Sensor Beacon is a technology that can be researched at the Command Center in Tech Level 2. It costs 75 Food, and gives buildings +4 Line of Sight. It must be researched before Sensor Array becomes available.

This a useful and cheap technology, and makes it easier to spot approaching enemy units, but it's not vital.

"All buildings gain a substantial bonus to their lines of sight with this Tech Level 2 technology."
—Manual description