The Transit navigational satellite system is expected to be in operational use during the second half of 1962, though not perhaps with its full quota of four satellites. The system, which will be of considerable significance to both air and surface navigation, is here described by its originators, Dr. Kershner and Dr. Newton, of Johns Hopkins University in the United States. Using a single of the two frequencies transmitted by the satellite, the mean navigation error will be about one mile, limited by refraction. Using both frequencies the mean error will be about ½ mile, limited by the accuracy of the satellite coordinates that will be transmitted. With elaborate equipment, it will be possible to navigate to about 0·1 mile, although the satellite coordinates that will be available to the ordinary user will not reflect this full accuracy. When all four satellites are in orbit, a fix will be available every 110 minutes, on the average, with occasional intervals of 220 minutes between fixes.