WASHINGTON — The current U.S. territorial missile shield has limited effectiveness against the threats likely to emerge from North Korea and Iran in the next decade or two and should be upgraded with higher-speed interceptors, a third interceptor site located in the northeastern United States, and improved integration of radar and optical tracking data for better discrimination between incoming warheads and decoys or debris, a new report concludes.

The congressionally mandated report, prepared by an expert panel of the National Research Council, also dismissed so-called boost-phase missile interceptors as impractical and said a constellation of missile tracking satellites proposed by the U.S. Missile Defense Agency (MDA) would not be worth the likely multibillion-dollar cost. The report, “Making Sense of Ballistic Missile Defense: An Assessment of Concepts and Systems for U.S. Boost-Phased Missile Defense in Comparison to other Alternatives,” was released Sept. 11.

The report also said the MDA’s Phased Adaptive Approach to protecting Europe, which relies and builds upon the existing Aegis Ballistic Missile Defense system, is on track to be able to defend U.S. and allied troops there and in the Middle East against limited attacks from Iran. However, the latter phases of that system, ostensibly intended to augment the U.S. territorial shield, would not be very effective in that capacity.

The report, called for in the Duncan Hunter National Defense Authorization Act of 2009, was intended primarily to assess the practicality and affordability of boost-phase missile defenses against other alternatives, such as midcourse and terminal defenses. Boost-phase refers to the powered phase of ballistic missile flight; midcourse is when the missile is coasting through space; terminal is the period between atmospheric re-entry and impact.

Boost-phase systems are impractical because to be able to engage a missile during its brief liftoff phase of flight, they must be so far forward deployed as to be vulnerable themselves. Moreover, they cannot be deployed close enough to the missile-launching sites of large countries like Iran, the report said.

Space-based systems can overcome some of these geographical disadvantages but their cost would be exorbitant and they would be vulnerable to countermeasures such a salvo launch, the report said. Currently the MDA has no boost-phase missile defense systems in development. The high-speed Kinetic Energy Interceptor was canceled in 2010 while the Airborne Laser was relegated to the role of research and development testbed.

Terminal systems, meanwhile, might be useful as backup defenses for high-value assets, but their range is too limited to protect large swaths of territory, the report said.

“In short, any practical missile defense system must rely primarily on intercept during the midcourse phase of flight,” the report said. “The attraction of midcourse (exoatmospheric) defense is interceptors at a few sites can protect an entire country or even an entire continent, committing the first intercepts only after multiple phenomenology attack assessment.”

The current U.S. territorial shield, called the Ground-based Midcourse Defense system, relies on two interceptor sites, one at Vandenberg Air Force Base, Calif., and the other at Fort Greely Alaska. Some U.S. lawmakers have argued that a third site somewhere in the northeastern United States is necessary and have proposed legislation that would require the Pentagon to study possible locations.

The National Research Council report appears to support that idea, suggesting Fort Drum, New York, and Maine as possible third-site locations. But the third site is just one of several evolutionary upgrades that in combination would make the U.S. shield more effective, said one of the report’s main authors.

“We do not believe that it makes any sense to simply create a third site in the eastern United States without putting the right interceptor and the radars that we recommend in place at the same time,” L. David Montague, a former Lockheed Martin executive and co-chair of the committee that prepared the report, said during a Sept. 11 teleconference with reporters.

The new interceptor would build upon the work done for the Kinetic Energy Interceptor, a super-high-speed boost-phase interceptor that was being developed by Northrop Grumman before its cancellation. The new interceptor would have two stages and feature a heavier, more capable kill vehicle than the one that tops the current three-stage interceptors.

The third site also would require ground-based X-band radars for tracking. The report said that in general, the MDA needs to do a better job of exploiting and integrating data from its ground-based X-band radars and the optical sensors aboard its interceptors to discriminate between missile warheads and decoys or debris such as spent rocket stages. What is needed is a so-called shoot-look-shoot capability in which multiple interceptors can be launched to maximize the probably of hitting an incoming warhead, the report said.

A proposed constellation of tracking satellites known as the Precision Tracking Space System (PTSS) would not help with the challenge of target discrimination and in general is not worth the cost, the report said. The 20-year life cycle cost for PTSS could reach $18.2 billion for a nine-satellite constellation and $37 billion for a 12-satellite constellation, according to the report. The investment is not justified because the Defense Department could use the Space Based Infrared System missile warning satellites in concert with X-band radars to perform the mission more effectively at a lower cost, according to the report.

The MDA disagreed with that assessment, according to Walter Slocombe, co-chairman of the study committee and a former U.S. undersecretary of defense for policy.

“One of the places where there was a specific difference of view is on PTSS, and particularly on the cost estimates,” Slocombe said during the media teleconference. “We believe, and I think you will find in the report’s discussion of cost, we [took] a very thorough and careful look at the cost issues. But cost estimates in this business are uncertain and we have said that we welcome the idea of another genuinely independent review of the costing and of the technical factors that drive the costing.”

In a written response to questions, MDA spokesman Rick Lehner said PTSS is the only “technology available” to provide persistent tracking of missiles globally. Line-of-sight radars are crucial for an integrated missile defense but do not have the capability that PTSS has to provide the tracking the military requires, he said.

The MDA agrees that boost-phase missile defense programs are not practical for the reasons outlined in the report, Lehner said. The Ground-based Midcourse Defense system is an effective defense against threats from North Korea and Iran, he said. There are no plans for a two-stage ground-based missile interceptor, Lehner said.

U.S. Rep. Michael Turner (R-Ohio), chairman of the House Armed Services strategic forces subcommittee, issued the following statement in response to the report:

“Today’s report by the National Research Council highlights that we as a nation have much work to do in countering the threat of long-range missiles to the homeland. As we have seen in report after report, the President’s European phased adaptive approach was ill-conceived and prematurely rolled out by a President more focused on Russia’s concerns than defense of the United States.”