Propagation, Electronic Warfare and Spectrum Coexistence

Introduction

Radar performance is determined not only by transmitted power, aperture and signal processing, but also by the electromagnetic environment through which signals propagate. Atmospheric gases, precipitation, refractive structure, terrain and multipath can alter coverage, measurement accuracy and detection probability. At the same time, deliberate electronic attack and unintentional interference may conceal targets, create false information or saturate a receiver.

Modern radar engineering therefore requires an integrated treatment of propagation, electronic warfare, electromagnetic compatibility and spectrum governance. A radar that performs well in free-space simulation may fail when deployed near a coastline, inside an anomalous propagation duct, close to communication transmitters or against a coherent digital jammer. Conversely, suitable propagation modelling, electronic protection and coordinated spectrum access can substantially improve operational effectiveness without increasing transmitter power.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Calculate atmospheric and precipitation attenuation on one-way and two-way radar paths.

2. Explain standard refraction, sub-refraction, super-refraction and ducting.

3. Assess multipath, diffraction, terrain masking and clutter effects.

4. Derive and apply jammer-to-signal ratio and burn-through range expressions under stated assumptions.

5. Distinguish noise jamming, coherent deception and expendable and non-expendable countermeasures.

6. Select electronic protection measures against representative threats.

7. Explain low-probability-of-intercept radar principles and limitations.

8. Analyze electromagnetic compatibility, aggregate interference and spectrum-sharing measures.

1. Propagation Loss Beyond Free Space

For a monostatic radar in free space, the received echo power is

S = PₜGₜGᵣλ²σ / [(4π)³R⁴Lₛ],

where Pₜ is transmitted power, Gₜ and Gᵣ are transmit and receive antenna gains toward the target, λ is wavelength, σ is radar cross-section, R is target range and Lₛ ≥ 1 represents system and propagation losses not already included elsewhere. Peak or average quantities must be used consistently.

The R⁻⁴ dependence results from outward spreading to the target and return spreading to the radar. Atmospheric, terrain and interference effects modify this ideal result and may be time-varying or spatially nonuniform.

1.1 Gaseous absorption

Oxygen and water vapour absorb electromagnetic energy around molecular resonance frequencies. Specific gaseous attenuation γg is usually expressed in dB/km and depends on frequency, atmospheric pressure, temperature and water-vapour density. Oxygen absorption is especially prominent near 60 GHz, while water-vapour absorption is strong near 22 and 183 GHz.

For a path of length R,

A_g = ∫₀ᴿ γg(r) dr dB.

If γg is approximately uniform, A_g = γgR. A monostatic radar traverses the path twice, so the two-way attenuation is 2A_g. Standardized line-by-line or approximate models, such as those maintained by the ITU-R, should be used for high-frequency system design.

1.2 Rain, cloud and fog attenuation

Rain attenuation becomes important at microwave frequencies, particularly above approximately 10 GHz. A common model is

γR = kRr^α dB/km,

where Rr is rain rate in mm/h and k and α depend on frequency, polarization and temperature. The symbol Rr denotes rain rate and must not be confused with range. Because rain cells are nonuniform, attenuation should be integrated over the portion of the path occupied by rain.

Cloud and fog attenuation arises mainly from suspended liquid water. It is normally modelled from liquid-water density and a frequency-dependent specific attenuation coefficient. Wet radomes can introduce additional loss and antenna-pattern distortion that may exceed clear-air atmospheric loss.

Snow attenuation is generally lower than rain attenuation for the same precipitation rate, although wet snow and melting-layer particles can produce strong scattering and enhanced reflectivity.

Worked Example 1: Two-way atmospheric attenuation

A 35 GHz radar observes a target at 20 km. Clear-air specific attenuation is 0.08 dB/km. A 5 km segment contains rain producing 4 dB/km. Assuming the same medium on the outward and return paths:

Clear-air contribution = 2(20)(0.08) = 3.2 dB.

Rain contribution = 2(5)(4) = 40 dB.

