Localizer, glideslope, and other instrument landing aids (such as microwave transmitters) are both high power consumers and mission-critical, and cannot be reliably operated from a battery supply, even for short periods. Hence, when absolute reliability is required (such as when Category 3 operations are in force at the airport) it is usual to run the system from a diesel generator with automatic switchover to the mains supply should the generator fail. This avoids any interruption to transmission while a generator is brought up to operating speed.