Four parameters define the requirements for navigational aids that operate in civil airspace: accuracy, integrity, continuity, and availability. Accuracy refers to how well the system can provide correct position information in the absence of a failure. Integrity refers to the ability of the system to inform the user in a timely manner of a latent failure that may cause a hazardous condition. (A latent failure is an undetected degradation of the navigational aid operation.)

Continuity refers to the ability of the system to let the user navigate without interruption. For example, radio-frequency interference (jamming) and blockage of radionavigation signals are potential causes of a loss of continuity. Availability is the probability that the system will function and provide required levels of accuracy, integrity, and continuity.