to estimate position, such as a Global Navigation Satellite System (GNSS) or DME/DME. Relative geometry of the aircraft position to the supporting navigation aids: All radio navaids have this basic variability, although the specific characteristics change. GNSS performance is affected by the relative geometry of the satellites compared to the aircraft. DME/DME navigation solutions are affected by the inclusion angle between the two DMEs at the aircraft (90° being optimal) and the distance to the DMEs, since the aircraft DME transponder can have increasing range errors with increasing distance. Inertial reference units: Errors increase over time since last updated. Application of performance monitoring and alerting to aircraft Although the TSE can change significantly over time for a number of reasons, including those above, the RNP navigation specifications provide assurance that the TSE distribution remains suitable to the operation. This results from two requirements associated with the TSE distribution, namely: The requirement that the TSE remains equal to or better than the required accuracy for 95% of the flight time. The probability that the TSE of each aircraft exceeds the specified TSE limit (equal to two times the accuracy value) without annunciation is less than 10−5. Typically, the 10−5 TSE requirement provides a greater restriction on performance. For example, with any system that has TSE with a normal distribution of cross-track error, the 10−5 monitoring requirement constrains the standard deviation to be 2 × (accuracy value)/4.45 = accuracy value/2.23, while the 95% requirement would have allowed the standard deviation to be as large as the accuracy value/1.96. It is important to understand that while these characteristics define minimum requirements that must be met, they do not define the actual TSE distribution. The actual TSE distribution may be expected to be typically better than the requirement, but there must be evidence on the actual performance if a lower TSE value is to be used. In applying the performance monitoring requirement to aircraft, there can be significant variability in how individual errors are managed: Some systems monitor the actual cross-track and along-track errors individually, whereas others monitor the radial NSE to simplify the monitoring and eliminate dependency on the aircraft track, e.g. based on typical elliptical 2-D error distributions. Some systems include the FTE in the monitor by taking the current value of FTE as a bias on the TSE distribution. For basic GNSS systems, the accuracy and 10−5 requirements are met as