GNSS based positioning and navigation always require perfect synchronization between the receiver and satellites clock. Further, due to the limited frequency stability of the GNSS receiver’s internal oscillator, an additional receiver clock error has to be estimated along with the coordinates. Thus, the observation geometry is changed; it results in some disadvantages which are: at least four satellites are required for positioning or navigation, high correlations are generated among the estimated receiver clock, the up-component and tropospheric delay, and the up-component is estimated less precisely than the horizontal coordinates. Research has shown that these drawbacks can be avoided by replacing the receiver internal oscillator with a more stable external clock and modelling its operation in a physically meaningful way over intervals in which the oscillator noise is far less than the observation noise. This method is known as receiver clock modelling (RCM). In this contribution, we will present a simulation study which is done to evaluate the gain in performance by RCM in code-based GNSS flight navigation where the height component is of relevance. Different flight test trajectories are simulated with code observation of a multi-GNSS system. Observations for different test trajectories are evaluated with and without RCM using different types of external clocks. The gain in precision of the coordinates for different trajectories w.r.t different clocks will be presented.