WASHINGTON In the struggle to modernize air traffic control systems, the most crucial missing link is the selection of a digital data link for two-way communication between aircraft and controllers. The airline industry sees digital communications combined with global navigation satellite systems as essential to improving the efficiency and capacity of air transport operations.

McDonnell Douglas recently flight-tested its DC-X vehicle, successfully demonstrating key operational features that would be needed by a reusable, rocket-powered, single-stage-to-orbit space launcher. These tests have sparked a new round of debate on the merits and risks of SSTO launch systems.

BOSTON The U. S. Technology Reinvestment Project is funding a major effort to speed the development of fly-by-light and power-by-wire technology for use in military and commercial aircraft. The project, which is a key part of President Bill Clinton's defense conversion initiative, also chose to invest in rocket and jet engine technology as well as satellite antennas.