This article deals with the topic of pose and sensor bias estimation based on global navigation satellite system (GNSS) and inertial measurement unit (IMU) measurements, which is a widely discussed topic but usually by omitting the rigorous check of state observability leading to the suspicion of invalid assumptions. The current work underlines this suspicion by pointing out through observability calculations that to estimate position, velocity, attitude and acceleration, and angular rate biases of a system, attitude related measurements should be included besides GNSS position and velocity data. Such measurement can be the magnetic vector. Theoretical results are underlined by tests considering real-flight GNSS and IMU data of a DJI M600 Pro multicopter and comparing the results to the onboard DJI pose estimator.