Total two-way attenuation = 43.2 dB.

The received echo is therefore reduced by a factor of 10^(43.2/10), approximately 2.09 × 10⁴, relative to a calculation that omits these losses. This illustrates why millimetre-wave radar availability must be designed statistically rather than from clear-air conditions alone.

2. Atmospheric Refraction and Ducting

2.1 Refractivity

Atmospheric refractive index n is close to unity, so radio refractivity is defined as

N = (n − 1) × 10⁶.

A commonly used approximation for moist air is

N = 77.6P/T + 3.73 × 10⁵e/T²,

where P is total atmospheric pressure in hPa, e is water-vapour pressure in hPa and T is absolute temperature in kelvin. This empirical expression applies within the atmospheric conditions for which it was developed.

Because pressure and humidity usually decrease with altitude, N normally decreases with height. The downward bending of radio rays extends the radar horizon relative to the geometric horizon.

2.2 Effective-Earth-radius model

For an approximately constant refractivity gradient dN/dh, with h in kilometres, the effective-Earth-radius factor is approximated by

kₑ = 157 / [157 + dN/dh].

The effective radius is kₑa, where a is the true Earth radius. The approximation assumes horizontally stratified refractivity and moderate path lengths.

Under the standard gradient dN/dh ≈ −39 N-units/km, kₑ ≈ 4/3. A more-negative gradient causes stronger downward bending and super-refraction. A less-negative or positive gradient causes weaker downward bending or upward bending and therefore sub-refraction. It would be incorrect to describe a more-negative gradient as sub-refractive.

For antenna heights h₁ and h₂,

d ≈ √(2kₑah₁) + √(2kₑah₂),

provided heights and Earth radius use consistent units. Under standard refraction, with heights in metres and distance in kilometres, this is often approximated as

d ≈ 4.12(√h₁ + √h₂) km.

Worked Example 2: Classification of refractive conditions

For dN/dh = −80 N-units/km,

kₑ = 157/(157 − 80) = 2.04.

The gradient is more negative than standard, so propagation is super-refractive and the radio horizon increases.

For dN/dh = +20 N-units/km,

kₑ = 157/(157 + 20) = 0.887.

This is sub-refraction, producing a shorter radio horizon and potentially unexpected low-altitude coverage gaps.

2.3 Ducting

Modified refractivity is defined by

M = N + 157h,

where h is measured in kilometres. A layer for which dM/dh < 0, equivalently dN/dh < −157 N-units/km, can trap rays and form a radio duct. Depending on its height and structure, a duct may be surface-based, elevated or associated with rapid humidity changes near an evaporation layer.

Ducting can extend detection far beyond the normal horizon. It can also create blind zones beneath elevated ducts, increase sea clutter and carry interference from distant transmitters. Duct existence alone does not guarantee trapping: antenna height, frequency, duct thickness, launch angle and polarization affect coupling into supported propagation modes.

Operational prediction uses radiosondes, numerical weather prediction, refractivity profilers, sea-surface meteorology and propagation solvers. The effective-Earth approximation is inadequate for strongly varying or layered refractivity; parabolic-equation or ray-tracing models are then preferred.

3. Multipath, Terrain and Surface Effects

3.1 Multipath propagation

Multipath occurs when direct and reflected or diffracted components arrive through different paths. For a direct field E₀ and one reflected field,

E = E₀[1 + Γexp(−jΔφ)],

where Γ is the complex reflection coefficient and Δφ includes the additional path phase. Received power is proportional to |E|². Depending on phase, multipath can produce enhancement or deep fading.

Over smooth ground or sea, a two-ray model may be useful. Roughness, surface curvature, polarization and grazing angle determine the reflection coefficient. At very low grazing angles, small errors in surface height or atmospheric profile can shift interference nulls significantly.

Multipath consequences include:

• range and angle bias;
• false elevation tracks;
• fading and intermittent detection;
• increased sea or ground clutter;
• target-image ambiguity in low-angle tracking.

