RF measurements of the speed of light

Modern RF measurements of the speed of light are reviewed, with emphasis on those methods capable of attaining an accuracy of 1.0 km/s or better. The geodimeter and microwave interferometer methods are discussed in some detail because of the significant role they have played in arriving at our present value for c. Serious limitations of the microwave resonant cavity method are pointed out and brief mention is made of the electrical units and band spectrum methods. A recent measurement by Karolus and Helmberger, who obtained the value 299 792.1±0.2 km/s, is mentioned. Some experimental factors that should be considered in planning future speed-of-light measurements are presented, and an experiment involving lasers, now in progress at the Joint Institute of Laboratory Astrophysics, is described briefly. The conclusion is that the presently accepted value of 299 792.5±0.3 km/s is still valid. Only two or three measurements have been completed since its adoption nearly ten years ago and they are not of sufficient accuracy to warrant a re-evaluation of c.