Technique for improving the accuracy of air-to-ground radio distance measurement systems

A subsystem is described that can be added to existing radio distance measuring systems, such as SHIRAN, to increase the accuracy. The modification gives the accuracy characteristics of optical ranging systems but requires light transmission in only one direction over the path being measured. The optical subsystem can be used as seeing conditions permit, and when it cannot be used the radio system performance is not affected. Available components and technology indicate operational ranges up to several hundred kilometers.