Mitigation techniques include frequency diversity, spatial diversity, monopulse processing designed for low-angle multipath, elevation-pattern control, polarization selection, track filtering and terrain-aware siting.

3.2 Terrain masking and diffraction

Hills, buildings and vegetation block or scatter radar energy. A line-of-sight test alone is insufficient because diffraction can introduce measurable energy into a geometric shadow.

For an isolated knife edge, the normalized diffraction parameter is

ν = h√[2(d₁ + d₂)/(λd₁d₂)],

where h is the obstacle height above the straight line joining transmitter and receiver, and d₁ and d₂ are distances from the obstacle to the two endpoints. Distances must be expressed consistently.

A widely used approximation to excess knife-edge loss for ν > −0.78 is

L_d = 6.9 + 20log₁₀{√[(ν − 0.1)² + 1] + ν − 0.1} dB.

For ν ≤ −0.78, the additional loss is approximately zero. At ν = 0, the obstacle lies on the direct line and the loss is approximately 6 dB. Multiple ridges require multi-edge diffraction methods or numerical terrain propagation tools.

Terrain also creates clutter with spatially varying statistics. Urban structures may generate strong specular returns and ghost tracks, while wind turbines create Doppler-modulated clutter. Digital elevation models, land-cover databases and site measurements should therefore form part of radar coverage design.

4. Electronic Warfare Framework

Electronic warfare involving radar is commonly divided into three interacting functions:

• Electronic support: detection, interception, identification, geolocation and analysis of electromagnetic emissions.
• Electronic attack: use of electromagnetic energy, decoys or anti-radiation effects to degrade or mislead an adversary.
• Electronic protection: measures that preserve friendly use of the spectrum despite electronic attack or interference.

The terminology does not imply that every interference source is hostile. A radar receiver may experience almost identical symptoms from a jammer, a communication transmitter, an oscillator fault or intermodulation in its own front end. Threat attribution requires waveform, direction-of-arrival and contextual evidence.

5. Noise Jamming and Jammer-to-Signal Ratio

Noise jamming raises the effective receiver noise or masks echoes. It may be spot, barrage, swept or responsive. Spot jamming concentrates power in a narrow band; barrage jamming covers a broad band at lower spectral density; swept jamming moves through frequency; responsive jamming transmits after detecting the radar.

For a self-screening jammer collocated with a target, the jammer power entering the radar receiver is

J = αPⱼGⱼGᵣλ² / [(4π)²R²Lⱼ],

where Pⱼ is jammer transmitted power, Gⱼ is jammer gain toward the radar, Gᵣ is radar receive gain toward the jammer, Lⱼ is jammer-path loss and α is the fraction of jammer power admitted by the receiver and processing bandwidth.

For uniform noise fully overlapping a rectangular receiver passband, α may be approximated by min(1, Bᵣ/Bⱼ), where Bᵣ is effective receiver bandwidth and Bⱼ is jammer bandwidth. This approximation is not valid for nonuniform spectra, partial overlap or processing that weights frequency nonuniformly.

Dividing by the monostatic echo power gives the general same-range result

J/S = [4π αPⱼGⱼGᵣR²Lₛ] / [PₜGₜGᵣ,ₛσLⱼ].

Here Gᵣ and Gᵣ,ₛ distinguish receive gain toward the jammer and target. If the jammer is collocated with the target and both are in the same main-beam direction, the receive gains cancel:

J/S = 4παPⱼGⱼR²Lₛ / (PₜGₜσLⱼ).

There is no wavelength-squared term in this special-case ratio: the λ² factors in the radar and jammer received-power equations cancel. The formula must not be generalized to stand-off jammers with different ranges or antenna directions without retaining the corresponding geometry and receive gains.

Worked Example 3: Self-screening jammer

A radar has Pₜ = 1 MW and Gₜ = 30 dB = 1000. A target with σ = 1 m² carries a jammer having Pⱼ = 4 kW and Gⱼ = 10 dB = 10. The range is 100 km, α = 1 and Lₛ/Lⱼ = 1.

