The Pentagon’s missile defense review is now underway, incorporating mandates from both the White House and Congress. One of its considerations, at presidential direction, is whether there should be a relative “rebalancing” between homeland and regional missile defense. Regional defense has received a relatively greater share of the Missile Defense Agency (MDA) budget over the past decade, so increased emphasis on protecting the homeland may well be in order, especially given missile advances by North Korea, including the launch of a Hwasong-14 ICBM earlier this week. But another, crosscutting kind of rebalance may be even more important to consider: a rebalancing of the sensor architecture to the space domain. Fielding a space-based sensor layer would provide unique improvements to homeland and regional missile defenses alike.
Sensors and ground systems do not as easily capture the popular imagination, so they do not get as much attention as the interceptors themselves. Commentators typically tend to focus on the numbers and characteristics of interceptors, and understandably so. For the homeland mission in particular, improvements to the reliability, capability, and capacity of ground-based interceptors are certainly in order. Interceptors are only as good as the sensors that tell them where to go and what to kill. We currently rely almost entirely upon sensors based on sea or land. A space layer would add unique capabilities not merely to this or that interceptor, but across the entire ballistic missile defense system—including Patriot/PAC-3, THAAD, Aegis, and Ground-based Midcourse Defense (GMD).
The utility of space for this mission is no secret. Each of the last five administrations has had space-based infrared sensors as a key component of its planned missile defense architecture, at least on paper, but none has yet deployed one. Ground- and sea-based radars will and should continue to play critical roles, but the first and possibly most important step to improving global missile defense capabilities would be to create and field a space sensor layer for persistent, birth-to-death missile tracking and discrimination.
The Defense Support Program and space-based infrared satellites in high orbit provide strategic warning that a missile has launched, but once a missile’s motors stop burning, other sensors are required to track the missile’s trajectory more precisely. Right now, that means ground and sea-based radars. Two demonstrator satellites called the Space Tracking and Surveillance System are currently on orbit, but they do not operationally contribute to the mission. For that, a larger or alternative constellation would be needed. Today, five TPY-2 radars are deployed in forward-based mode: two in Japan, one in Turkey, one in Israel, and one with U.S. Central Command. The floating Sea-based X-band radar has a powerful if focused view, but it cannot always be on station, and will not be around forever. Several Cold War early warning radars have been upgraded to tie into missile defense command and control. Aegis missile defense ships can also support the tracking mission if they are in the right place at the right time. Finally, an S-band long-range discrimination radar under construction at Clear Air Force Station, in Alaska, will come online around 2020, narrowing but not fully closing the “midcourse gap” over the Pacific.
The deployed and planned future sensor architecture will fall short, however, especially as missile threats become more numerous and complex. Terrestrial radars have considerable advantages, but they also have limitations. Even a proliferation of ground-based radars would be limited to a single technology, and their single upward-looking perspective. The curvature of the earth restricts the field of view of even the most powerful radars based on sea or land, thereby requiring numerous and geographically dispersed radars, with a substantial number forward-based. But there are only so many islands in the Pacific to put such radars, let alone other oceans. The emerging threat of boost-glide vehicles skimming atop the atmosphere could further circumvent much of this terrestrial radar coverage.
So what is the advantage of space sensors? Simply put, they provide a better vantage point, more persistence, and a different kind of look at the threat cloud created by a missile in midcourse. Infrared satellites in low-Earth orbit, or even high altitudes, can view the threat cloud from the side, observing not merely the radar cross section of the warhead and other objects but also their heat signature contrasted against the cold backdrop of space. Combining terrestrial radars below with infrared/electro-optical eyes orbiting above would dramatically help interceptors find their target.
Anything in orbit is subject to efforts to blind or destroy it, no military domain is a sanctuary. For this reason, resilience and redundancy would naturally be key criteria for such a layer. Space-basing has the benefit of avoiding air defenses, missile strikes against ground-based radars, and other area-denial threats. Orbits are furthermore unconstrained by host nation agreements. Forward-based radars also involve numerous operational and political considerations, evidenced by China’s recent objections to a single TPY-2 radar in South Korea. As Russia and China like to remind our partners, such large radars are themselves targets.
One obstacle has been cost, but several innovative concepts are being explored to enable less expensive launching and operation. MDA’s Space-based Kill Assessment experiment aims to put a sensor payload on a commercial satellite to help determine whether interceptor missiles have destroyed their target, or whether another salvo is needed. This or some other model could help defray launch costs and lead to a larger, more distributed, and more resilient constellation of smaller and less-expensive satellites.
The GMD program had a successful intercept test May 30 against an ICBM target. It was also the first intercept test for the newest configuration of the kill vehicle, clearing the way for eight new interceptors to be deployed to meet the Pentagon’s directive to deploy 44 ground-based interceptors by the end of this year. After that test, the Office of the Director of Operational Test and Evaluation took the unusual step of updating its formal assessment of GMD, upgrading it from “limited capability” to “demonstrated capability to defend the U.S. homeland from a small number of intermediate-range or intercontinental missile threats with simple countermeasures.” Capacity growth is important, but sensor and other improvements for discrimination are necessary to outpace a more complex and growing threat scene.
Without a space layer, U.S. missile defenses will have a harder time adequately distinguishing warheads from debris or decoys, which means that more interceptors will need to be fired to ensure a kill, in turn reducing the effective supply of interceptors. Even 44 ground-based interceptors could be challenged by a larger missile salvo because of the number of interceptors that would need to be fired at the threat cloud produced by an ICBM. Space sensors would better address the discrimination challenge, helping to deal with decoys and countermeasures, and reducing the number of interceptors that would have to be fired at any given missile.
Momentum seems to be growing around such a path. “Given where the threat is going,” MDA director Vice Admiral James Syring said last year, “persistent tracking and discrimination capability from space is a must.” A campaign memo for then-candidate Donald Trump last year pledged to create “a comprehensive ballistic missile defense system with a heavy emphasis on space-based early warning and missile tracking technologies.” A recently introduced bipartisan bill in the Senate singles out space sensors as an important capability, and would direct MDA to produce an acquisition plan. The benefits of such an architecture have also been the topic of a series of articles and reports from the CSIS Missile Defense Project.
As the forthcoming missile defense review considers rebalancing the missile defense enterprise, it should consider not merely the tradeoffs between the homeland and regional defense parts of the budget pie, but also between the Earth’s surface and above it. Every interceptor program fielded today by the United States could benefit from such a constellation, as might those of allies and partners. It may not sound like the most attention-grabbing proposal to improve missile defense capability, but it would be among the most significant. A space sensor layer has thus far existed almost entirely on paper. Now is the time to make it a reality.
