Mending Fences: Strengthening Homeland Defense through Integrated Civil-Military Air Surveillance
A 1953 advertisement for the U.S. Air Force’s civilian Ground Observer Corps described America’s air defenses as a “10 mile high fence full of holes.” Seventy years later, the United States again finds itself unable to reliably detect and identify threats from the air. One need look no further than the 2023 Chinese spy balloon incident, the unattributed aerial incursions over Langley Air Force Base and other U.S. military installations at home and overseas, and recent reports of mysterious drone activity over several U.S. states to see the urgency of the issue.
Vulnerability to aerial attack poses a serious threat to America’s ability to defend its citizens or to sustain a war effort. Lest this sound alarmist, U.S. military commanders have testified as much, and independent analysis confirms a “near-complete lack of homeland cruise missile defense and related forms of air defense.” Meanwhile, Russia and China have fielded “robust and redundant” integrated air defense systems within their own borders, ominously suggesting that the two countries are preparing for the possibility of reciprocal homeland strikes in wartime.
With storm clouds gathering over the Taiwan Strait and the ever-present risk that Russia’s war on Ukraine could escalate unpredictably, the United States should move swiftly to mend its aerial fence. To get a running start, the Department of Defense should rapidly integrate all relevant civil sensors into its homeland air defense network. From weather radars to 5G towers and FM radio stations, U.S. territory is dotted with thousands of transmitters and receivers that could contribute to aerial surveillance. Such integration would be both feasible and affordable. It would also reflect a long tradition of civil-military cooperation in watching America’s skies.
A Fence Full of Holes
For nearly seven decades America has lived with the prospect of nuclear warheads plunging down from hundreds of miles above. As a result, the Department of Defense has focused on building a ballistic missile defense umbrella over the United States. From the Nike interceptors of the late 1950s to the 1980s’ Strategic Defense Initiative and today’s Ballistic Missile Defense System, cumulative U.S. investment in countering exo-atmospheric missile threats is estimated at more than $450 billion in 2024 dollars.
Far less attention has been paid to endo-atmospheric threats like cruise missiles, balloons, and drones. If ballistic missiles are likened to enemy paratroopers that can land with little warning far behind friendly lines, threats flying at hundreds or thousands of feet in altitude (rather than hundreds of miles) are akin to ground troops menacing a nation’s flanks. Comprehensive surveillance of those aerial flanks is the first requirement of an effective fence. Unfortunately, comprehensive aerial surveillance is exactly what the United States lacks.
From the 1950s onward, the main U.S. and Canadian homeland air radars (the Distant Early Warning Line and its replacement, the North Warning System) have been oriented to detect Soviet or Russian bombers approaching over the North Pole, leaving the coasts and internal airspace largely unguarded. The end of the Cold War put the brakes on any new investments in America’s aerial fence, with then-Chairman of the Joint Chiefs of Staff Gen. Colin Powell declaring in 1993 that the United States no longer needed a dedicated air defense force. While the 9/11 terrorist attacks understandably led to major investments in transportation security, homeland air defense against military attack remained a low priority. Air defense investments that the Defense Department did make were intended for U.S. forces involved in distant “regional” conflicts, inspiring the rueful title of a 2022 paper, “North America is a Region, Too.” The department has moved in recent years to develop over-the-horizon radars capable of detecting aerial threats to the United States, but tangible investments have been deferred to the point that these radars are unlikely to become operational before 2030.
Consequently, the Federal Aviation Administration — not the Department of Defense — provides the lion’s share of the surveillance infrastructure for the “world’s largest and busiest airspace.” Air traffic radars, though numerous, are not ideally suited to detect modern threats. Mechanically rotating radars cannot scan fast enough to reliably track fast, low-altitude cruise missiles, which is why U.S. warships and missile defense batteries use electronically steered phased array radars. Additionally, the radar horizon prevents the detection of low-flying targets at a distance, leaving significant holes in U.S. air surveillance, especially below 5,000 feet, including several complete gaps in radar coverage below 1,000 feet along every coastline. This is precisely the vulnerability that Chinese and Russian cruise missiles and drones could exploit.
Moreover, U.S. air traffic control radars are aging, with some being more than 70 years old. The Federal Aviation Administration has embarked on a modernization program that will reduce reliance on radar in favor of a transponder-based system called the Automatic Dependent Surveillance-Broadcast. This requires that both military and civilian aircraft transmit their own GPS position data to receivers on the ground. As a result, the Federal Aviation Administration will retire 14 percent of the terminal radars that monitor airspace near airports by the end of 2025, adding yet more holes to the aerial fence.
