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However, Mr. Coyle said one problem with the radar is that its resolution is so fine it needs to be “cued,” or directed where to look. That may be a reason it was not deployed, he said.

“Both the [Government Accountability Office] and my former office have questioned whether this radar can survive the maritime environment,” said Mr. Coyle, now with the Center for Defense Information.

The administration's restrictions on missile defenses were disclosed as Mr. Gates announced last week that he is planning a $1.4 billion cut in missile defense funding.

Sen. Jon Kyl, Arizona Republican, and Sen. Joe Lieberman, Connecticut independent, wrote to Mr. Obama on April 6, urging him to reject the missile defense cuts.

The senators warned that the planned missile defense funding cut would undermine international cooperation with Japan, Israel and other states at a time when missile threats are growing.