CSIS Finds Passive Ground‑Based Sensor Networks Enhance Air and Missile Defense Resilience
CSIS Finds Passive Ground‑Based Sensor Networks Enhance Air and Missile Defense Resilience
Published:
July 21, 2025
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Updated:
July 21, 2025
Air & Missile Defense Systems
Michael Adler
US Air National Guard photo by Senior Airman Colin Simpson
A new study from the Center for Strategic and International Studies suggests that using a network of ground-based sensors could make U.S. and allied air and missile defenses much more effective. Defense officials call it the most detailed public report on passive mesh sensing so far, and they think it’s coming at the right time – since enemy missiles are getting faster, smarter, and more numerous every year.
The report pushes back against the old idea of relying on a handful of powerful radars. Those big radars have to send out strong signals, which makes them easy targets for modern anti-radiation weapons. Instead, the study recommends spreading out a bunch of simple electro-optical, infrared, acoustic, and passive radio-frequency sensors. These stay quiet, just listening for aircraft, drones, or missiles. Because they don’t send out any signals, they’re much harder for enemies to find or jam.
Lead author Masao Dahlgren and project director Dr. Tom Karako came up with a model network covering Poland. Their simulations show that about 400 affordable electro-optical/infrared towers, connected wirelessly, could cover most of the gaps created by the landscape and the curve of the earth. They picked Poland because its size and terrain are similar to some regions in the Indo-Pacific, so the lessons can apply elsewhere.
U.S. Army and Air Force analysts say this fits well with their idea of “affordable mass.” Instead of protecting an area with just a few expensive radars that are critical single points of failure, you can have hundreds of cheap, low-maintenance sensors. If one gets destroyed, the others still work together to track threats.
Missile threats have changed a lot in the last five years. Mass-produced attack drones fill up radar screens. Hypersonic glide vehicles give less time to respond. Ballistic missiles can release decoys or maneuver in flight. Radars that light up every few seconds are at risk of being detected before they can even identify what’s coming.
A network of silent sensors turns that around. The report points out three main benefits:
Emission control: Passive sensors never give away their position by sending out signals.
Geometric resilience: With so many lines of sight, it’s hard for decoys to fool the network or for terrain to hide threats like low-flying cruise missiles.
Cost flexibility: Off-the-shelf sensors and wireless tech are cheap and easy to scale up.
Defense officials say these features line up with the military’s push for distributed sensing.
The CSIS team looked at several types of sensors:
Electro-optical/infrared (EO/IR): Thermal cameras can spot a rocket launch from far away, day or night.
Passive multistatic radar: Sensors use commercial broadcast signals bouncing off targets to form images without sending anything themselves.
Acoustic arrays: Microphone clusters pick up the sounds of drones or other threats.
Passive RF receivers: Antennas detect stray signals from enemy equipment.
Tests show that combining all these types of data cuts down on false alarms compared to using just one kind of sensor.
Ukraine’s “Sky Fortress” program demonstrates how this can be acheived. Volunteers set up more than 14,000 low-cost microphones across key routes, helping air defenses shoot down waves of drones last winter. Visitors to the control center in Kyiv described a simple interface that spots threats like Shahed drones before the radars even turn on.
The U.S. Army is already testing its ALPS system in Southwest Asia. ALPS listens passively and shares data over lightweight mesh radios. Reports say the sensors run for days on a single battery, and everything fits in one case, making it easy to set up in remote areas.
One limitation: ground sensors can only see what’s above the horizon. Low-flying drones can sneak in, especially near coasts or in cities. To help with this, the engineers suggest putting sensors on cell towers or wind turbines, but they warn that no system can cover every direction perfectly. Software and machine learning help planners choose the best spots for overlap.
Bad weather can also be a problem, blocking infrared or muffling sounds. The report suggests using a few small, mobile radars as backup, turning them on only when the passive network needs confirmation, which saves energy and avoids giving away their position.
Power and data are other challenges. Each sensor doesn’t use much power, but hundreds of them can stress old power grids. The authors recommend using solar or hydrogen fuel cells, plus radios that automatically reconnect if one link goes down. Cybersecurity is a must too, since a single hacked sensor could mess up the whole picture. Officials say secure GPS timing and backup systems are also on their wish lists.
