BOSTON — A rigorous and comprehensive system-verification program adopted for the U.S. territorial missile shield in the wake of back-to-back test failures has Pentagon officials feeling much better about the flight demonstrations that lay ahead.

The Missile Defense Agency (MDA) and its contractors are scrutinizing the interceptor rockets of the Ground Based Midcourse Defense System (GMD) more carefully than ever as they prepare for upcoming tests, said U.S. Army Brig. Gen. Patrick O’Reilly, the program director at Redstone Arsenal in Huntsville, Ala.

The MDA has three GMD flight demonstrations scheduled for 2006, two of which include planned intercepts. In preparation, the agency and its contractors are doing just about every conceivable test on the rockets short of launching them, O’Reilly said in a March 1 telephone interview.

The MDA suffered successive embarrassments in GMD demonstrations in late 2004 and early 2005, when interceptors that were supposed to engage or knock down target missiles failed to launch. At the time, the MDA was deploying GMD interceptors at sites in Alaska and California and was preparing to declare the system ready to defend U.S. territory against missile attacks.

Following the failures, the MDA and the White House came under a hail of criticism for rushing an insufficiently tested system into the field.

The MDA’s response to the failures was to charter a review team to develop a new test strategy, and to divert some interceptors slated for operational deployment to be used for testing purposes.

O’Reilly said the more-rigorous GMD scrutiny is being applied by MDA as well as the contracting team. Boeing Co. of Chicago is the GMD prime contractor; Orbital Sciences Corp. of Dulles, Va., supplies the main interceptor rocket; and Raytheon Co. of Waltham, Mass., is responsible for the warhead-seeking kill vehicle that goes atop the interceptor rocket.

Every aspect in the process of preparing the interceptors, down to the ordering and assembly of individual components, is being looked at closely, O’Reilly said.

Scott Fancher, Boeing vice president and GMD program director, said the more-rigorous system scrutiny, both on hardware and software, paid off with the successful test flight of the GMD rocket in December. Fancher came to the GMD program in November after leading Boeing’s work on the Airborne Laser, a 747 aircraft being equipped with a high-power laser to shoot down missiles as they lift off from the ground.

Boeing’s team completed its pre-test check-out earlier than usual in the lead-up to the December flight test, and that helped avoid a rush at the end in which a potential problem might have escaped notice , Fancher said.

But Fancher stressed that Boeing is wary of getting overconfident. “It doesn’t mean that it’s going to be smooth sailing,” he said. “We expect continued challenges.”

The December test did not attempt to hit a target missile, but did represent the first successful flight of the operationally configured GMD interceptor. The Orbital Sciences-built booster flies much faster than the prototype boosters — assembled from Minuteman strategic missile motors — that have been used in previous intercept tests, O’Reilly said.

The next attempt to knock down a missile using the operational interceptor is scheduled for late spring or early summer.

Meanwhile, the MDA is looking ahead to future improvements to the GMD system. Among them is the Multiple Kill Vehicle system, which is scheduled to begin flight testing in 2009.

Being developed by Lockheed Martin Space Systems of Sunnyvale, Calif., the Multiple Kill Vehicle is designed to solve the sticky problem of discriminating between missile warheads and decoys with a shotgun approach that destroys all likely looking targets in the immediate vicinity.

The MDA is weighing the possibility of testing the Multiple Kill Vehicle atop the Kinetic Energy Interceptor, a super-fast rocket designed to destroy missiles in their boost phase. Northrop Grumman Mission Systems of Fair Lakes, Va., is developing the Kinetic Energy Interceptor in preparation for a 2008 flight test that could determine the program’s future.

O’Reilly noted that the unit cost of the Multiple Kill Vehicle system will be reduced if it is used on both the GMD and Kinetic Energy Interceptor programs.

The GMD program will continue testing for the foreseeable future, and there is neither a deadline for declaring the system operational nor a specific set of criteria for doing so, O’Reilly said.