Despite their limitations, these air traffic control radars do in fact make an important contribution to U.S. air surveillance. Yet, the Department of Defense’s command-and-control system can ingest air tracks from only 40 percent of them, requiring operators to use separate military and Federal Aviation Administration systems simultaneously to monitor the same airspace. The fence therefore relies not on the latest AI algorithms, but on operators “yelling missile status updates” to each other inside an operations center. Moreover, since air defense “shooters” — primarily aging F-15 and F-16 fighter aircraft — are spread thin, any delay or errors in integrating the air picture could do great harm as operators work to vector scarce interceptors toward rapidly closing targets.
Arrayed against this flimsy air defense shield is a growing range of airborne threats. Perhaps the most severe is Russia’s combat-tested Kalibr cruise missile, launched from Severodvinsk-class submarines — and soon to be a continuous presence off U.S. shores. China, too, is developing submarine-launched land attack missiles. The speed, stealth, and terrain-hugging flight of modern cruise missiles make them difficult to detect. Yet, they are not invulnerable: Ukraine’s wartime experience shows that such missiles can be shot down by determined defenders — but only if they are detected in time.
Drones pose similarly vexing challenges to air defense. Stealthy, long-range variants such as China’s GJ-11 and Russia’s S-70 could be used to attack U.S. territory. Adversaries could also attack using smaller platforms such as the Iranian Shahed-136 that Russia has employed in Ukraine, perhaps launched from ships operating in the U.S. littorals or even within the homeland. The dazzling drone-based light show that kicked off the 2022 Beijing Winter Olympics portends the kind of militarized drone swarms that China is rapidly developing.
Watching the Skies, Together
The United States should focus more eyes on the sky, and it should do so quickly. The Department of Defense cannot afford to wait decades for its acquisition system to deliver the perfect solution. Fortunately, civil-military air defense cooperation has been a longstanding American tradition. During World War II, federal, state, and local authorities cooperated to deploy hundreds of thousands of air raid wardens to supplement Army air defenders. After the war, the Defense Department initially estimated a need for 160,000 Ground Observer Corps volunteers to scan the skies for Soviet bombers. When the Soviet Union acquired the atomic bomb, the Air Force redoubled Ground Observer Corps activity, and more than 300,000 civilian volunteers helped to provide 24-hour, year-round visual surveillance until 1958, when the first national air defense network came online.
Today, as in the past, the Department of Defense could achieve rapid and meaningful improvements by harnessing existing civilian resources. This is not an entirely new idea. For example, a 2022 Center for Strategic and International Studies report claims that civil sensors “could supply raw data, which when fused with others nationwide and processed through machine learning, could identify anomalous activity and nominate possible tracks.” Indeed, this vision of AI-enabled air surveillance is implicit in the Defense Department’s plan to link all sensors to all shooters through its Combined Joint All-Domain Command and Control concept. Unfortunately, this capability will likely take years to come online.
Moreover, exquisite battlespace awareness at the headquarters level does not automatically yield total battlespace awareness for air defense shooters, which, with few exceptions, cannot receive raw data from offboard sensors. Thus, any AI-generated air surveillance data must be translated into standard data link message formats to provide essential target data such as position, altitude, course, and speed to front-line shooters.
Fortunately, there is no need to wait years for an ideal but hard-to-implement solution. With little more than a modest investment in software development, along with link terminals that cost less than $250,000 apiece, the U.S. military could consolidate and verify tracks generated by civil sensors, then distribute them over standard data links using procedures familiar to U.S. air defenders in cockpits and command centers across the force. Rather than waiting for AI to produce a detection capability that could be achieved sooner and more affordably with off-the-shelf capabilities, those more advanced techniques could be focused on the tougher challenges of target classification, identification, and prioritization.
A Land of Opportunity
As quickly as possible, the Department of Defense should deploy the ability to generate air tracks from civil sensors at the source, rather than in still-hypothetical data fusion clouds, and to incorporate those tracks into the air defense picture using off-the-shelf data link hardware. Specifically, track data could be extracted from civilian sensor network nodes, or directly from individual sensors themselves, using algorithms tailored to each sensor, run on computing systems that the Defense Department could provide at negligible cost using off-the-shelf components. These tracks could then be incorporated into the air defense picture via Link 16, using equipment already mass-produced for U.S. and NATO forces.