On the budget side, passive sensors are way cheaper. One PAC-3 interceptor missile costs more than $4 million. A basic passive sensor station costs under $50,000. Even with all the networking gear, the Poland example is still less expensive than replacing one Patriot missile battery. But nothing’s totally free – there are hidden costs like digging trenches for fiber, licensing frequencies, and training operators.
European defense ministries are weighing these trade-offs now. Industry insiders say Poland may test a mesh network with its Pilica short-range batteries by 2027. Lithuania and Finland have shown interest, hinting at a possible Baltic partnership.
For this to work, sensor data needs to flow smoothly into fire-control systems. The Army’s IBCS system already connects sensors and shooters. New software can translate passive sensor data into standard messages in about two seconds – fast enough to direct defensive weapons.
This approach helps conserve missiles by matching the right weapon to each threat. For example, a drone might be taken out with a laser, while a complex missile still gets intercepted with a hit-to-kill weapon.
Passive sensors are also easy to move and set up quickly. Troops can reposition them overnight to confuse enemy planners. Decoys are cheap, too – an empty camera case can force the enemy to waste time and resources. Recent Red Flag exercises used this tactic, and pilots reported having trouble finding the real sensors without electronic help.
Mobility comes with its own headaches – moving sensors means recalibrating and updating the network. The authors say these tasks need to be automated with AI to avoid overloading operators.
Not many U.S. companies build rugged passive EO/IR sensors in large numbers. Without large, steady orders, they can’t ramp up production or obtain the necessary parts. Companies involved in the study say longer-term contracts would help lower prices. Sourcing parts is tricky too, since many cheap infrared sensors come from China. Switching to allied suppliers cuts dependency but can slow things down until U.S. or partner factories can catch up. The report suggests using defense incentives and tech-sharing deals with Japan and South Korea to help.
CSIS’s roadmap:
Pilot networks: Set up small test networks in the U.S. within a year.
Allied demos: Work with Poland or Romania to prove cross-border data sharing by 2028.
Faster buying: Use special contracting to skip slow military approval processes for passive gear.
Doctrine update: Update military tactics to treat passive and active sensors as equals.
Officials say these steps will be included in the next major air and missile defense strategy.
Events in the Black Sea and Red Sea have shown how cheap drones can disrupt shipping. A continent-wide passive sensor network could give NATO commanders early warning without making them easy targets for missile strikes. Plus, it makes enemy misinformation harder, since you can’t easily prove a sensor is there until it’s too late. The Pentagon sees this uncertainty as a good thing – a deterrent in itself.
The Philippine Army is studying mesh networks for island defense, and Australia is testing passive radars that use FM radio signals to spot low-flying threats over water. Making all these systems work together will require sharing data formats and agreeing on electromagnetic rules.
REFERENCE SOURCES
https://www.leidos.com/insights/leidos-building-advanced-air-defense-sensors-marines
https://www.csis.org/events/mesh-sensing-air-and-missile-defense
https://breakingdefense.com/2025/07/passive-ground-based-sensor-networks-could-bolster-air-missile-defense-resilience-csis/
https://csis-website-prod.s3.amazonaws.com/s3fs-public/2025-07/250717_Dahlgren_Mesh_Sensing.pdf
https://united24media.com/war-in-ukraine/sky-fortress-ukraines-acoustic-detection-system-that-tracks-drones-cheap-and-fast-9451
https://www.defenseone.com/defense-systems/2024/07/ukraines-cheap-sensors-are-helping-troops-fight-waves-russian-drones/398204/
https://centcomcitadel.com/en_GB/articles/ssc/features/2023/10/10/feature-07
The post CSIS Finds Passive Ground‑Based Sensor Networks Enhance Air and Missile Defense Resilience appeared first on defense-aerospace.
A new study from the Center for Strategic and International Studies suggests that using a network of ground-based sensors could make U.S. and allied air and missile defenses much more effective. Defense officials call it the most detailed public report on passive mesh sensing so far, and they think it’s coming at the right time – since enemy missiles are getting faster, smarter, and more numerous every year.
The post CSIS Finds Passive Ground‑Based Sensor Networks Enhance Air and Missile Defense Resilience appeared first on defense-aerospace.
