Northrop Grumman-built Space Tracking and Surveillance System (STSS) demonstrator satellites are on-orbit, demonstrating capabilities required for birth-to-death tracking of ballistic missiles and other cold objects in space. Credit: Northrup Grumman artist’s concept

Ballistic missiles travel in space, and the missile defense task is by definition largely a challenge in and through the space domain. For all but very short range missiles, a considerable part of the ballistic trajectory is spent in space, after the motors burn out and before the warhead re-enters the atmosphere. Exoatmospheric midcourse intercept is the exclusive realm for two of the four currently deployed U.S. missile defense programs, Aegis Standard Missile-3 and Ground-based Midcourse Defense.

Space is thus the place for a variety of missile defense tasks — including launch detection, tracking, discrimination, intercept, and kill assessment. Space-based sensor concepts have been underway since the beginning of the missile age, from the early Missile Defense Alarm System, to Brilliant Eyes, to more recent efforts such as the Space Tracking and Surveillance System demonstrators. It is therefore unfortunate that U.S. funding for space- and near-space missile defense assets is at an all-time low. It may be time to reverse that trend and renew efforts for a space sensor layer. The concept of a “layered” defense applies, after all, not just to interceptors, but also to sensors.

To intercept a missile in its midcourse phase, one must detect its launch, track its flight, and then differentiate or “discriminate” the threatening warhead target from any countermeasures and from the flying junk pile of debris created from launching it. For launch detection and warning, the United States relies on the 1970s-era Defense Support Program, two Space-Based Infrared System-GEO satellites, and two highly elliptical SBIRS payloads. Tracking tells the interceptor and other sensors where to look, and discrimination determines what interceptors need to kill. In the 1960s and 1970s, the way to compensate for discrimination shortcomings was with nuclear-armed interceptors, which besides frying satellites within line of sight would also damage the defenders’ own radars. Nobody wants to go back to that, and missile defense efforts have for decades focused on hit-to-kill.

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The quality of discrimination directly relates to both shot doctrine and effective inventory capacity. In the absence of better discrimination, a larger number of kill vehicles must be fired at the threat cloud to overcome uncertainty. If several North Korean missiles each produce an insufficiently discriminated threat cloud containing multiple targets that must be engaged, and if several interceptors are assigned to each target, the available Ground-based Interceptor inventory might soon be consumed.

One way to improve tracking and discrimination is to expand the current terrestrial network with more and different kinds of radars, efforts for which are underway. Today, five TPY-2 transportable radar surveillance and control modules 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 an extremely powerful but focused view. A wider-looking S-band long-range discrimination radar under construction at Clear Air Force Station, Alaska, will come online around 2020, helping track and discriminate incoming missiles, narrowing (but not fully closing) the “midcourse gap” over the Pacific.

With long-range discrimination radar and other improvements, the U.S. Missile Defense Agency is taking a number of shrewd steps to improve tracking and discrimination at the margin. Additional sensors to close the midcourse gap and sharpen discrimination would, however, be beneficial. By way of comparison, the Clinton administration’s proposed Capability-3 national missile defense plan included nine X-band radars co-located with Upgraded Early Warning Radars and elsewhere. A 2012 National Academies study recommended a similar increase in ground-based X-bands, including stacked TPY-2s co-located at each of the early warning radar sites and in the continental United States.

The deployed and currently planned future radar architecture still falls short by failing to incorporate a space layer of sensors. While terrestrial radars have considerable advantages, they also have inherent limitations. Even a proliferation of ground-based radars would be limited to a single technology, and its 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 a more substantial number to be forward-based. Forward-basing in turn carries operational and political considerations, evidenced by China’s recent objections to a TPY-2 transportable radar in South Korea. As our friends in Russia and China regularly remind our host partners, these large radars are themselves targets.

Given these factors, it’s time to reexamine the utility of a space layer of missile defense sensors. Infrared satellites in low Earth orbit or even high altitudes can look “sideways” at a threat cloud, viewing the heat of the warhead and surrounding objects not relative to the surface of the Earth but in contrast to the much colder backdrop of space. Combining terrestrial radars of various frequencies with infrared sensors would engage both the different vantage points and different technologies to identify what to shoot at.

