Hybrid laser and RF communications system to link space, aircraft, and surface sites reliably in atmospheric turbulence

by John Keller

Optical communications experts at the Johns Hopkins University Applied Physics Laboratory, Laurel, Md., will design and demonstrate a hybrid free-space optical communications and radio frequency data modem to provide a reliable, fast communications link among spacecraft, aircraft, military vehicles, ground installations, and ships at sea.

Johns Hopkins scientists are doing the RF and laser communications work under terms of a $35.9 million contract awarded Tuesday by the U.S. Defense Advanced Projects Agency (DARPA) Strategic Technology Office in Arlington, Va., as part of the DARPA Optical Modem project.

Johns Hopkins engineers will design and test an optical modem that will allow multi-gigabit per second, hybrid laser-RF communications at long range, and under extreme atmospheric turbulence and cloudy conditions.

Hopkins experts will blend free-space optical (FSO) and radio frequency (RF) communications terminals and non-deterministic network routers. DARPA is sponsoring this research together with the Naval Research Laboratory in Washington and the Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio.

Hopkins will provide as many as eight free-space optical terminals with advanced optical modems, as many as four hybrid FSO/RF network routers and node controllers, and related equipment for three test aircraft and one ground station for field testing.

The prototype system should be able to operate on large aircraft such as the Navy P-3 Orion, at altitudes as high as 25,000 feet, and distances as far as 125 miles air-to-air and 30 miles air-to-ground, at data rates of 10 gigabits per second and 270 megabits per second, respectively, and near-perfect reliability and error-free operations.



The optical modem, including its optical automatic gain control (OAGC) and forward error correction, is the key subsystem for compensating for large channel fading to yield error-free 10-gigabit-per-second data rate in atmospheric turbulence.
 Officials from DARPA, the Navy, and the Air Force will evaluate the integrated system at places like the Nevada Testing and Training Range (NTTR) or White Sands Missile Range.