Despite their discreet design, stealth aircraft can be detected by certain radars. Here are the most effective technical methods.

A constant technological race

Stealth technology is based on one promise: to evade radar detection. Since the 1980s, designers of stealth fighter jets such as the F-117 Nighthawk, the B-2 Spirit and, more recently, the F-35 Lightning II have optimized the shape, materials and thermal signatures of their aircraft to reduce their radar footprint. This electromagnetic stealth is based primarily on reducing the radar cross section (RCS), sometimes down to 0.001 m² for certain aircraft, which is equivalent to a golf ball.

However, advances in radar sensors, signal processing methods, and the integration of multi-band systems are challenging this superiority. Detecting a stealth aircraft is not impossible: it is a question of physics, exploited blind spots, and trade-offs. No system is undetectable. In turn, modern radars exploit flaws that are sometimes costly for aircraft to conceal: frequency, dynamics, altitude, weather conditions, and infrared signature.

In this article, we analyze the principles that enable radar to detect stealth aircraft, focusing on technical weaknesses, radar innovations (UHF/VHF, multistatic), and the strategic issues involved in this technological confrontation.

Radar operates on unalterable physical principles

Radar physics and the reduced signature of stealth aircraft

Radar relies on the emission of electromagnetic waves, which are reflected by objects and picked up by a receiving antenna. The strength of the reflected signal depends on many parameters: transmission power, antenna gain, distance, and above all, the radar cross section (RCS) of the target. This RCS, expressed in square meters, measures the amount of energy returned to the radar.

A stealth aircraft is designed to minimize this RCS. For example:

• The F-15 has an RCS of approximately 10 m²
• The Su-27 exceeds 12 m²
• The F-117 Nighthawk is less than 0.01 m²
• The F-35 is close to 0.005 m²

This reduction is achieved through angular shapes that deflect waves, absorbent materials (RAM), and extreme attention to reflective parts (leading edges, fins, air intakes).

However, these reductions are most effective against X-band radars (8 to 12 GHz), used by guided missiles or conventional interception radars. But the higher the frequency, the shorter the wavelength, and the easier it is for a small object to hide.

Low-frequency radar can penetrate stealth

The use of VHF and UHF bands

Low-frequency radars, operating in the VHF (30 to 300 MHz) and UHF (300 MHz to 1 GHz) bands, use wavelengths that are much longer than the size of fighter aircraft. In this range, geometric stealth loses its effectiveness.

A stealth aircraft with a SER reduced to 0.001 m² in the X band can increase to 0.1 or 1 m² in the VHF band, or even more if the aircraft is flying at low altitude or is poorly oriented relative to the beam.

The Soviet Union, and later Russia, have retained a doctrine based on this type of long-wave radar. The P-18 Spoon Rest, Nebo-M and more recent Rezonans-NE systems use these frequencies to detect stealth targets at distances of up to 300 km.

The disadvantages remain significant:

• Low angular resolution
• Sensitivity to jamming
• Bulky antennas (up to 20 m high)
• Data difficult to use for missile guidance

However, when networked, these systems can alert fighter aircraft or higher-frequency radars. In this sense, they act as strategic approach detectors.

Multistatic radar circumvents stealth blind spots

The principle of receiver offset

A conventional radar is called monostatic: the transmitter and receiver are in the same place. Stealth aircraft are optimized for this geometry. The waves are reflected at precise angles to avoid direct return.

Multistatic or bistatic systems separate these components: the receiver is sometimes offset by several kilometers. This geometry eliminates the need to optimize the radar angle and allows secondary returns that are normally absorbed or deflected to be detected.

Current examples:

• Silent Sentry (Lockheed Martin), based on civilian FM/TV waves
• Kolchuga (Ukraine), a network of multistatic passive sensors
• Passive listening systems based on FM radio, 4G or digital terrestrial television

These sensors do not require active transmission. They are therefore undetectable by aircraft radar warning systems. They provide presence data, sometimes trajectory data, but rarely of sufficient quality to fire a weapon.

The objective is therefore early detection, not necessarily targeting.

Doppler radar can reveal stealth movements

Exploiting speed rather than reflectivity

Doppler radar analyzes not only the returning wave, but also its frequency variation caused by the target’s movement (the Doppler effect). This method is effective for filtering out stationary targets (ground, clouds, mountains) and isolating moving ones.

Even a stealth aircraft, if moving quickly, generates a measurable Doppler signature. This phenomenon is exploited in particular by:

• Airborne radars such as the AN/APG-77 on the F-22
• Ground-to-air defense radars such as the Thales Ground Master 400
• Multirole naval systems

The main drawback is the angle of approach. An aircraft flying directly toward the radar (bearing 0°) can mask its Doppler effect. Modern sensors therefore use active antenna arrays (AESA), which can quickly scan the airspace to look for tiny variations.

Digital signal processing can then correlate several phenomena: radar signature, relative speed, estimated altitude, and thermal emissions. This cross-referencing makes it possible to reconstruct a track, even if it is incomplete.

A network of cross-referenced sensors reduces blind spots

Data fusion, satellites, optics, and artificial intelligence

Today, radar alone is no longer sufficient to guarantee the detection of a stealth fighter jet. The trend is toward multi-sensor fusion:

• Low-frequency radar for early warning
• Electro-optical sensors for visual confirmation
• High-altitude infrared (such as US SBIRS satellites)
• Passive electronic warfare networks

Artificial intelligence is playing an increasing role in interpreting radar signals. It can detect anomalies in noise patterns, identify repetitions in reflections, and recognize abnormal signatures through machine learning. China, the United States, and Israel are investing heavily in these technologies.

Finally, HALE (High Altitude Long Endurance) drones enable constant surveillance, with multi-band sensors and an off-center position that optimizes the chances of picking up weak reflections.

This cross-approach does not aim to make stealth obsolete, but to reduce its tactical effectiveness by increasing the detection rate in contested areas.

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