The first priority is to incorporate all of the Federal Aviation Administration’s more than 600 air traffic control radars into the homeland air defense network. In addition, weather radars such as the Federal Aviation Administration’s Terminal Doppler Weather Radar and Next Generation Radar could make a substantive contribution. Designed to detect near-ground weather hazards, these radars could help to fill the low-altitude surveillance void that currently exists.
A potential limitation of weather radars is that they prioritize extremely fine target resolution rather than the rapid search rates needed to detect fast-moving threats. One such radar, for instance, can “determine the shape of a 6-millimeter raindrop from more than 8 miles away.” Fortunately, the precision/speed tradeoff can be adjusted with software modifications: Weather radars could be programmed to scan more rapidly, though at a cost in precision. While this would result in degraded weather-forecasting performance, it could be a reasonable tradeoff under attack.
Regarding maritime sensors, the National Oceanic and Atmospheric Administration uses the Integrated Ocean Observing System to monitor ocean currents up to 200 nautical miles offshore. With suitable processing, high-frequency radars like this could be redirected to detect airborne objects at low altitudes. In fact, Canada has deployed a similar system to monitor its coastlines. The Defense Department should partner with the National Oceanic and Atmospheric Administration to extract air tracks from its maritime radars, and Canada’s co-leadership of the North American Aerospace Defense Command provides a fine opportunity to merge capabilities.
National Oceanic and Atmospheric Administration weather satellites could also produce air targeting data using processors at the downlink sites. Future weather satellites could be configured to detect solid objects on demand, transmitting data directly to the ground receiver. The proven rapid launch capabilities of the U.S. Space Force and the speedy development cycles of U.S. satellite manufacturers suggest that such modifications to weather satellites could be achieved in months rather than years.
Additional low-altitude aerial sensors could help too. U.S. Customs and Border Protection employs a variety of sensors to detect illicit air traffic approaching the United States, including the Tethered Aerostat Radar System, which is optimized for detecting low-altitude threats. Associated terminals are installed at North American Air Defense Command centers, but are not integrated electronically — leaving operators once again to shout updates to one another. The Department of Defense should field a translation capability to bring Customs and Border Protection data into the common air picture. Since the terminals are already physically co-located, this might not even require dedicated algorithms or link hardware — only a simple network interface.
The U.S. military also partners with numerous universities to fund cutting-edge research facilities such as the University of Oklahoma’s Advanced Radar Research Center. Research radars, some of which might provide exquisite capabilities, could easily be enlisted in a “Radar Observer Corps.”
Finally, the Defense Department could take advantage of the ubiquitous emissions from cellular towers, radio and television stations, and even geostationary satellites such as those of Sirius XM satellite radio. NATO-sponsored researchers have demonstrated that low-cost, off-the-shelf recievers combines with simple processing can detect small aerial targets via reflected satellite TV transmissions. Further, the 5G sector is rapidly deploying multi-static beamforming technology to improve connectivity by detecting airborne obstructions and rerouting beams around them. Collaboration between the Department of Defense, Federal Aviation Administration, and U.S. telecommunications companies could produce a nationwide ability to pull tracks from the 5G airwaves.
Move Out!
The United States is not currently prepared to face a growing number of national security threats and challenges, including from the air. Lacking a comprehensive homeland air defense network, the Department of Defense should move with urgency to mend America’s hole-riddled aerial fence. Affordable and realistic fixes could be achieved by integrating civil and military sensors into a better aerial threat common operating picture.
There are a number of steps the Department of Defense can take now. First, it should assess civil sensors to determine which provide the most value, and prioritize and pursue civil partnerships accordingly. As part of this effort, it should review the Federal Aviation Administration’s Radar Divestiture Program and determine whether radars slated for decommissioning should be retained, using Defense Department funds if necessary.
Second, the department should review authorities for military employment of civil systems in wartime, and recommend changes where appropriate. Federal law provides for the Coast Guard to be transferred to the Defense Department during war. Executive orders and law establish similar measures for the Federal Aviation Administration and National Oceanic and Atmospheric Administration. More such arrangements may be necessary.
Finally, it should develop software to extract track data from civil sensor data and deploy off-the-shelf hardware suites, with associated funding, personnel, and training for installation, maintenance, and operations. This would require relatively little effort. Indeed, the entire project might be delegated to a government laboratory or defense contractor.
In both World War II and the Cold War, hundreds of thousands of civilian volunteers turned up to help plug the holes in America’s aerial fence. Today, civil technology could play a similar role in securing the skies. If history is any indication, the American public will answer the call to action.
Thane C. Clare is a senior fellow at the Center for Strategic and Budgetary Assessments.
Image: Lt. Scott Handlin via DVIDS.
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