In a letter sent Tuesday to follow up on questions arising from a March 29 House Space and Aeronautics Subcommittee hearing on NextGen, Government Accountability Office Director of Physical Infrastructure Issues Dr. Gerald Dillingham reiterated that “the current FAA funding structure can provide sufficient funding for NextGen?with some caveats.” He said that the FAA itself has estimated that “if the current taxes remain in effect at their current rates, revenues will continue to increase.” According to projections prepared by the Congressional Budget Office (CBO), revenues obtained from the existing FAA funding structure will increase substantially. “Assuming the General Fund continues to provide about 19 percent of FAA’s budget, CBO estimates that through 2016 the ? Trust Fund ? can support about $19 billion in additional spending over the baseline FAA spending levels CBO has calculated for FAA ? provided that most of the spending occurs after fiscal year 2010,” Dillingham noted. He said that how far this money will go to fund ATC modernization is subject to several uncertainties, including NextGen investment costs, air-traffic volume, future NAS operating costs and future Airport Improvement Program appropriations. Dillingham is also concerned about whether the FAA and other federal partners have the resources and capability to fill the aeronautical research and development that once was the domain of NASA, which has all but been eliminated due to budget cuts at the space agency.