Project studies by both firms started in 1964, at about the same time the US Army started a project which resulted in the TOW missile, but unlike the TOW which entered service in 1973, the development and testing phase for the HOT took considerably longer.
After the missile is fired, all the gunner has to do is keep the target in the sight's cross hairs, and the system will automatically track the missile's rear-facing flares, gather the missile into the gunner's sight, and send commands to steer the missile into the gunner's line of sight.