In this paper, a complexity indicator for 4D flight trajectories is developed
based on conflict probability. A 4D trajectory is modeled as piecewise linear
segments connected by waypoints. The position of each aircraft is modeled as a
2D Gaussian random variable and an approximation of the conflict probability
between two aircraft is deduced analytically over each segment. Based on such
conflict probability, a complexity indicator is constructed for the whole
trajectory. Numerical examples show that the proposed complexity indicator is
able to reflect the complexity of 4D trajectories perceived by air traffic
controllers