J/S = [4π(4000)(10)(100,000)²] / [(10⁶)(1000)(1)]

= 5.03 × 10⁶.

Therefore,

J/S in decibels = 10log₁₀(5.03 × 10⁶) ≈ 67.0 dB.

The result is intentionally based on ideal main-beam alignment and complete spectral overlap. Polarization mismatch, antenna pointing, processing gain and nonuniform bandwidth occupancy would alter the effective ratio.

5.1 Burn-through range

Burn-through occurs when decreasing range makes the target echo sufficiently strong relative to self-screening jamming. If acceptable operation requires J/S ≤ γ, the boundary range under the preceding assumptions is

R_BT = √[γPₜGₜσLⱼ/(4παPⱼGⱼLₛ)].

For R < R_BT, J/S is below the specified threshold. This expression applies to a self-screening jammer and should not be used unchanged for a stand-off jammer.

As a separate example, let Pₜ = 1 MW, Gₜ = 40 dB = 10⁴, σ = 10 m², Pⱼ = 100 W, Gⱼ = 1, α = 1, equal losses and γ = 1. Then

R_BT = √[(10⁶ × 10⁴ × 10)/(4π × 100)] ≈ 8.92 km.

Quadrupling jammer power would halve the burn-through range because R_BT is proportional to 1/√Pⱼ.

6. Deception and Coherent Electronic Attack

Deception jamming attempts to alter radar information rather than merely obscure it. A digital radio-frequency memory, or DRFM, can sample an intercepted waveform, apply delay, Doppler, phase or amplitude changes and retransmit a coherent replica.

Representative techniques include:

• Range-gate pull-off: a false echo initially captures the tracking gate and is gradually delayed away from the true target.
• Velocity-gate pull-off: controlled Doppler shifts draw a velocity tracker away from the true echo.
• False-target generation: multiple delayed and Doppler-shifted replicas simulate additional targets.
• Angle deception: amplitude or phase manipulation exploits scanning, monopulse or interferometric angle estimation.
• Repeater jamming: an intercepted waveform is amplified and retransmitted to exploit radar processing gain.

Coherent jammers can match pulse compression and therefore enter the same range-Doppler cells as genuine echoes. Their effectiveness is limited by interception latency, dynamic range, waveform knowledge, spatial separation and the radar’s ability to compare measurements across frequency, aperture, polarization or multiple sensors.

7. Expendable and Non-Expendable Countermeasures

Countermeasures may be classified by whether the deployed resource is consumed during the engagement.

7.1 Expendable countermeasures

Expendables are released or consumed and normally cannot be recovered. Important radar-related examples include:

• Chaff: clouds of conductive fibres or strips designed to resonate over selected radar bands. Chaff creates strong distributed returns, obscures targets and can form false range-angle structures. Its motion subsequently follows wind and gravity.
• Expendable active decoys: compact transmitters ejected from aircraft, ships or vehicles. They may repeat or synthesize radar signals to create an attractive off-board target.
• Corner-reflector or inflatable decoys: devices presenting deliberately enhanced radar cross-section.
• One-shot obscurants and cartridges: deployed to modify the electromagnetic scene for a limited period.

Expendables provide spatial separation from the protected platform and can seduce a tracker away from it. Their disadvantages are finite inventory, short endurance, environmental sensitivity, logistics and possible debris or unexploded-device hazards. Deployment timing is crucial: a decoy must become more attractive than the protected platform without revealing its tactical intent prematurely.

7.2 Non-expendable countermeasures

Non-expendable systems remain installed, attached or recoverable and can normally be reused. They include:

• onboard noise and DRFM jammers;
• electronic support receivers and threat-warning systems;
• active electronically steered arrays used for directional electronic attack;
• recoverable unmanned decoys;
• towed decoys that remain attached to the platform during normal operation;
• radar-absorbing structures and signature-control measures.

These systems offer repeated use, adaptive control and greater available processing or power. Their disadvantages include cost, platform integration, power and cooling demand, antenna-placement constraints and the risk that emissions reveal the platform.

