No missile defense is better than the sensors that tell the interceptors where to go and what to kill. The Ground-based Midcourse Defense system, or GMD, draws upon considerably more sensors for homeland defense than when operations began in 2004, but shortfalls remain. The North Korean and other missile threats are not diminishing, and it’s time to get this right.

In a forthcoming report, we recommend that the Defense Department and Missile Defense Agency take several steps to improve the sensor backbone of America’s homeland missile defenses, including fielding a space layer, filling radar gaps, adding omnidirectional focus, and improving command and control. Unfortunately, the budget for missile defense sensors has fallen considerably over the past decade, exactly the wrong trend for our changing security environment.

Gaps in the Architecture

The sensors supporting homeland missile defense include both some of the newest and oldest ones currently operating. An enemy missile launch would be detected first by satellites tuned to spot the heat of a firing rocket motor. Yet their infrared sensors cannot track a missile much after its engine burns out, and do not in any case provide enough data to conduct an intercept.

For the demanding tasks of missile tracking and discrimination, the global Ballistic Missile Defense System largely relies on large ground-based radars: five upgraded Cold War-era Early Warning Radars; the Cobra Dane surveillance radar on Shemya, Alaska; and five newer forward-deployed TPY-2 X-band radars. When they are deployed to the right place, the Navy’s SPY-1 radars on Aegis ships and floating Sea-Based X-band radar, or SBX, also provide important help.

These radars help track incoming missiles in different ways, depending on their location and capabilities. Lower-frequency radars like the early warning radars are better for tracking over great distances, while the higher-frequency X-band radars are best at discriminating the warhead from accompanying debris. All these terrestrial sensors, however, are limited by the curvature of the earth.

To be sure, ground- and sea-based radars will continue to play critical roles, but the first and possibly most important step to improving global missile defense capabilities is to create and field a space sensor layer for persistent, birth-to-death missile tracking and discrimination. “Given where the threat is going,” MDA director Vice Adm. James Syring said last year , “persistent tracking and discrimination capability from space is a must.”

Each of the last five presidential administrations has had space-based infrared sensors as a key component of their planned missile defense architectures—at least on paper—but none has yet deployed one. Two demonstrator satellites called STSS are currently on orbit, but they do not operationally contribute, and a larger constellation would be needed to do so. Last year, a campaign memo for president-elect Donald Trump pledged to create just that, endorsing “a comprehensive ballistic missile defense system with a heavy emphasis on space-based early warning and missile tracking technologies.”

This is exactly what is needed, but there’s still no concrete plan to deploy it.

One obstacle is cost, which is why MDA’s Space-based Kill Assessment experiment aims to put a sensor payload on a commercial satellite, to help determine whether interceptor missiles had destroyed their target, or whether another salvo is needed. If successful, this model of commercial hosting could help defray launch costs and lead to a larger, more distributed constellation of smaller and less-expensive satellites.

But until a space layer is fielded, U.S. missile defenses will have too few sensors to definitively discriminate warheads from debris or decoys, which means that more interceptors will need to be fired to ensure a kill, which in turn reduces the effective magazine. The 37 Ground-based Interceptors currently deployed in Alaska and California could be challenged by a larger missile salvo. The emergence of hypersonic boost glide vehicles maneuvering in the high endo-atmosphere and other new threats will further tax terrestrial radars.