than suffering shallow-water boundary reflections.

Several conditions limit this effect. Both the transmitting and receiving platforms must be above the thermocline for surface-refracted convergence paths to function; a below-layer platform does not contribute to or benefit from convergence zone propagation. In littoral waters with depths around 1,600 ft or less, the water column is too shallow to support full channel development — convergence effects are present but modest, producing a marginal detection edge rather than the dramatic extended ranges achievable in deep ocean. The effect is most operationally significant when maneuvering into or out of known zone geometries in deep water.

Doppler Processing

Both radar and active sonar use Doppler shift — the frequency change caused by relative motion between the sensor and the target — to separate moving target returns from stationary or slow-moving background returns. A target with significant radial velocity relative to the sensor produces a clear frequency offset and is readily discriminated. A target with near-zero radial velocity relative to the sensor blends into the background.

The underlying principle is identical across domains, but the background environment differs. Radar contends with ground clutter; active sonar contends with reverberation. In both cases, Doppler discrimination is the primary mechanism for extracting target returns from the noise floor.

Radar

Pulse-Doppler radar separates target returns from ground clutter by frequency. A target with high radial velocity toward or away from the radar produces a large Doppler shift and stands out clearly. A target maneuvering to minimize its radial velocity relative to the radar — a technique known as “notching” — causes its return to fall within the clutter rejection filter, where it is indistinguishable from terrain returns.

Aircraft can exploit notching by flying perpendicular to the radar’s line of sight during critical phases of an approach. The effect is transient — the relative geometry between the platform and the radar changes continuously, so sustained notching requires continuous maneuvering to maintain the perpendicular aspect.

Active Sonar

Active sonar pings produce returns from the seabed, sea surface, and volume scatterers throughout the water column — collectively termed reverberation. Doppler discrimination separates moving target echoes from this reverberation background. A target with meaningful closing speed produces an echo offset in frequency from the reverberation, making it detectable. A target with low radial velocity produces an echo at nearly the same frequency as the reverberation returns, rendering detection reverberation-limited.

Unlike radar notching, reverberation limiting is less dependent on deliberate target maneuvering. Any geometry that produces low relative radial velocity — including a target on a parallel course at similar speed — degrades active sonar discrimination.

Sensor Reference

For complete sensor specifications, see the Platform Reference.