Basics of Radar and Stealth
Stealth has two main components: absorption and deflection. The shape of an object can deflect radar waves away from the source. Special materials, often ferrites or other radar-absorbent materials, can absorb much of the incident energy. Infrared is another detection modality. Stealth aircraft also use infrared suppression by mixing external air with exhaust and generally flying subsonic to keep skin temperatures lower. Infrared is less effective for long-range detection, so the primary concern at range is defeating radar-guided threats and short-range infrared missiles.
Electromagnetic Scattering Mechanisms
Complex targets interact with electromagnetic waves through specular reflection, edge diffraction, tip diffraction, creeping-wave diffraction, guided-wave diffraction, and diffraction due to abrupt electromagnetic discontinuities on non-slender bodies. For conventional non-stealth aircraft, the scattered field includes reflection and diffraction components, with specular reflection and edge diffraction dominating. Stealth aircraft use multiple measures to suppress specular reflection and edge diffraction. A typical fighter without stealth may have an RCS around 1 m2, whereas stealth aircraft aim for RCS values near 0.01 m2 or less.
Radar Operation and Echo Detection
Radar essentially detects echoes. The analogy is shouting into a canyon: sound spreads, bounces off hard rock surfaces, and some of that scattered sound returns to you as an echo. Radar uses radio waves instead of sound.
Radar antennas emit pulsed radio waves and then listen for returned signals, commonly called radar returns. Although the reflected energy is only a small fraction of the transmitted pulse, amplifiers increase the received signal strength. A sufficiently large return appears on a radar display and can be detected at long ranges.
Traveling and Surface Wave Scattering
Traveling or surface-wave scattering: incident radar waves on an aircraft can induce surface currents. These surface currents travel along paths to discontinuities, such as leading edges or gaps. Surface boundaries can cause backscattering or scattering into multiple directions. This reflection can be reduced with radar-absorbent materials, radar-absorbent structures, and by minimizing surface gaps or aligning edges.
Doppler and Detection Range
The Doppler effect allows calculation of target speed and direction. If a target moves toward the detector, radar waves are compressed; if it moves away, the waves are stretched. The same principles now apply in many civilian systems, like automotive braking assist and weather radar. Radar can be defeated: stealth techniques deflect or absorb incoming radar so little or no energy returns to the source. While the concept is simple, practical implementation is technically demanding because conventional aerodynamic shapes—rounded fuselages, wings, tails—are efficient for flight but also very effective reflectors of radio signals.
Frequency Dependence and Beamwidth
As radar wavelength increases (frequency decreases), the intensity of specular reflection decreases and the beamwidth widens if aperture size remains constant. Wider specular lobes make it harder to direct reflected energy away from the radar, and reflected energy is distributed over a broader angular region.
Faceting and Smooth Curvature
In the late 1970s, the Lockheed "Skunk Works" produced the F-117 Nighthawk. Its unusual faceted design and materials reduced returns to a small fraction of a conventional aircraft’s radar signature. Many small flat surfaces scatter radio energy away from the transmitter, and the aircraft used radar-absorbent materials.
Two main shape-based approaches reduce RCS. The first is faceting, where the exterior is composed largely of flat panels to limit specular returns in the direction of the radar. The second uses smoothly varying curvature to achieve a similar effect across a wide frequency band but requires more computation to predict the correct curvature. Engine compressor blades are another major contributor to RCS; frontal radar illumination on compressor discs yields strong returns. Stealth aircraft hide engines from direct view using S-shaped inlets.
Countermeasures and Low-Frequency Radar
Options against stealth are limited. Increasing radar power is one approach, but required power increases are large. There are reports that some modern multi-radar, high-power ground-air missile systems can detect older stealth designs at close ranges. Low-frequency radars with very long wavelengths are less affected by shaping-based stealth, though they have other limitations such as coarse angular resolution and large antenna size. Parasitic or incidental electromagnetic emissions can also betray a target if a baseline radiation pattern of an area is monitored for changes; this is conceptually similar to monitoring at airport security checkpoints. Continuous environmental electromagnetic interference must be managed to mitigate such detection techniques.
Infrared Detection and Signatures
Infrared signatures can be suppressed using special nozzle designs that mix and disperse exhaust heat and with low-emissivity coatings or materials. Infrared detection is strongly dependent on atmospheric conditions; IR behaves more like visible light than radio waves, so range is limited and clouds or fog reduce performance. Because of these limitations, most infrared-guided missiles are short-range, but modern IR systems have improved and can have substantial effective ranges under favorable conditions. If a stealth aircraft gets within IR detection range, it becomes highly vulnerable to IR-guided weapons.
Weapon Bays and Signature Changes
All operational stealth aircraft lose some stealth when deploying external stores. F-117, B-2, F-22, and F-35 carry weapons internally to preserve low observability; weapon bay doors must open to release weapons, and when open the aircraft’s stealth shaping is compromised and returns increase. Most stealth platforms mitigate this by using standoff weapons such as cruise missiles so that weapons can be released at long range; the radar signature of the weapon bay door at hundreds of miles is negligible. After weapon release, doors close and the aircraft’s low observability is largely restored. Radar systems generally require sustained returns to track and engage targets effectively.
Detection Tactics and Persistent Search
Some counter-detection strategies include passive sensing and distributed surveillance. A passive receiver located opposite a transmitter will notice a temporary signal drop when a stealth aircraft crosses the path; such anomalies can indicate a stealth target. Persistent rotating surveillance or airborne early warning platforms can reduce angular vulnerability because stealth effectiveness varies by aspect. Unmanned aerial systems can also perform loitering searches with randomized flight paths to increase coverage. Increasing signal concentration via focused beams effectively raises power density on a small region, which can be used to detect low-observable targets, though the scanned area is small. Rapid scanning or pulsed beams can help mitigate the narrow coverage drawback.
Concept and Scope of Low Observability
The term stealth or low observable (LO) technology covers techniques applied to aircraft, ships, submarines, satellites, and rockets to reduce their detectability by radar, sonar, infrared, and other sensors. For military aircraft, LO primarily addresses radar and infrared signatures. The governing principles include shaping to redirect radar energy away from the transmitter, use of radar-absorbent materials, careful alignment of edges and gaps, and management of thermal and acoustic emissions. Modern advances in sensors and counter-detection continue to influence LO design and operational tactics.
