Companies currently use product lines to enhance products qualities and to lower the development and marketing costs. Many approaches available to support product lines, but most of them fail to address safety critical requirements in their approaches. In this paper we present DOPLER⁺, an approach to handle safety critical system requirements and the addition of new safety critical requirements, and their influences on other features and requirement within the product line. To illustrate our approach, we apply DOPLER⁺ to mobile robot case study.