This paper introduces a concept, a baseline design, and a trade study for a new space-based global continuous disaster monitoring system composed of a dual-mode satellite constellation and on-orbit propellant depots. The proposed constellation operates in two different modes: a normal mode and a disaster mode, which are responsible for atmospheric/oceanic imaging and disaster monitoring, respectively. The dual-mode concept enables the system to manage the uncertainties associated with the unknown time and location of a disaster and to enhance its operational efficiency by improving its utilization. The mode-change requires orbit transfers accompanying large amounts of fuel consumption, and this challenge is addressed by an on-orbit refueling system to support the constellation. A reference design for the proposed satellite constellation and the orbiting depot is presented. Orbital parameters and the options for mode-change transfers are explored considering the trade-off relationships among the propellant consumption (to minimize), the response time (to minimize), and the access area in normal mode (to maximize). Options for the number of on-orbit propellant depots and the drift rate for the refueling operation are also explored considering the time to complete the preparation and associated probability to get ready for the next disaster outbreak.