In an experimental analysis of a free-space optical communications system, researchers at the University of California, San Diego, and the Army Research Laboratory in Adelphi, Md., have determined that chaotic pulse position modulation enables robust transmission through atmospheric turbulence. They reported their findings in the Dec. 30 issue of Physical Review Letters.The setup employed a 10-mW, 690-nm diode laser as a light source. The signal traveled 2.5 km through the atmosphere to a corner reflector and 2.5 km back to a PIN photodetector. A chaotic transceiver controller generated a series of 1-µs pulses from the laser, and a timer circuit in the receiver generated a chaotic decoder map based on the interpulse intervals of the received signal.The researchers estimated that the contribution to bit error rate by the chaoticity of the transmitted signal in the 60-kb/s system was approximately 5.5 x 10-5.