Passive sensors do not require energy emissions to detect and track targets, making it more difficult for adversaries to detect, track, and target them.
On June 21, 2024, at the remote ballistic missile surveillance radar building of the 213th Space Warning Squadron at the Clear Air Force Station in Alaska, which is part of a solid-state phased array radar system.
A new study by the Center for Strategic and International Studies (CSIS) indicates that moving away from reliance on large ground radars in favor of a network of small passive sensor layers can significantly improve the U.S. air defense and missile defense capabilities while reducing the likelihood of being attacked.
“In the face of threats from complex integrated attacks, the existing air and missile defense systems would benefit from a more decentralized, passive, and ultimately more resilient posture. This approach will better adapt air and missile defense operations to today’s strategic environment,” the authors of CSIS assert.
This study, titled “Networked Sensing for Air and Missile Defense,” is a highly technical examination of how to design and implement such a network composed of multiple passive sensors, along with its advantages and disadvantages.
The study proposes a conceptual network covering Poland as a case study, finding that developing such a network would require 400 electro-optical and infrared sensors connected through robust communication and power grids.
Poland was chosen as a case study due to its geographic location, which “provides a useful test case for considering networked sensors, with relevant terrain combinations and an area comparable to the Indo-Pacific region of interest.”
Most of the U.S. military’s air and missile defense systems currently rely on a small number of high-power radars that precisely locate targets by emitting electromagnetic waves and receiving reflected signals. Radars have many advantages, including all-weather operational capability and long-range detection. However, the energy emitted by radar radiation is increasingly detectable by enemy weapon systems. The farther the radar detection range, the greater the power required, and the more energy emitted.
In contrast, passive systems (such as electro-optical, infrared, acoustic, and dedicated radio frequency sensors) do not need to emit energy to detect and lock onto targets, as they only collect energy from incoming weapon systems. Therefore, it is more difficult for adversaries to detect, track, and target them.
Examples of such systems include Ukraine’s “Aerial Fortress” network, which relies on acoustic sensors to listen for sound energy and uses microphones that “do not emit easily detectable signals.” CSIS notes that the U.S. Army is also using passive radio receivers in its Army Long-Range Persistent Surveillance (ALPS) system, which has been deployed in various theaters.
Nevertheless, passive systems also face numerous challenges, with the primary issue being performance degradation due to cloud cover and adverse weather conditions. Therefore, CSIS’s research concludes that the optimal architecture for constructing a comprehensive AMD sensor network should be hybrid, utilizing multiple types of systems.
The study states: “The challenges posed by weather highlight the necessity of integrating networked sensors with other passive sensing modes and existing active architectures. Passive multi-static radar, passive radio frequency detection, acoustic systems, and other methods all have potential roles to play.”
The research acknowledges that even hybrid systems may have blind spots. One of these is that various ground sensors “have limitations in tracking low-flying targets. The curvature of the Earth and surrounding terrain can obstruct the line of sight of ground sensors at lower angles, limiting their maximum detection range. This is why deployment at higher altitudes is often preferable,” the study explains. (In fact, this is also a key reason why satellites have traditionally been used to detect and track global ballistic missile launches.)
The study emphasizes that placing various sensors in such a hybrid system is not as simple as dispersing them across hills and mountains.
However, the challenge lies in planning a grid deployment, as even the siting of a single radar is a complex process that requires consideration of potential missile launch sites, tactical mobility, and other factors. As the number of sensors increases, the complexity of these issues can rise dramatically. Computational methods are needed to determine the deployment locations of a large number of sensors,” the study notes.
The research finds that another issue facing the burgeoning AMD sensor architecture is ensuring access to the power grid and robust communication networks.
“At the operational level, achieving the diffusion of sensor networks requires further integration of existing mobility planning analysis tools with air and missile defense mission planning. Sensors connected to a centralized power source may expose the civilian power grid to enemy attacks, necessitating distributed grids equipped with high-performance batteries, solar panels, generators, or fuel cells,” the study emphasizes. “Additionally, the engineering design and layout of the physical communication backbone connecting grid assets must be as carefully considered as the sensors themselves.”
In summary, while CSIS’s case study of the hypothetical Polish system found that using a network composed of numerous passive sensors can enhance performance and resilience, it also clearly indicates that developing such a multi-node architecture is not easy (or, by inference, inexpensive).
The research concludes: “There is much work to be done to bring this concept to fruition.”
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