Aircraft stealth is not a single technology but a combination of many techniques that make platforms harder to detect, intercept, and target. In modern conflict, onboard fire-control radars, navigation, communication, and data-link systems emit electromagnetic energy. Adversary electronic surveillance can intercept emissions from a platform's radar and communication transmitters, and use frequency measurement, direction finding, localization, and signal identification to determine attributes such as emitter parameters, type, and spatial location. Radio-frequency (RF) stealth refers to reducing RF signal signatures, including radar, so that adversary sensors remain uncertain and unable to timely detect or identify the target.
Purpose and Threats
The goal of RF stealth is to use low-probability-of-intercept (LPI) techniques to counter passive threats such as anti-radiation missiles (ARM), direction-of-arrival (DOA) systems, radar warning receivers (RWR), electronic countermeasure (ECM) devices, and electronic intelligence (ELINT) systems, thereby reducing target signature and improving survivability. Passive threats rely on intercept receivers that detect, classify, and identify emissions. LPI system design must reduce the intercept receiver's ability to perform all three functions. Most RF stealth research uses the Schleher intercept factor, defined as the ratio of the distance at which an adversary intercept receiver can detect an LPI radar to the maximum distance at which the LPI radar can be detected by an intended receiver.
The smaller the Schleher intercept factor alpha, the better the aircraft's RF stealth performance. A larger alpha increases the risk of RF interception. When alpha < 1, the intercept receiver's detection range is less than the radar's own detection range, so the airborne radar is difficult to intercept and RF stealth performance is good. When alpha > 1, RF stealth performance is poor. Alpha = 1 represents a critical state.
RF stealth techniques model the detection range of emitters such as radar and data links against the intercept range of adversary receivers, and then adjust parameters via power control, waveform design, beam control, and other methods so that adversary intercept receivers cannot capture the emissions, or if captured, cannot identify them reliably.
Core RF Stealth Technology Categories
Making emitted electromagnetic signals difficult to intercept is the primary objective of RF stealth. Factors that affect RF stealth include time domain, spatial domain, frequency domain, energy domain, polarization domain, and waveform domain. Key technical approaches include low-probability-of-intercept waveform design, emitter power control, directional antenna techniques, and maximizing signal uncertainty. RF stealth is therefore a system-level capability involving multiple devices and techniques rather than a single technology or onboard system.
1. Emitter Power Control
Transmit power management, i.e., using only the power necessary to achieve required detection performance, is a fundamental RF stealth technique. A minimum-radiation-energy strategy requires the active emitter to radiate at the lowest energy needed at any moment, keeping emitted signals below the intercept receiver threshold whenever possible. This involves managing emission characteristics, propagation loss, and receiver sensitivity.
2. Ultra-Low Sidelobe Antenna Technology
LPI antenna design focuses on two aspects: first, controlling main-lobe and sidelobe gain to illuminate targets using minimal power while maintaining normal operation; second, controlling beam direction or platform maneuvering to avoid interception by adversary sensors. Techniques such as narrow beams and ultra-low sidelobe patterns concentrate energy in the main lobe and reduce sidelobe radiation, lowering the probability of interception.
3. Low-Probability-of-Intercept Waveform Design
LPI waveform design aims to produce signals with low peak power and large instantaneous bandwidth so that power remains at the minimum level consistent with acceptable performance. If the signal is detected, waveform complexity and randomness make intercept receivers harder to recognize, reducing their operational effectiveness. Pulsed compression techniques targeting large time-bandwidth products have evolved through linear FM (LFM), nonlinear FM (NLFM), phase-coded signals, random radar signals, and various hybrid forms.
4. Maximizing Emitter Signal Uncertainty
Maximizing uncertainty in emitter signals is an important RF stealth approach. Introducing chaos concepts and entropy features into signal design increases parameter uncertainty, disrupting intercept receiver pulse sorting and classification because of irregularities in frequency, time, and spatial domains. This improves resistance to sorting and recognition, minimizing the information adversary receivers can extract. Techniques such as frequency agility and random beam scanning fall into this category.
Integration with Other Signatures
RF stealth is one component of electronic protection. Coordinated development of radar, infrared, and RF stealth capabilities is required to maintain relative advantage in stealth and counter-stealth engagements. Developing techniques specifically aimed at adversary RF detection systems is a critical factor in modern stealth platform design.