The complementary relationship between space and ground-based sensor layers is no new discovery, and has long been identified as a logical way to address the missile defense problem. The above-mentioned Clinton-era Capability-3 architecture, for instance, included five SBIRS-High and 24 SBIRS-Low satellites in addition to nine X-bands and 250 ground-based interceptors.

Although space-based assets would potentially be subject to counterspace efforts to blind or destroy them, no military domain is a sanctuary. 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.

For the time being, two Space Tracking and Surveillance System (STSS) demonstrators in low orbit are also available to support the tracking mission, but these are likely to soon go away. STSS is an attempt to provide “birth-to-death” tracking of a missile, from launch to intercept. Besides supporting Ground-based Midcourse Defense, in a 2013 test STSS provided firing data to support Aegis launch-on-remote, significantly extending the reach of Standard Missile-3 interceptors. The STSS demonstrators were intended to be the beginning of a larger constellation, but that effort has unfortunately been discontinued

A planned follow-on, the Precision Tracking and Surveillance System, was also canceled in 2013. This program would have included a constellation of nine to 12 satellites carrying a larger telescope and a replacement for STSS’s gimbaled tracking system.

When the STSS demonstrators conclude their mission next year, there will be little left besides high and geosynchronous-orbit launch detection. The Missile Defense Agency currently has no plan for a follow-on program to get that valuable sideways infrared look. The basic STSS concept, however, still makes much sense.

A clear, cold vantage point from space provides benefits even without using satellites in orbit. Past tests of sub-orbital exoatmospheric sensors include the Midcourse Space Experiment and the Queen’s Match. The old Strategic Defense Initiative architecture included a Ground-based Surveillance and Tracking System element, essentially a popup rocket carrying sensors to provide a resilient if fleeting means to observe incoming threats when other satellites are degraded. A future Ground-based Interceptor carrying Multiple Object Kill Vehicles will benefit substantially from on-demand updates from ground radars, but a bus carrying a cluster of small kill vehicles might have a modular option, including one less kill vehicle in favor of an additional dedicated sensor.

Other characteristic benefits of space-based sensors might be had from airborne platforms at high altitude. The High Altitude Learjet Observatory experiment is one example of a past near-space sensor, as are other airborne infrared concepts. The Missile Defense Agency’s more recent pursuit of a persistent unmanned aerial vehicle near 18,000 meters also shows considerable promise as a platform for both tracking sensors and potentially boost-phase intercept using directed energy.

The utility of space sensors is nothing new, but getting there in today’s budget environment may nonetheless require imaginative thinking. This may include commercially hosted payloads like the Air Force’s CHIRP and the Missile Defense Agency’s Space-based Kill Assessment effort, or launching sensors in cooperation with other agencies. International cooperation and hosted infrared payloads on the satellites of Japan, NATO members, and other missile defense partners might further defray costs and offer a more diversified sensor base.

Besides tracking and discrimination, the space domain may invite other opportunities to counter missile threats. It may in particular make sense to restore the space test-bed that existed in the recent past, to explore the benefits of kill vehicles in low orbit. Pre-acceleration to orbital velocity could buy time and battlespace, adding defense depth and avoiding much of the midcourse discrimination problem altogether. Such a test bed might explore the potential for boost and early midcourse intercept options against limited threats.

Of course, there are no silver bullets and no all-seeing eyes. Just as land and sea-based platforms can be countered, space-based systems have their own vulnerabilities, such as from jamming, dazzling, or shadowing by microsatellite mines. Again, resilience against growing anti-satellite threats would also here be at a premium. Life-cycle maintenance, upgrades, concepts of operation, and repairs also present potential considerations.

The missile defense challenge crosses every military domain, but a healthy share of it involves the space domain. Various kinds of terrestrial radars provide significant capabilities, but also have inherent limitations, and incorporating additional sensor technologies from orbit or near-space platforms could contribute substantially to the missile defense mission. When a new administration takes a fresh look at programs and policy as part of next year’s overall missile defense review, a space sensor layer should be among its considerations.

Thomas Karako is director of the Missile Defense Project and a senior fellow in the International Security Program at the Center for Strategic and International Studies, a Washington think tank.

Thomas Karako is director of the Missile Defense Project and a senior fellow in the International Security Program at the Center for Strategic and International Studies, a Washington think tank.