This paper describes the processes for estimat-ing and analyzing the position states onboard small unmanned aerial vehicles in the low-altitude simulation environment. Those processes are explained using two simulation platforms, Robot Operating System and Gazebo. They comprise different system functionalities from trajectory generation, linear-kinematic trajectory conversion, path controllers to the modeling and configuration of various aerial vehicles, onboard positioning and navigation sensors, and creation and visualization of the simulation environment. We model and simulate the vehicle trajectories to determine 3-D positions from the GPS sensor, along with estimated positions from the fusion of GPS/IMU and Altimeter. The simulated results have provided a dataset on lateral and vertical trajectory profile guidance and prediction in the low altitude airspaces for follow-up research on the common reference altitude determination, as well as the definition of the well clear and collision states for the detect and avoid functions of the small unmanned aerial vehicles.