Broadband Photonics Incorporated proposes to develop a patent-pending fiber optic continuous liquid sensor for low-thrust level settled mass gauging with measurement uncertainty <0.5% over fill levels from 2% to 98%. The fiber optic liquid sensor has significant advantages over the existing liquid sensors, including Delta-P pressure sensors, capacitance probes, ultrasonic sensors, and silicon diode point sensors in terms of gauging accuracy, reliability, simplicity, and maintenance. The proposed sensor is able to replace silicon diode point sensors currently used for propellant gauging without any modification on the tank. In Phase 1, we will prove the feasibility of the liquid sensor, including demonstration of 1 mm liquid level spatial resolution and development of the robust sensing fiber for cryogenic temperature applications. In Phase 2, we will further develop the prototype of the fiber optic liquid sensor.