In this blog post, we’ll examine the principles of radar detection, the radar-evasion technologies used by stealth aircraft, radar cross-section (RCS), and the principles behind aircraft geometry and internal weapon bays, and explore the significance of stealth technology in modern air combat.
How Do Stealth Aircraft Evade Radar Detection?
For some time, South Korea was worried and fearful about the possibility of war between the two Koreas due to North Korean provocations. Of course, war must never happen, but if war were to break out unavoidably, the belligerents would quickly mobilize their air power to seize air superiority over the enemy. To prevent the other side from seizing air superiority, each side would deploy fighter jets and use radar to monitor the enemy’s aircraft. However, there are fighter jets that make radar detection difficult—namely, stealth aircraft. Stealth aircraft are not completely invisible to radar; rather, they are designed to minimize the probability of being detected by radar and to reduce the distance at which they can be detected and tracked.
The English word “stealth” refers to secrecy or moving without being detected. Radar is used to detect and track these stealth aircraft. Radar is classified into various types based on the frequency band used, with the most common being S-band radar, C-band radar, and X-band radar. Here, S, C, and X refer to frequency bands. Generally, the S-band uses a frequency range of approximately 2–4 GHz, the C-band approximately 4–8 GHz, and the X-band approximately 8–12 GHz.
To reduce the likelihood of detection by such radars, stealth aircraft are designed to minimize the amount of radio waves that bounce off the airframe and return to the radar receiver. In particular, the airframe’s shape and surface are designed to reduce scattering—where radar waves disperse in multiple directions—and to prevent reflected waves from returning to the radar’s location. So, what technologies should be used to reduce this radar reflection? Broadly speaking, there are two main methods. One involves using materials that absorb radar waves, and the other involves designing the aircraft’s shape to control the direction in which the radar waves are reflected.
How are radar-absorbing materials and the shape of stealth aircraft created?
The first method involves using materials that absorb radar waves. These are known as RAM (Radar Absorbing Material), and typical applications include using absorbers in the form of tiles or panels, or applying them as special coatings and paints. Absorbers are used on specific parts of the aircraft to reduce radar wave reflection, and special coatings are sometimes applied to the exterior surfaces. Radar-absorbing materials work by dissipating radio wave energy into other forms, such as heat, or by reducing reflection. Various composite materials and radio wave-absorbing materials have been researched for a long time, and today’s stealth aircraft also utilize these low-observable materials and surface treatments in conjunction with their airframe design.
Furthermore, the shape of a stealth aircraft also plays a crucial role in minimizing radar reflection. A prime example is the B-2 stealth bomber. The B-2 adopts a flying-wing configuration without vertical stabilizers, and its airframe shape and surface are meticulously designed to reduce radar reflection. The engines and various structural components are also designed to be integrated into the interior of the airframe so that they are not visible from the outside. According to the U.S. Air Force, the B-2’s low-observability capabilities are the result of a combination of technologies—including its flying-wing design, composite materials, and special coatings—and a significant portion of the specific low-observability technologies remains classified to this day.
What is RCS, and how do radars track fighter jets?
So, what are the characteristics and principles of the radars used to monitor fighter jets? Radars operating across various frequency bands are used for fighter jet detection and tracking, and relatively high-frequency bands, such as the X-band, are among the most commonly used for precise detection and tracking. However, the distance at which a specific radar can identify an object of a certain size depends on the radar’s power, antenna, operating environment, the target’s size and shape, altitude, and radio wave conditions; therefore, it is difficult to generalize based solely on a specific distance. Radar uses a concept called RCS (Radar Cross Section) when detecting and tracking objects; this is an indicator of how strongly radar waves are reflected back by a target. RCS is measured in square meters (㎡), and generally, the smaller the value, the more difficult it is for radar to detect the target. In particular, RCS is not a fixed number but varies depending on the radar’s frequency, the direction of the beam, and the target’s attitude. Therefore, it is difficult to pinpoint specific figures for undisclosed performance details, such as the exact RCS value of the B-2. The U.S. Air Force has also stated that a significant portion of the B-2’s low-observability technology remains classified.
Looking more closely at how radar tracking works, a radar system transmits radio waves toward a target and receives the signals that bounce back from it. Different parts of the target’s airframe and structure produce reflections of varying intensities; the phenomenon where the target’s position as seen by the radar shifts slightly due to these scattering centers is called “glint.” Consequently, the signal received by the radar may appear as a combination of multiple reflected signals rather than a single, perfectly fixed point.
Radar tracks aircraft by synthesizing information from these multiple reflected signals, along with the target’s distance, speed, and direction. In other words, it does not identify a target simply by the appearance of a single reflection point on the radar screen, but rather determines the target’s movement by analyzing changes in the signal over time.
Today’s widely used AESA (Active Electronically Scanned Array) radar can rapidly steer its beam by electronically controlling numerous transmit-receive modules and digitally processes the received signals to detect and track targets. Even with stealth aircraft, radar signals can change when reflections increase from a specific direction or when the aircraft changes its attitude during flight; therefore, simply reducing the average RCS does not guarantee the same level of low-observability performance from all directions. Consequently, stealth technology has evolved by comprehensively considering airframe geometry, absorptive materials, and structural design to reduce reflections from specific directions and make detection and tracking more difficult.
Why are internal weapon bays necessary for stealth aircraft?
Finally, let’s examine the armament of stealth aircraft. Since stealth aircraft must also attack enemy aircraft or ground targets in combat, they need to carry weapons such as missiles. This is where the internal weapon bay plays a crucial role.
While conventional fighter jets can mount weapons under their wings or on the exterior of the airframe, mounting weapons externally can cause the weapons and their mounting mechanisms to reflect radar waves, thereby compromising stealth performance. For this reason, stealth aircraft utilize an internal weapons bay system to store weapons inside the airframe as much as possible. Although radar reflections may increase when the internal weapons bay opens—as structural elements are exposed to the outside—the bay can be closed again after the weapon is fired, thereby maintaining the aircraft’s low-observability characteristics. On the other hand, because space and structural elements are required to store weapons internally, there may be more limitations on weapon operations compared to external mounting methods.
How will stealth technology evolve in the future?
In past live-fire exercises and simulations, there have been reports of stealth fighters achieving very high engagement results against conventional fighters. A notable example is the 2006 U.S. “Northern Edge” exercise, where the F-22 recorded a simulated kill ratio of 144 to 0 against F-15s, F-16s, and F/A-18s. However, these results should not be interpreted as solely attributable to the effectiveness of stealth technology itself. This is because, at the time, various factors—including generational differences between aircraft, as well as sensors, armaments, tactics, and training conditions—all played a role. Therefore, it is difficult to interpret the 144-to-0 figure as meaning that stealth aircraft hold the same advantage over non-stealth aircraft in all combat situations.A stealth aircraft is not one that is completely invisible to radar, but rather one that makes it difficult for enemy radar to detect and track it. Consequently, as stealth technology advances, radar and sensor technologies designed to counter it must inevitably advance as well. In fact, modern air defense systems are evolving to detect and track targets by integrating multiple sensors and sources of information, rather than relying on a single radar. In the future, as the design, absorptive materials, sensors, and electronic warfare capabilities of stealth aircraft continue to advance, so too will the radar technology used to detect and track them. Even as the technological competition to secure an advantage in war continues, it is clear that the best scenario is one in which war does not occur at all.
