Hi Bob, Sig

The Harrier uses two "systems" when in hover. The primary system is the four nozzles on the fuselage. Two "cold" nozzles just behind the intakes and two"hot" nozzles just before the trailing edge of the wing. These four combine to provide the thrust necessary to keep the harrier hovering in the air. When in hover, these nozzles are rotated down, and the thrust out of each one is constant.

There is a separate "puffer" system with ports on each wing tip, bottom of the nose, bottom of the tail and one lateral port in the tail linked to the rudder. These ports provide the manoverability and stability control in hover. Each of these ports is a valve in its' own right, and are directly connect to the appropriate flying control surfaces. The amount the valves open is proportional to the direction and amount of travel of the connected flying surface.

Although the Harrier doesn't move as quickly in hover as a helicopter, for a fixed wing aircraft standing still it does pretty well.

Ed