A mathematical collision risk model can be used for evaluating air traffic control (ATC) separation standards. As one of the parameters required for calculating the expected number of collisions by this model, the probability of overlap in the vertical dimension based on height-keeping errors of aircraft must be estimated.

However, there are few examples of empirical data on height-keeping errors because data collection is expensive and time-consuming. In order to collect these data, the author et al. started data collection using a newly developed simple height-finding radar, the navigation accuracy measurement system (NAMS) which has been installed under a trunk airway in Japan.