Robust and reliable navigation of Unmanned Aerial Vehicles (UAVs) is an upcoming and promising field of research. In this paper we describe tightly-coupled initialization procedure using a monocular camera to resolve the GPS carrier phase integer ambiguities. This helps us obtain heading estimates during flight, using the GPS receivers. Further, we describe a sensor fusion structure which uses the measurements from multiple GPS receivers and multiple sensors on-board a UAV. We implement an unscented Kalman filter (UKF) to fuse information from the GPS receivers, the monocular camera, the inertial measurement unit (IMU) and the motor tachometers. In the filter we use pseudoranges and carrier phases from the GPS receivers, as opposed to directly using the position updates. Experiments were conducted outdoors with an Ascending Technologies Firefly hexacopter. We validate the integer ambiguity resolution by our tightly coupled initialization, and the robust sensor fusion structure.