A layered countermeasure plan often combines onboard non-expendable sensing and jamming with expendable off-board decoys. Selection should consider threat seeker type, engagement geometry, inventory, collateral spectrum effects and rules of engagement.

8. Electronic Protection

Electronic protection is most effective when designed into the radar rather than added after deployment. Major techniques include:

1. Frequency agility and wide tuning: reduce vulnerability to narrowband or predictive jamming.

2. Waveform diversity: vary pulse width, modulation, pulse repetition interval and coding while preserving coherent processing.

3. Low antenna sidelobes: reduce reception of off-axis interference.

4. Sidelobe blanking and cancellation: use auxiliary channels to detect or estimate sidelobe interference.

5. Adaptive beamforming: place spatial nulls toward interference sources, subject to calibration and degrees-of-freedom constraints.

6. Polarization diversity: exploit differences between target, clutter and jammer polarization.

7. Adaptive filtering: suppress interference in time, frequency, Doppler or space-time domains.

8. Track-level validation: test kinematic consistency, acceleration, amplitude history and multisensor correlation.

9. Home-on-jam and passive angle estimation: exploit jammer emissions where operationally appropriate.

10. Receiver protection: limiters, filtering and high-dynamic-range design prevent damage, saturation and intermodulation.

Adaptive systems introduce risks. Training-data contamination can cause cancellation of genuine targets, while a jammer may deliberately manipulate adaptation. Machine-learning classifiers can assist waveform recognition and anomaly detection, but they require representative data, uncertainty estimates and protection against adversarial inputs. Safety-critical decisions should not depend solely on opaque model outputs.

9. Low-Probability-of-Intercept Radar

Low-probability-of-intercept, or LPI, design seeks to perform the radar mission while reducing the probability that an intercept receiver detects, identifies or geolocates the emission. It does not imply invisibility.

Common LPI measures include:

• low peak power with long coherent integration;
• wideband spread-spectrum or noise-like modulation;
• frequency agility and irregular timing;
• narrow, low-sidelobe transmit beams;
• adaptive power control;
• short transmission schedules and emission control;
• distributed or multistatic architectures;
• waveforms with low autocorrelation sidelobes and low spectral features.

Radar detection and interception are an asymmetric problem. The radar benefits from knowledge of its own waveform and expected target delay-Doppler region. An intercept receiver may compensate through wide instantaneous bandwidth, sensitive channelization, long observation and cooperative sensor networks. Increasing bandwidth reduces jammer spectral density and may conceal narrow spectral features, but it also increases receiver noise bandwidth and spectrum occupancy. LPI design is therefore a joint optimization of energy, processing gain, aperture, mission time and detectability.

10. Electromagnetic Compatibility

Electromagnetic compatibility, or EMC, is the ability of equipment to function acceptably in its electromagnetic environment without creating intolerable disturbance for other systems. It includes emissions, susceptibility, coupling and coexistence at equipment, platform and network levels.

Interference paths include antenna coupling, cable conduction, enclosure leakage, harmonics, transmitter noise, receiver local-oscillator radiation, intermodulation and nonlinear front-end overload. Out-of-band signals can be damaging even when the desired-channel filter is selective, because saturation may occur before filtering.

For independent interference sources, average interference powers are real quantities and must be summed in linear units:

I_total = Σ Iᵢ.

They must not be directly added in dB. Each dBm value should first be converted to milliwatts, summed, and converted back using 10log₁₀. If signals are mutually coherent with stable relative phases, electric fields rather than average powers may need to be combined; this is a distinct special case.

An interference-to-noise ratio is

INR = I/(kTₛB),

where I is interference power at the relevant detector input, k is Boltzmann’s constant, Tₛ is system noise temperature and B is effective noise bandwidth. The acceptable INR depends on the permitted degradation in detection, false-alarm rate or measurement accuracy.

EMC engineering practices include frequency planning, shielding, grounding, cable segregation, filtering, antenna isolation, linearity analysis, time-domain coordination and testing under representative simultaneous-operation conditions.

