One of the more interesting aspects of ILS systems is how they are powered when Category II or III operations are in force. These low ceiling and visibility approaches obviously cannot afford to have even a second or two of off-line time. So obviously a backup power supply is needed, and there is a standby generator in nearly every ILS set up. However, the time to change over from mains to standby power takes some time, and this is too long a time during Category II or III operations. The solution is quite ingenious- run the system on the standby during those periods, with the changeover time to using the mains being measured in fractions of a second in the event that the standby suddenly goes down.