Beyond the Radar Archipelago: A New Roadmap for Missile Defense Sensors


In missile defense circles, commentators frequently remark that that there are only so many islands or ships in the Pacific on which to put radars. Reading through recent missile defense budget requests, however, one is struck by the fact that the Pentagon seems to have doubled down on a strategy of building a chain of sea- and ground-based radars, both on Pacific islands and elsewhere.

Call it the radar archipelago.

The expansion of long-range missile defense sensors over the past 16 years has, with some exceptions, been nearly synonymous with a gradual increase of large, surface-based radars. And this virtual island chain of radars is growing. In addition to a nearly complete radar in Alaska, $2.5 billion has been allocated over the next five years for the construction of two Pacific radars just to address the threat from North Korea. Once built, these will supplement the current handful of terrestrial radars that include one at Clear, Alaska, the floating Sea-Based X-Band Radar, and two additional ground-based radars in Japan.

Although well-enough suited to limited ballistic missile threats, a thinly layered sensor architecture with many single points of failure is ill-equipped for the specter of complex and integrated air and missile attack. In short, today’s architecture is all too susceptible to suppression.

The joint force faces a more complex and contested aerial threat environment than ever before. Threats have become more diverse, including drones and cruise missiles that can get around sectored sensors with a limited field of view, maneuvering ballistic missiles, radiation-seeking missiles, and hypersonic glide vehicles. As seen with tactics and techniques employed in Yemen, Syria, and Ukraine, sophisticated adversaries attack from various directions, altitudes, and velocities — and combine with electronic countermeasures to degrade radars.

Meeting this challenge requires a new roadmap towards a more distributed, diverse, layered, and survivable missile defense sensor suite.

Today’s Sensor Shortcomings

When it comes to missile defense, it’s all about sensors. An interceptor is only as good as the sensors that tell it where to go and what to kill.

Today’s missile defense sensors have several major limitations: a relatively small number of dedicated assets with high-emission signatures that can be easily identified and themselves targeted; sectored coverage that makes it possible for enemy missiles to get through; over-reliance on one phenomenology, namely radiofrequency or radar; surface-basing limited by the geographic horizon; insufficient force protection of high-value assets against asymmetric threats like unmanned aerial vehicles; and insufficient integration with non-dedicated sensor assets.

These shortcomings represent gaps that adversaries can exploit. To borrow a phrase from Gen. John Hyten, head of U.S. Strategic Command, reliance on any small handful of assets makes them “juicy targets.”

The Way Forward

The roadmap for a more capable and survivable sensor architecture should incorporate at least five characteristics: domain rebalance, with a more extensive use of platforms at high altitudes and in space; disaggregation and dispersal of more numerous, smaller, and cheaper sensors; diversity of technologies and use of a wider range of the electromagnetic spectrum, especially with passive and low-emitting sensors; more integration of sensor data from non-dedicated tactical assets; and advanced radar capabilities and operations.

Domain rebalance

Surface radars look out and up, but the threat requires sensors that also look down and across.

The first sensor shift should be a domain rebalance, a shift from today’s near-total reliance on surface-based assets to a much broader mix of platforms in a variety of domains; specifically, a shift upwards to platforms in the air, at high to very high altitudes, and in orbit.

The top priority remains a space-based sensor layer for persistent, birth-to-death tracking and discrimination. Each of the past six administrations has has been committed on paper to the utility of a space layer for long-range missile defense — but one has not yet been fielded. The time for studies is over.

A mix of elevated assets provides numerous benefits. Orbits are fixed and predictable, so unmanned fixed-wing aircraft and lighter-than-air vessels can make a substantial contribution in addition to the space sensor layer. High-altitude platforms can make up in mobility, speed of fielding, and cost what they cannot provide in global coverage. Australia’s E-7A Wedgetail, for instance, has demonstrated significant potential for air defense applications. Especially for cruise missile defense, some alternative overhead solution is needed to replace the Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System (JLENS) aerostat radar.


The architecture must also be more disaggregated. For the U.S. Navy, distributed lethality (or distributed maritime operations) complicates the targeting and surveillance problem of an adversary by distributing strike assets on everything that floats. The same applies to sensors: Broadening the attack surface improves survivability and resilience, presenting an adversary with many aimpoints rather than few. But with resources being finite, we cannot simply multiply the number of large and expensive assets. A set of more numerous, smaller, cheaper targets would make it considerably harder for an enemy to defeat them all.