11. Spectrum Sharing and Regulation

Radar bands increasingly overlap or border satellite, mobile, fixed-link, navigation and scientific services. Coexistence is not achieved solely by meeting transmitter emission masks; receiver blocking, aggregate interference and anomalous propagation must also be considered.

Sharing mechanisms include:

• geographic exclusion or coordination zones;
• frequency separation and dynamic channel selection;
• time sharing and synchronized quiet periods;
• power-flux-density and equivalent isotropically radiated power limits;
• antenna pointing and elevation restrictions;
• database-assisted access;
• sensing-based dynamic spectrum access;
• waveform and receiver-filter coordination.

Aggregate interference assessment must account for the spatial distribution, duty cycle, antenna orientation and propagation of many emitters. Monte Carlo simulation is often more informative than a single worst-case link, but rare ducting events may dominate high-consequence outages and must not be averaged away.

National regulators implement frequency allocations consistent with international frameworks such as the ITU Radio Regulations. Military and civil radars may operate under different authorization processes, but neither is exempt from responsible interference management. Logs, calibrated measurements, configuration control and cross-organizational incident procedures are essential.

12. Integrated Illustrative Case: Coastal Surveillance Radar

Background

This fictional case concerns an S-band coastal surveillance radar located 40 m above sea level. The system must detect small vessels while sharing the region with fixed wireless links. A hostile vessel may carry a coherent self-screening jammer.

Problem

Operators report three intermittent effects: long-range false plots during calm evenings, loss of low vessels on hot afternoons and receiver desensitization when nearby communication sites are active. A DRFM jammer also produces plausible delayed targets during exercises.

Analysis

Meteorological profiles show that calm evening conditions frequently create a surface duct. Energy coupled into the duct produces beyond-horizon sea clutter and allows distant communication emissions to reach the radar. Hot afternoon profiles are less negative than the standard refractivity gradient, producing sub-refraction and a reduced low-altitude horizon.

Spectrum measurements reveal that several communication carriers are outside the nominal radar channel but collectively overload an early receiver amplifier. This is a front-end linearity problem rather than purely an in-band filtering problem. Independent carrier powers are converted to watts and summed before calculating aggregate INR.

The deceptive tracks preserve the radar waveform but show inconsistent angle estimates between the main aperture and an auxiliary spatial channel. Small frequency changes also alter the false-target amplitude more rapidly than genuine target returns.

Approach and Solution

The engineering team implements four coordinated measures:

1. A refractivity-aware coverage service uses meteorological profiles and a parabolic-equation model to predict ducting and sub-refractive gaps.

2. Front-end preselection and improved amplifier linearity reduce blocking, while coordinated frequency assignments limit simultaneous high-power communication emissions.

3. Adaptive sidelobe cancellation suppresses off-axis interference, with constraints that protect the main-beam target region.

4. Waveform agility, auxiliary-angle validation and multiscan kinematic tests reduce DRFM false-track capture. Expendable active decoys remain available for high-threat missions, while the shipborne jammer and electronic support receiver provide reusable non-expendable protection.

Outcome

The false-alarm rate during ducting is reduced, although it cannot be eliminated because duct-enhanced clutter is physically present. Afternoon coverage predictions become more realistic, and receiver overload events decline substantially. Deception rejection improves without relying on a single classifier.

Learning Points

Propagation, hostile jamming and civil interference can create similar operational symptoms. Effective diagnosis requires meteorological, spectral, spatial and waveform evidence. No single electronic protection method is sufficient; robust performance comes from propagation awareness, receiver engineering, adaptive processing, coordination and disciplined operational procedures.

13. Challenges, Governance and Emerging Directions

Propagation predictions remain uncertain because small-scale humidity structure, sea state and terrain data may be unavailable. Electronic-warfare models are also limited by incomplete knowledge of adversary algorithms. Testing should therefore include uncertainty ranges and adversarial cases rather than only nominal scenarios.

