Why Auctioning Military S-Band Spectrum Is a Bad Idea

Electromagnetic spectrum is a scarce national resource with significant implications for national security and economic growth. The S-band, spanning from 2–4 gigahertz (GHz), is a crucial frequency for air and missile defense radar. Over the years, large portions of what were previously military bandwidths have been auctioned off. Some telecommunication associations and Chinese-dominated international telecom bodies are now pressing the Federal Communications Commission (FCC) to auction off the low-3 range of the S-band.

The low 3 S-band (3.10–3.45 GHz) is one area that has been preserved for military use alone, and for good reason. For air and missile defense, the S-band is a Goldilocks zone within the electromagnetic spectrum: low enough to propagate freely through rain and other forms of weather, and high enough to accurately classify and track inbound missiles, drones, and aircraft. Preserving continuous and incumbent military use of the low-3 S-band is necessary for the operation of numerous defense assets, including the Golden Dome initiative. The risks of an S-band auction have been widely recognized by congressional, military, and senior defense officials across both the Biden and Trump administrations. Similar issues apply to space and intelligence applications in the 7-8 GHz band.

Advocates of selling the low-3 have proposed transitioning military radars out of the S-band at the cost of hundreds of billions of dollars, years of delay, new layers of bureaucratic procedures, and the disruption of decades of intelligence gathering about how missiles appear to radars operating in the S-band. Auctioning off the low-3 S-band spectrum is a bad idea.

Civilian use of this bandwidth for 5G would disrupt the operation of critical U.S. air and missile defense radars, including the Navy’s Aegis SPY family, the Army’s TPQ-53, the Marine Corps’ Ground/Air Task-Oriented Radar, and the Space Force’s Long Range Discrimination Radar. High-power radars that must track faint missile signatures thousands of miles away would be adversely affected by ambient noise from telecom use. Notably, China does not permit outdoor commercial use of the low-3. Those countries that do allow it degrade the ability of S-band radars to operate in their defense.

Several legislative proposals have emerged to auction off the low-3, and there is a debate underway about the prospect of sharing. The House-passed reconciliation bill included a provision to reauthorize the FCC’s spectrum auction authority through 2034. The bill mandates that the FCC identify and auction 600 megahertz of mid-band spectrum for exclusive, licensed use in mobile or fixed broadband services. Fortunately, the legislation in its current form explicitly excludes certain frequency bands from this auction requirement, notably the 3.10–3.45 GHz band.

Language has more recently emerged from the Senate that would explicitly protect both the 3.10–3.45 S-band and 7.4–8.4 GHz in the X-band while authorizing the auction of 500 MHz from other bands. Besides the difference with the House version, the issue is not yet settled. Both in the near term and the future, debates are nevertheless expected to continue regarding the scope of the spectrum auction authority and the prospect for sharing these bands. These debates will be better informed by clarifying the basic trade-off between commercial and military use, including for missile defense radars.

Auction advocates tout the Citizens Broadband Radio Service (CBRS) as an example of sharing, but CBRS has been an effective sharing system precisely because national security assets can compress from the mid-3 to the low-3 band when they must share the 3.5–3.7 GHz band. If the low-3 is sold, the Department of Defense will have nowhere else to go.

Understanding the needs of the missile defense mission provides a better appreciation for why the military must keep the undisrupted use of 3.10–3.45 GHz and why proposed spectrum sharing messages are not currently viable.

Nearly all missile defense engagements have discrete functions that a radar must perform: continuous surveillance, threat detection, classification and identification compared to a threat library, tracking trajectory precisely enough to develop an interceptor solution, discriminating between lethal and nonlethal objects, and sending updates to interceptors in flight.

Rigorous testing has shown the incompatibility of 5G and high-powered radars; the former has to be off for the latter to be on. Proposals for dynamic sharing fail to account for the need to continuously surveil airspace. The CBRS approach to this has not proven effective, as it requires a central clearinghouse to adjudicate power switching. Even assuming that switching could work reliably, an enemy will not give advance notice about when an attack will come. For the foreseeable future, unproven proposals for “dynamic sharing” remain impracticable. The potential should continue to be studied, but no such capability has yet been demonstrated to overcome the national security threshold.

Auction advocates have said that alternative technologies, like infrared or thermal sensors, could substitute for S-band radar. While infrared sensors hold promise for tracking hypersonic glide vehicles, they are less useful for ballistic missile warheads or low-flying cruise missiles. Both for discrimination and developing fire control–quality tracks sufficient for interception, it is difficult to substitute for the range, power, and resolution that comes from depositing radar energy onto an incoming missile and having it reflected.

It has been suggested that artificial intelligence (AI) can somehow solve the problems of sharing. AI and machine learning hold significant promise for missile defense, especially for threat classification and discrimination of a threat cloud. AI cannot, however, overcome the need for radar or the interference issue.

Despite claiming that dynamic sharing will solve the jamming problems, telecom advocates have recommended that military radars be redesigned and rebuilt to operate in an entirely different part of the spectrum. This is especially impractical. Redesigning radars and building new ones would be prohibitively expensive, take many years, and may not even be possible.

Even if it were possible, however, it would likely incur losses in operational effectiveness. Vacating the S-band would forgo the specific ability of that bandwidth to send and receive radar energy through weather. X-band radars, for instance, have much lower weather penetration. It would also degrade the threat classification function, which relies on a library populated by decades of observations of how missile signatures appear in the S-band.

In January 2018, thousands of Americans in Hawaii received a text message that there was an incoming ballistic missile from North Korea headed to Hawaii. Fortunately, the text message was only a test that was inadvertently sent. If that incoming threat missile had been real, however, it would have been small comfort to U.S. citizens to receive the text message slightly sooner thanks to boosted 5G, if it meant that U.S. missile defense radars were blinded, unable to track the incoming threat and do something about it.

In short, auctioning off the low-3 S-band spectrum remains a bad idea. Commercial use of this bandwidth for 5G risks jamming military radars and degrading missile defense capability at a time when air and missile threats have never been worse. For the foreseeable future, national security needs counsel that the United States preserve exclusive and incumbent military access to the low-3 band while continuing to study the long-term feasibility of spectrum sharing.

Thomas Karako is the director of the Missile Defense Project and a senior fellow with the Defense and Security Department at the Center for Strategic and International Studies in Washington, D.C.