With respect to space sensors, Under Secretary of Defense for Research and Engineering Michael Griffin recently stated, “I want us to be as widely distributed over as many choices of orbital regime as we can effectively use…to pose the adversary with such a difficult problem that they will choose not to fight.” Sensors in both low- and medium-earth orbits are more capable and survivable than a constellation at one altitude.

The disaggregation logic also applies to terrestrial assets. As the National Defense Strategy notes, the survivability and resilience of the force requires a transition “from large, centralized, unhardened infrastructure to smaller, dispersed, resilient, adaptive basing.” The multiplication of sensors within each domain likewise complicates an adversary’s surveillance and targeting. Adding appropriate active decoy systems and other means of deception would further complicate an enemy’s battlefield awareness. In short, distribution forces an adversary to engage in its own sort of Scud hunting, which is never easy.

Whether on the surface, in the air, or in space, the principle of distribution should be widely applied to missile defense sensors and interceptors alike.


Third, the sensor architecture should be more technologically diverse. In little-noticed remarks earlier this year, Vice Chairman of the Joint Chiefs of Staff Gen. Paul Selva warned about overreliance on radiofrequency for communication and command and control: “It doesn’t have to be a [radiofrequency] game. It’s an RF game because we choose to make it so.”

The same goes for sensors. Radar has been around nearly a century, and adversaries have had decades to monitor and develop countermeasures to air and missile defense radars. There will always be a place for radar to cut through weather and sharply illuminate a target, but non-radiofrequency emitters, including lasers, would improve tracking and discrimination and complicate targeting, surveillance, and countermeasure tasks for an adversary.

The future sensor set should prioritize sensor types that are passive, lower-emitting, and harder to negate. Electro-optical, infrared, lasers, and other forms of directed energy have benefits against certain threats and may have much lower emission signatures.

The multispectral targeting system on the MQ-9 Reaper is a kind of poster child for more diversified, elevated, and lower-emitting sensors. A constellation of high-altitude, long-endurance unmanned aerial vehicles with various sensor payloads could be operated over the Pacific to both fill gaps in radar coverage and buy time until a space layer is orbited. Unlike ground-based radars and orbiting sensors, their locations would not be easily predictable.

(Source: CSIS Missile Defense Project)

Opportunistic integration

Another key improvement lies with integration: the idea of opportunistically fusing sensor data from non-dedicated platforms across the joint force.

Today’s missile defense architecture relies on a fairly static, closed set of dedicated sensor assets that are more or less assigned to the mission. An alternative approach would be to accept a bit more day-to-day risk while anticipating increased capability in the event of a crisis. Elevated tensions with, say, North Korea would result in aerial or maritime sensors surging to the area. Platforms that are not assigned to the missile defense mission, say, an artillery radar or F-35s in the area, may very well pick up a missile launch, acquiring information that should then be relayed to air defense commanders.

Requiring missile defense command and control to integrate sensor data from a much wider set of sources would be a sort of culture change for the missile defense community. But it would also be a concrete application of the National Defense Strategy’s approach to “dynamic force employment.” Several months ago, Missile Defense Agency Director Lt. Gen. Samuel Greaves gave voice to this need: “Our job is to look outside of the classic missile defense system…and look for sensors and shooters that would be able to contribute when integrated into the [Ballistic Missile Defense System].”

The Army’s Integrated Air and Missile Defense Battle Command System and the Navy’s Cooperative Engagement Capability have made progress in fusing disparate sensor data into a single air defense picture. But much more should be done, especially with non-radar sources and those employing different frequencies, waveforms, and timing characteristics. The Navy has experimented with integrating the F-35 Multifunction Advanced Data Link into its Aegis Combat System, but this has not yet translated to an operational configuration that can be fielded. Wringing additional data out of existing sensors would yield significant improvements in situational awareness and capability.