Cognitive radar and dynamic spectrum systems can learn interference patterns and select waveforms adaptively. Their benefits include more efficient spectrum use and faster response, but certification is difficult when behaviour changes after deployment. Governance should define permitted adaptation, protected bands, fallback modes, audit logs and human authorization.

Electronic attack can disrupt safety-of-life, navigation and civil communication services beyond the intended area. Engineers and operators must comply with authorization, proportionality, frequency-allocation rules and applicable law. Spectrum measurements may also reveal sensitive operational or commercial information and should be governed accordingly.

Best practice is to treat propagation, electronic protection and coexistence as a single systems-engineering problem. Models should be validated with calibrated field data; losses and gains should be traceable; peak and average quantities should not be mixed; and special-case equations should be applied only when their geometry and bandwidth assumptions hold.

Chapter Summary

Radar propagation is modified by gaseous absorption, precipitation, refractive gradients, ducting, multipath and terrain. A more-negative refractivity gradient produces super-refraction, while a less-negative or positive gradient produces sub-refraction. Ducts occur when modified refractivity decreases with height and can extend both radar coverage and harmful interference.

Electronic attack includes noise masking and coherent deception. Under self-screening, same-direction assumptions, the jammer-to-signal ratio increases as R², and wavelength factors cancel. Burn-through range follows from the required J/S threshold and varies inversely with the square root of jammer power. Expendable countermeasures such as chaff and active decoys complement reusable onboard jammers, warning receivers and adaptive arrays.

Electronic protection combines waveform, spatial, spectral, polarization and track-level measures. LPI radar reduces intercept opportunities through energy management and waveform design but cannot guarantee undetectability. EMC and spectrum coexistence require front-end linearity analysis, aggregate interference calculations, regulatory coordination and propagation-aware risk assessment.

Key Terms

Atmospheric attenuation: Reduction of electromagnetic power by gaseous absorption, precipitation and suspended particles.

Burn-through range: Range at which target echo strength becomes sufficient relative to self-screening jamming.

Chaff: Expendable conductive material deployed to create strong distributed radar returns.

Deception jamming: Electronic attack that introduces false or altered radar measurements.

Diffraction: Bending and spreading of waves around terrain or structural obstructions.

DRFM: Digital radio-frequency memory used to capture and coherently reproduce radar waveforms.

Ducting: Trapping of radio energy within a layer whose modified refractivity decreases with height.

Electronic attack: Use of electromagnetic energy or countermeasures to degrade or mislead an opponent.

Electronic protection: Measures that preserve radar operation in interference or electronic attack.

Electronic support: Interception and analysis of electromagnetic emissions for situational awareness.

Expendable countermeasure: A deployed countermeasure that is consumed or normally not recovered.

Interference-to-noise ratio: Ratio of received interference power to receiver thermal-noise power.

Low probability of intercept: Design approach intended to reduce the probability of emission detection or identification.

Modified refractivity: Refractivity adjusted for Earth curvature and used to identify ducting layers.

Multipath: Reception through multiple propagation paths having different amplitudes and phases.

Non-expendable countermeasure: Reusable, attached or recoverable electronic countermeasure equipment.

Refractivity: Scaled measure of atmospheric refractive index, N = (n − 1) × 10⁶.

Spectrum coexistence: Operation of multiple radio services with acceptably controlled mutual interference.

Sub-refraction: Propagation with weaker downward bending than standard, usually reducing the radio horizon.

Super-refraction: Propagation with stronger downward bending than standard, usually extending the radio horizon.

Review Questions

Short Answer Questions

1. Why is atmospheric attenuation counted twice for a monostatic radar echo?

2. Distinguish sub-refraction from super-refraction in terms of the refractivity gradient.

3. State the condition for ducting using modified refractivity.

4. Why must independent interference powers be summed in linear units rather than directly in decibels?

5. Give two examples each of expendable and non-expendable countermeasures.

Long Answer Questions

1. Explain how gaseous absorption, rain and wet-radome loss affect microwave and millimetre-wave radar design.

2. Derive the self-screening main-beam jammer-to-signal ratio and identify every assumption required for receive-gain and wavelength cancellation.

