WASHINGTON — The Missile Defense Agency is planning to develop a layered homeland intercontinental ballistic missile defense architecture, but it must clear a range of hurdles to get after an approach that addresses emerging threats and fills a gap while a next-generation interceptor is developed, according to the agency’s director.

The agency unveiled plans in its fiscal 2021 budget request in February to create a more layered homeland defense system that would include regional missile defense capability already resident with the Navy and Army to bolster homeland defense against ICBMs.

The plan would include establishing layers of defensive capability relying on the Aegis Weapon System, particularly the SM-3 Block IIA missiles used in the system, and a possible Aegis Ashore system in Hawaii. The underlay would also include the Terminal High Altitude Area Defense (THAAD) system. The Army is already operating a THAAD battery in South Korea and Guam.

The layered approach would buy time while the Pentagon scrambles to field a new interceptor to replace older ground-based interceptors — after canceling its effort to redesign the kill vehicle for the GBIs — in its Ground-based Midcourse Defense system located primarily at Fort Greely, Alaska.

With little detail conveyed in MDA’s budget request for a layered homeland missile defense plan, Congress is pressuring the agency to come up with a strategy and approach to putting the architecture in place in both the House and Senate passed FY21 defense authorization bills.

Much is riding on the success of an upcoming test of the SM-3 Block IIA missile, Vice Adm. Jon Hill said during a Heritage Foundation virtual event on Aug. 18. The missile has seen several test failures in the recent past.

“We’re going to really stress the SM-3 Block IIA outside its design space,” Hill said. “It was designed for medium and intermediate range. Now we’re going against long-range intercontinental ballistic missiles. The analysis says we’ll be successful, but nothing is real to any of us until we actually get the empirical data from being out in the flight range.”

The test will involve several time zones on several different ranges and the same ICBM target used in the most recent GMD test, Hill said.

And while the test is still on track to happen by the end of the year, Hill said, challenges in coordination and travel due to the coronavirus pandemic could possibly have an impact on schedule.

When the pandemic hit, “we were ready and postured to go to the Pacific to execute” Flight Test Maritime-44, Hill said, “so we did get the target on station. So the target’s out there in the Pacific and it’s ready to roll.”

Following FTM-44, the agency would like to execute another test against a very complex ICBM target unlike the “simple” one being used in the upcoming test. That target will have “a lot of separation debris and that has a lot of countermeasures,” Hill said.

“We want to make sure that the system in total, from the space assets to the radar to the engage-on-remote capability that passes that information to the ship, that ship can actually sift through all that and say, ‘That’s the [reentry vehicle] and that’s the where the missile’s going to go,‘” he added.

Success in the upcoming test doesn’t mean the agency’s work is done, Hill said. Upgrades will be required based on threats, combat system certifications will need to be conducted and work with the Navy to determine where Aegis ships would deploy will have to occur.

The agency will also have to determine how quickly it can ramp up its production line for SM-3 Block IIA missiles.

Then “if we succeed with Aegis, then we’ll go right down the path with THAAD,” Hill said.

The second big challenge, after ensuring all the parts work to provide layered coverage, is developing engagement coordination between the different layers, according to Hill.

“Let’s just say that step one is a ship off the coast as a complement to GMD. Those systems today talk already, but they’re not talking in terms of being layered defenders,” Hill said. “So if GMD, for example, decides he’s going to wait this first shot out and let the ship take it, we have to have the communication network to go do that. We have to have the technical architecture with the Command and Control Battle Management system, but in that context of layered defense and engagement coordination.”

Aegis ships are already able to do engagement coordination among each other and the work the Pentagon is doing to get THAAD and the Patriot air-and-missile defense system to coordinate are “extensible to that problem,” Hill said, “but we still need to do that kind of engineering and that sort of architecture work.”

And while a layered missile defense architecture for the homeland is an intricate one, “you don’t have to solve the whole problem” at once, Rebecca Heinrichs, a missile defense analyst at the Hudson Institute, said during the Heritage Foundation event. But she cautioned that she did foresee challenges in establishing such an architecture on the political front rather than on the technical side.

“Congress always wants to vet these kinds of things, so even though it is Congress that wanted the SM-3 Block IIA test, whenever you start talking about which district it is going to be in, where it’s going to go and that kind of thing, that is a political challenge that takes a lot of debate and conversation.”