To realize this vision, the Command and Control, Battle Management, and Communications network — which manages all elements of the Ballistic Missile Defense System — will require substantial improvement to connect with non-dedicated sensors. Given that the spectrum of air and missile threats differ dramatically by trajectory, range, and altitude, it is difficult to orchestrate engagements from today’s battle management structure. Further challenges include how to move between centralized and federated command and control for the engagement of threats that are very close or half a world away.

Better radars

Finally, radars should perform better. Surface-based radars will remain a critical element of the missile defense architecture for the foreseeable future. Given the Pentagon’s urgency to increase the speed at which new capabilities are fielded, marginal improvements here could yield relatively more substantial improvements in the near term.

A sort of radar renaissance is underway, the results of which are only now beginning to reach the field. Many radars fielded today date to the 1970s and still use vacuum tubes. But emerging solid-state radar technologies, scalable construction, and increasingly digitized concepts hold considerable promise for more efficient energy use and beam direction, the ability to use multiple frequency bands, advanced waveforms to support multiple missions, and improved resistance to jamming and other countermeasures. With digitization, of course, comes the perennial need for robust cyber protection.

Low-hanging fruit in the area of radar improvement includes incremental changes to the program of record. Long-serving Aegis SPY-1 radars could benefit from near-term modernization, but the Navy could also accelerate the SPY-1’s planned replacement with the SPY-6, both afloat and ashore. Today’s Patriot and THAAD radars also need upgrading.

The Pentagon should also explore more imaginative concepts. In contrast to today’s reliance upon fixed radars that both emit and receive pulses of energy, enormous potential may be found with network cooperation and extensive use of semi-active, bi-static, or multi-static configurations. In these configurations, some elements would emit energy, but most would be passive. Smaller and less capable radars would acquire resilience through numbers on the principle of distribution, whereas the handful of larger ones could have dedicated force protection against aerial threats. Swarms of cheap, disposable emitters could be used to illuminate targets, the reflected signals from which other passive receivers would then detect. High-value emitters must be protected or disguised, but lower cost emitters and passive receivers might not be.

Another option is to disperse radar modules — the elements of a radar that emit and receive energy — and then coherently integrate their returns. One might, for instance, take apart the modules of a Long Range Discrimination Radar or a SPY-6 and distribute them across an area. Instead of one big, self-designating target, an adversary would have to pick among many targets to discern critical elements to attack. Although coherent integration of disaggregated beams presents significant challenges for precision timing and computing, the boost to survivability and resilience could be tremendous. Large radars will surely retain an important place, but the overall landscape may be more of a high-low mix: a few high-end assets working together with many more of lesser capability.

It is also time to decisively move away from sectored radars — those with a limited field of view. The radars for Patriot, THAAD, and Ground-based Midcourse Defense are almost entirely sectored, but air and missile threats come from all directions. Taking a page from the Aegis Combat System, the default expectation should be that all dedicated air and missile defense radars have 360-degree coverage to prevent being attacked from behind. Current plans for the sectored Hawaii radar should be adjusted so it can later be expanded to have radar faces in all directions, rather than only one. And whether for budgetary reasons or urgency to field, the Army should reconsider its apparent decision to go soft on relaxing 360-degree coverage for its Low Tier Air and Missile Defense Sensor. Plans to upgrade the capability of the TPY-2 radar for THAAD batteries might also incorporate some means to supplement its 180-degree coverage.

The Sensor Archipelago

The roadmap that brought about the radar archipelago supporting today’s Ballistic Missile Defense System was designed in a world focused on limited ballistic missile threats. The future will require much more than just big radars on islands and ships. The image of an archipelago may, however, remain a useful guide, for it points to the crucial principle of distribution.

According to press reports, the Pentagon is on the verge of releasing its Missile Defense Review. Adapting today’s sensor architecture will be one of the most critical steps to reorient U.S. missile defenses to the complex realities of air and missile battle. Such an adaptation would benefit from more elevation, disaggregation, diversity, integration, and advanced capabilities.

Whether the forthcoming policy review endorses these characteristics remains to be seen, to say nothing of the budgetary and programmatic implementation needed to realize this kind of vision. But if the U.S. missile defense posture is to be reoriented to near-peer adversaries, it will require a radically different sort of sensor archipelago.

Thomas Karako is a senior fellow and director of the Missile Defense Project at the Center for Strategic and International Studies.

Image: Chief Petty Officer Brandon Raile