3. Compare noise jamming and coherent deception, including suitable electronic protection measures.

4. Critically evaluate the design trade-offs involved in an LPI radar.

5. Discuss how EMC engineering and regulatory coordination support radar-spectrum coexistence.

Analytical / Application Questions

1. A radar operates through a 12 km path with gaseous attenuation of 0.05 dB/km. Rain occupies 4 km and adds 2.5 dB/km. Calculate the two-way atmospheric attenuation.

2. For a measured gradient of −100 N-units/km, calculate the effective-Earth-radius factor and classify the refractive condition. Explain why the effective-radius model may fail in a layered atmosphere.

3. Develop an integrated mitigation plan for a coastal radar experiencing surface ducting, DRFM false targets and aggregate interference from nearby communication transmitters.

Multiple Choice Questions with Answers

Question 1

A refractivity gradient of −80 N-units/km corresponds approximately to which condition?

A. Sub-refraction with kₑ = 0.49

B. Standard refraction with kₑ = 1.33

C. Super-refraction with kₑ = 2.04

D. Ducting with kₑ = −2.04

Correct Answer: C. Super-refraction with kₑ = 2.04

Question 2

Which condition identifies a radio duct using modified refractivity M?

A. dM/dh < 0

B. dM/dh = 157 N-units/km

C. dN/dh > 0 only

D. kₑ = 1 exactly

Correct Answer: A. dM/dh < 0

Question 3

A 20 km monostatic radar path has gaseous attenuation of 0.08 dB/km, while 5 km has additional rain attenuation of 4 dB/km. What is the total two-way attenuation?

A. 21.6 dB

B. 40.0 dB

C. 41.6 dB

D. 43.2 dB

Correct Answer: D. 43.2 dB

Question 4

Under self-screening, main-beam and same-frequency assumptions, why does the simplified J/S expression contain no explicit λ² term?

A. The jammer does not experience spreading loss

B. The wavelength terms in received jammer and echo powers cancel

C. Radar cross-section always varies as λ²

D. Antenna gain is independent of direction

Correct Answer: B. The wavelength terms in received jammer and echo powers cancel

Question 5

A radar has Pₜ = 1 MW and Gₜ = 1000. A 1 m² target at 100 km carries a jammer with Pⱼ = 4 kW and Gⱼ = 10. With complete overlap and equal losses, what is J/S?

A. Approximately 27 dB

B. Approximately 47 dB

C. Approximately 67 dB

D. Approximately 87 dB

Correct Answer: C. Approximately 67 dB

Question 6

Under the self-screening burn-through model, what happens to burn-through range when jammer power is quadrupled and all other quantities remain fixed?

A. It doubles

B. It is halved

C. It is quartered

D. It remains unchanged

Correct Answer: B. It is halved

Question 7

Which is an expendable radar countermeasure?

A. Chaff cartridge

B. Installed DRFM jammer

C. Shipboard electronic support receiver

D. Reusable adaptive antenna array

Correct Answer: A. Chaff cartridge

Question 8

How should average powers from statistically independent interference sources be aggregated?

A. Add their dBm values directly

B. Add their electric-field phases only

C. Convert to linear power, sum, then convert back to decibels

D. Retain only the strongest interferer

Correct Answer: C. Convert to linear power, sum, then convert back to decibels

Question 9

Which measure is most consistent with LPI radar design?

A. High peak power with a fixed repetitive waveform

B. Broad sidelobes and continuous omnidirectional transmission

C. Low peak power with coherent integration and waveform agility

D. Increasing receiver noise bandwidth without changing processing

Correct Answer: C. Low peak power with coherent integration and waveform agility

Question 10

For an ideal knife edge with normalized diffraction parameter ν = 0, the approximate excess diffraction loss is:

A. 0 dB

B. 3 dB

C. 6 dB

D. 20 dB

Correct Answer: C. 6 dB