Which of the following is the most essential element of a VTS Centre's contingency plan for a complete power failure?

1. A procedure for notifying all vessels in the VTS area of the service outage.
2. A manual logbook and plotting charts to track vessel movements.
3. A list of senior management to be contacted in order of priority.
4. A backup power source, such as an Uninterruptible Power Supply (UPS) and a generator, capable of supporting critical systems.