Like ground-based radio-frequency communications systems, free-space laser communications systems can suffer from performance problems under adverse atmospheric conditions. In some situations, the apparatus will quit working altogether -- not the best scenario for those wanting a reliable communications system. Light beams encounter all sorts of impediments, such as dust particles, which scatter the light. Fog is one condition that can cause considerable problems for laser communications systems. "In terms of fog, there's not only scattering, but also absorption," said Brian R. Strickland, chief scientist, Advanced Technology Directorate, of the US Army Space and Missile Defense Command. His organization has been studying the effects of fog on these systems with its contractors, Trex Communications Corp. and AstroTerra Corp., both in San Diego. Despite the ultimate potential for system failure, what the researchers found was that the situation is not as bad as they had expected. Using a Trex 570-Mb/s laser communications transceiver, a long-path visibility transmissometer and meteorological instruments, they measured transmission capabilities along a 1207-m line of sight during fog. As detailed in the Jan. 20 issue of Applied Optics, they measured bit error rates of 1027 when the atmospheric transmission was as low as 0.25 percent, and the rate fell to 10210 when transmission was above 2.5 percent. These bit error rates are better than the 1026 that is considered acceptable for most uses, according to Strickland, although an acceptable rate depends on the application. "For video image rates, most people can live with 1022," he said, because the errors just cause a few random pixels to be off. On the other hand, some protocol financial data transmission may require bit error rates as low as 1029 or 10210, he added. The communications wavelength that the experiment used was 810 nm, which is more subject to scattering in fog than to absorption. The study just scratched the surface of what could be done to determine transmission capabilities under adverse atmospheric conditions, Strickland pointed out. Future tests may include analyzing the effects of haze over longer ranges and examining particle size in various types of fog. An optimized laser communications system -- with larger optics, higher laser power and error-correction techniques -- could operate under extremely unfriendly conditions, the group predicts. Design decisions will depend on trade-offs with space or cost constraints, though, Strickland noted. The trick is to keep high transmission rates without prohibitive cost increases, he said.